1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132
133
134
135
136
137
138
139
140
141
142
143
144
145
146
147
148
149
150
151
152
153
154
155
156
157
158
159
160
161
162
163
164
165
166
167
168
169
170
171
172
173
174
175
176
177
178
179
180
181
182
183
184
185
186
187
188
189
190
191
192
193
194
195
196
197
198
199
200
201
202
203
204
205
206
207
208
209
210
211
212
213
214
215
216
217
218
219
220
221
222
223
224
225
226
227
228
229
230
231
232
233
234
235
236
237
238
239
240
241
242
243
244
245
246
247
248
249
250
251
252
253
254
255
256
257
258
259
260
261
262
263
264
265
266
267
268
269
270
271
272
273
274
275
276
277
278
279
280
281
282
283
284
285
286
287
288
289
290
291
292
293
294
295
296
297
298
299
300
301
302
303
304
305
306
307
308
309
310
311
312
313
314
315
316
317
318
319
320
321
322
323
324
325
326
327
328
329
330
331
332
333
334
335
336
337
338
339
340
341
342
343
344
345
346
347
348
349
350
351
352
353
354
355
356
357
358
359
360
361
362
363
364
365
366
367
368
369
370
371
372
373
374
375
376
377
378
379
380
381
382
383
384
385
386
387
388
389
390
391
392
393
394
395
396
397
398
399
400
401
402
403
404
405
406
407
408
409
410
411
412
413
414
415
416
417
418
419
420
421
422
423
424
425
426
427
428
429
430
431
432
433
434
435
436
437
438
439
440
441
442
443
444
445
446
447
448
449
450
451
452
453
454
455
456
457
458
459
460
461
462
463
464
465
466
467
468
469
470
471
472
473
474
475
476
477
478
479
480
481
482
483
484
485
486
487
488
489
490
491
492
493
494
495
496
497
498
499
500
501
502
503
504
505
506
507
508
509
510
511
512
513
514
515
516
517
518
519
520
521
522
523
524
525
526
527
528
529
530
531
532
533
534
535
536
537
538
539
540
541
542
543
544
545
546
547
548
549
550
551
552
553
554
555
556
557
558
559
560
561
562
563
564
565
566
567
568
569
570
571
572
573
574
575
576
577
578
579
580
581
582
583
584
585
586
587
588
589
590
591
592
593
594
595
596
597
598
599
600
601
602
603
604
605
606
607
608
609
610
611
612
613
614
615
616
617
618
619
620
621
622
623
624
625
626
627
628
629
630
631
632
633
634
635
636
637
638
639
640
641
642
643
644
645
646
647
648
649
650
651
652
653
654
655
656
657
658
659
660
661
662
663
664
665
666
667
668
669
670
671
672
673
674
675
676
677
678
679
680
681
682
683
684
685
686
687
688
689
690
691
692
693
694
695
696
697
698
699
700
701
702
703
704
705
706
707
708
709
710
711
712
713
714
715
716
717
718
719
720
721
722
723
724
725
726
727
728
729
730
731
732
733
734
735
736
737
738
739
740
741
742
743
744
745
746
747
748
749
750
751
752
753
754
755
756
757
758
759
760
761
762
763
764
765
766
767
768
769
770
771
772
773
774
775
776
777
778
779
780
781
782
783
784
785
786
787
788
789
790
791
792
793
794
795
796
797
798
799
800
801
802
803
804
805
806
807
808
809
810
811
812
813
814
815
816
817
818
819
820
821
822
823
824
825
826
827
828
829
830
831
832
833
834
835
836
837
838
839
840
841
842
843
844
845
846
847
848
849
850
851
852
853
854
855
856
857
858
859
860
861
862
863
864
865
866
867
868
869
870
871
872
873
874
875
876
877
878
879
880
881
882
883
884
885
886
887
888
889
890
891
892
893
894
895
896
897
898
899
900
901
902
903
904
905
906
907
908
909
910
911
912
913
914
915
916
917
918
919
920
921
922
923
924
925
926
927
928
929
930
931
932
933
934
935
936
937
938
939
940
941
942
943
944
945
946
947
948
949
950
951
952
953
954
955
956
957
958
959
960
961
962
963
964
965
966
967
968
969
970
971
972
973
974
975
976
977
978
979
980
981
982
983
984
985
986
987
988
989
990
991
992
993
994
995
996
997
998
999
1000
1001
1002
1003
1004
1005
1006
1007
1008
1009
1010
1011
1012
1013
1014
1015
1016
1017
1018
1019
1020
1021
1022
1023
1024
1025
1026
1027
1028
1029
1030
1031
1032
1033
1034
1035
1036
1037
1038
1039
1040
1041
1042
1043
1044
1045
1046
1047
1048
1049
1050
1051
1052
1053
1054
1055
1056
1057
1058
1059
1060
1061
1062
1063
1064
1065
1066
1067
1068
1069
1070
1071
1072
1073
1074
1075
1076
1077
1078
1079
1080
1081
1082
1083
1084
1085
1086
1087
1088
1089
1090
1091
1092
1093
1094
1095
1096
1097
1098
1099
1100
1101
1102
1103
1104
1105
1106
1107
1108
1109
1110
1111
1112
1113
1114
1115
1116
1117
1118
1119
1120
1121
1122
1123
1124
1125
1126
1127
1128
1129
1130
1131
1132
1133
1134
1135
1136
1137
1138
1139
1140
1141
1142
1143
1144
1145
1146
1147
1148
1149
1150
1151
1152
1153
1154
1155
1156
1157
1158
1159
1160
1161
1162
1163
1164
1165
1166
1167
1168
1169
1170
1171
1172
1173
1174
1175
1176
1177
1178
1179
1180
1181
1182
1183
1184
1185
1186
1187
1188
1189
1190
1191
1192
1193
1194
1195
1196
1197
1198
1199
1200
1201
1202
1203
1204
1205
1206
1207
1208
1209
1210
1211
1212
1213
1214
1215
1216
1217
1218
1219
1220
1221
1222
1223
1224
1225
1226
1227
1228
1229
1230
1231
1232
1233
1234
1235
1236
1237
1238
1239
1240
1241
1242
1243
1244
1245
1246
1247
1248
1249
1250
1251
1252
1253
1254
1255
1256
1257
1258
1259
1260
1261
1262
1263
1264
1265
1266
1267
1268
1269
1270
1271
1272
1273
1274
1275
1276
1277
1278
1279
1280
1281
1282
1283
1284
1285
1286
1287
1288
1289
1290
1291
1292
1293
1294
1295
1296
1297
1298
1299
1300
1301
1302
1303
1304
1305
1306
1307
1308
1309
1310
1311
1312
1313
1314
1315
1316
1317
1318
1319
1320
1321
1322
1323
1324
1325
1326
1327
1328
1329
1330
1331
1332
1333
1334
1335
1336
1337
1338
1339
1340
1341
1342
1343
1344
1345
1346
1347
1348
1349
1350
1351
1352
1353
1354
1355
1356
1357
1358
1359
1360
1361
1362
1363
1364
1365
1366
1367
1368
1369
1370
1371
1372
1373
1374
1375
1376
1377
1378
1379
1380
1381
1382
1383
1384
1385
1386
1387
1388
1389
1390
1391
1392
1393
1394
1395
1396
1397
1398
1399
1400
1401
1402
1403
1404
1405
1406
1407
1408
1409
1410
1411
1412
1413
1414
1415
1416
1417
1418
1419
1420
1421
1422
1423
1424
1425
1426
1427
1428
1429
1430
1431
1432
1433
1434
1435
1436
1437
1438
1439
1440
1441
1442
1443
1444
1445
1446
1447
1448
1449
1450
1451
1452
1453
1454
1455
1456
1457
1458
1459
1460
1461
1462
1463
1464
1465
1466
1467
1468
1469
1470
1471
1472
1473
1474
1475
1476
1477
1478
1479
1480
1481
1482
1483
1484
1485
1486
1487
1488
1489
1490
1491
1492
1493
1494
1495
1496
1497
1498
1499
1500
1501
1502
1503
1504
1505
1506
1507
1508
1509
1510
1511
1512
1513
1514
1515
1516
1517
1518
1519
1520
1521
1522
1523
1524
1525
1526
1527
1528
1529
1530
1531
1532
1533
1534
1535
1536
1537
1538
1539
1540
1541
1542
1543
1544
1545
1546
1547
1548
1549
1550
1551
1552
1553
1554
1555
1556
1557
1558
1559
1560
1561
1562
1563
1564
1565
1566
1567
1568
1569
1570
1571
1572
1573
1574
1575
1576
1577
1578
1579
1580
1581
1582
1583
1584
1585
1586
1587
1588
1589
1590
1591
1592
1593
1594
1595
1596
1597
1598
1599
1600
1601
1602
1603
1604
1605
1606
1607
1608
1609
1610
1611
1612
1613
1614
1615
1616
1617
1618
1619
1620
1621
1622
1623
1624
1625
1626
1627
1628
1629
1630
1631
1632
1633
1634
1635
1636
1637
1638
1639
1640
1641
1642
1643
1644
1645
1646
1647
1648
1649
1650
1651
1652
1653
1654
1655
1656
1657
1658
1659
1660
1661
1662
1663
1664
1665
1666
1667
1668
1669
1670
1671
1672
1673
1674
1675
1676
1677
1678
1679
1680
1681
1682
1683
1684
1685
1686
1687
1688
1689
1690
1691
1692
1693
1694
1695
1696
1697
1698
1699
1700
1701
1702
1703
1704
1705
1706
1707
1708
1709
1710
1711
1712
1713
1714
1715
1716
1717
1718
1719
1720
1721
1722
1723
1724
1725
1726
1727
1728
1729
1730
1731
1732
1733
1734
1735
1736
1737
1738
1739
1740
1741
1742
1743
1744
1745
1746
1747
1748
1749
1750
1751
1752
1753
1754
1755
1756
1757
1758
1759
1760
1761
1762
1763
1764
1765
1766
1767
1768
1769
1770
1771
1772
1773
1774
1775
1776
1777
1778
1779
1780
1781
1782
1783
1784
1785
1786
1787
1788
1789
1790
1791
1792
1793
1794
1795
1796
1797
1798
1799
1800
1801
1802
1803
1804
1805
1806
1807
1808
1809
1810
1811
1812
1813
1814
1815
1816
1817
1818
1819
1820
1821
1822
1823
1824
1825
1826
1827
1828
1829
1830
1831
1832
1833
1834
1835
1836
1837
1838
1839
1840
1841
1842
1843
1844
1845
1846
1847
1848
1849
1850
1851
1852
1853
1854
1855
1856
1857
1858
1859
1860
1861
1862
1863
1864
1865
1866
1867
1868
1869
1870
1871
1872
1873
1874
1875
1876
1877
1878
1879
1880
1881
1882
1883
1884
1885
1886
1887
1888
1889
1890
1891
1892
1893
1894
1895
1896
1897
1898
1899
1900
1901
1902
1903
1904
1905
1906
1907
1908
1909
1910
1911
1912
1913
1914
1915
1916
1917
1918
1919
1920
1921
1922
1923
1924
1925
1926
1927
1928
1929
1930
1931
1932
1933
1934
1935
1936
1937
1938
1939
1940
1941
1942
1943
1944
1945
1946
1947
1948
1949
1950
1951
1952
1953
1954
1955
1956
1957
1958
1959
1960
1961
1962
1963
1964
1965
1966
1967
1968
1969
1970
1971
1972
1973
1974
1975
1976
1977
1978
1979
1980
1981
1982
1983
1984
1985
1986
1987
1988
1989
1990
1991
1992
1993
1994
1995
1996
1997
1998
1999
2000
2001
2002
2003
2004
2005
2006
2007
2008
2009
2010
2011
2012
2013
2014
2015
2016
2017
2018
2019
2020
2021
2022
2023
2024
2025
2026
2027
2028
2029
2030
2031
2032
2033
2034
2035
2036
2037
2038
2039
2040
2041
2042
2043
2044
2045
2046
2047
2048
2049
2050
2051
2052
2053
2054
2055
2056
2057
2058
2059
2060
2061
2062
2063
2064
2065
2066
2067
2068
2069
2070
2071
2072
2073
2074
2075
2076
2077
2078
2079
2080
2081
2082
2083
2084
2085
2086
2087
2088
2089
2090
2091
2092
2093
2094
2095
2096
2097
2098
2099
2100
2101
2102
2103
2104
2105
2106
2107
2108
2109
2110
2111
2112
2113
2114
2115
2116
2117
2118
2119
2120
2121
2122
2123
2124
2125
2126
2127
2128
2129
2130
2131
2132
2133
2134
2135
2136
2137
2138
2139
2140
2141
2142
2143
2144
2145
2146
2147
2148
2149
2150
2151
2152
2153
2154
2155
2156
2157
2158
2159
2160
2161
2162
2163
2164
2165
2166
2167
2168
2169
2170
2171
2172
2173
2174
2175
2176
2177
2178
2179
2180
2181
2182
2183
2184
2185
2186
2187
2188
2189
2190
2191
2192
2193
2194
2195
2196
2197
2198
2199
2200
2201
2202
2203
2204
2205
2206
2207
2208
2209
2210
2211
2212
2213
2214
2215
2216
2217
2218
2219
2220
2221
2222
2223
2224
2225
2226
2227
2228
2229
2230
2231
2232
2233
2234
2235
2236
2237
2238
2239
2240
2241
2242
2243
2244
2245
2246
2247
2248
2249
2250
2251
2252
2253
2254
2255
2256
2257
2258
2259
2260
2261
2262
2263
2264
2265
2266
2267
2268
2269
2270
2271
2272
2273
2274
2275
2276
2277
2278
2279
2280
2281
2282
2283
2284
2285
2286
2287
2288
2289
2290
2291
2292
2293
2294
2295
2296
2297
2298
2299
2300
2301
2302
2303
2304
2305
2306
2307
2308
2309
2310
2311
2312
2313
2314
2315
2316
2317
2318
2319
2320
2321
2322
2323
2324
2325
2326
2327
2328
2329
2330
2331
2332
2333
2334
2335
2336
2337
2338
2339
2340
2341
2342
2343
2344
2345
2346
2347
2348
2349
2350
2351
2352
2353
2354
2355
2356
2357
2358
2359
2360
2361
2362
2363
2364
2365
2366
2367
2368
2369
2370
2371
2372
2373
2374
2375
2376
2377
2378
2379
2380
2381
2382
2383
2384
2385
2386
2387
2388
2389
2390
2391
2392
2393
2394
2395
2396
2397
2398
2399
2400
2401
2402
2403
2404
2405
2406
2407
2408
2409
2410
2411
2412
2413
2414
2415
2416
2417
2418
2419
2420
2421
2422
2423
2424
2425
2426
2427
2428
2429
2430
2431
2432
2433
2434
2435
2436
2437
2438
2439
2440
2441
2442
2443
2444
2445
2446
2447
2448
2449
2450
2451
2452
2453
2454
2455
2456
2457
2458
2459
2460
2461
2462
2463
2464
2465
2466
2467
2468
2469
2470
2471
2472
2473
2474
2475
2476
2477
2478
2479
2480
2481
2482
2483
2484
2485
2486
2487
2488
2489
2490
2491
2492
2493
2494
2495
2496
2497
2498
2499
2500
2501
2502
2503
2504
2505
2506
2507
2508
2509
2510
2511
2512
2513
2514
2515
2516
2517
2518
2519
2520
2521
2522
2523
2524
2525
2526
2527
2528
2529
2530
2531
2532
2533
2534
2535
2536
2537
2538
2539
2540
2541
2542
2543
2544
2545
2546
2547
2548
2549
2550
2551
2552
2553
2554
2555
2556
2557
2558
2559
2560
2561
2562
2563
2564
2565
2566
2567
2568
2569
2570
2571
2572
2573
2574
2575
2576
2577
2578
2579
2580
2581
2582
2583
2584
2585
2586
2587
2588
2589
2590
2591
2592
2593
2594
2595
2596
2597
2598
2599
2600
2601
2602
2603
2604
2605
2606
2607
2608
2609
2610
2611
2612
2613
2614
2615
2616
2617
2618
2619
2620
2621
2622
2623
2624
2625
2626
2627
2628
2629
2630
2631
2632
2633
2634
2635
2636
2637
2638
2639
2640
2641
2642
2643
2644
2645
2646
2647
2648
2649
2650
2651
2652
2653
2654
2655
2656
2657
2658
2659
2660
2661
2662
2663
2664
2665
2666
2667
2668
2669
2670
2671
2672
2673
2674
2675
2676
2677
2678
2679
2680
2681
2682
2683
2684
2685
2686
2687
2688
2689
2690
2691
2692
2693
2694
2695
2696
2697
2698
2699
2700
2701
2702
2703
2704
2705
2706
2707
2708
2709
2710
2711
2712
2713
2714
2715
2716
2717
2718
2719
2720
2721
2722
2723
2724
2725
2726
2727
2728
2729
2730
2731
2732
2733
2734
2735
2736
2737
2738
2739
2740
2741
2742
2743
2744
2745
2746
2747
2748
2749
2750
2751
2752
2753
2754
2755
2756
2757
2758
2759
2760
2761
2762
2763
2764
2765
2766
2767
2768
2769
2770
2771
2772
2773
2774
2775
2776
2777
2778
2779
2780
2781
2782
2783
2784
2785
2786
2787
2788
2789
2790
2791
2792
2793
2794
2795
2796
2797
2798
2799
2800
2801
2802
2803
2804
2805
2806
2807
2808
2809
2810
2811
2812
2813
2814
2815
2816
2817
2818
2819
2820
2821
2822
2823
2824
2825
2826
2827
2828
2829
2830
2831
2832
2833
2834
2835
2836
2837
2838
2839
2840
2841
2842
2843
2844
2845
2846
2847
2848
2849
2850
2851
2852
2853
2854
2855
2856
2857
2858
2859
2860
2861
2862
2863
2864
2865
2866
2867
2868
2869
2870
2871
2872
2873
2874
2875
2876
2877
2878
2879
2880
2881
2882
2883
2884
2885
2886
2887
2888
2889
2890
2891
2892
2893
2894
2895
2896
2897
2898
2899
2900
2901
2902
2903
2904
2905
2906
2907
2908
2909
2910
2911
2912
2913
2914
2915
2916
2917
2918
2919
2920
2921
2922
2923
2924
2925
2926
2927
2928
2929
2930
2931
2932
2933
2934
2935
2936
2937
2938
2939
2940
2941
2942
2943
2944
2945
2946
2947
2948
2949
2950
2951
2952
2953
2954
2955
2956
2957
2958
2959
2960
2961
2962
2963
2964
2965
2966
2967
2968
2969
2970
2971
2972
2973
2974
2975
2976
2977
2978
2979
2980
2981
2982
2983
2984
2985
2986
2987
2988
2989
2990
2991
2992
2993
2994
2995
2996
2997
2998
2999
3000
3001
3002
3003
3004
3005
3006
3007
3008
3009
3010
3011
3012
3013
3014
3015
3016
3017
3018
3019
3020
3021
3022
3023
3024
3025
3026
3027
3028
3029
3030
3031
3032
3033
3034
3035
3036
3037
3038
3039
3040
3041
3042
3043
3044
3045
3046
3047
3048
3049
3050
3051
3052
3053
3054
3055
3056
3057
3058
3059
3060
3061
3062
3063
3064
3065
3066
3067
3068
3069
3070
3071
3072
3073
3074
3075
3076
3077
3078
3079
3080
3081
3082
3083
3084
3085
3086
3087
3088
3089
3090
3091
3092
3093
3094
3095
3096
3097
3098
3099
3100
3101
3102
3103
3104
3105
3106
3107
3108
3109
3110
3111
3112
3113
3114
3115
3116
3117
3118
3119
3120
3121
3122
3123
3124
3125
3126
3127
3128
3129
3130
3131
3132
3133
3134
3135
3136
3137
3138
3139
3140
3141
3142
3143
3144
3145
3146
3147
3148
3149
3150
3151
3152
3153
3154
3155
3156
3157
3158
3159
3160
3161
3162
3163
3164
3165
3166
3167
3168
3169
3170
3171
3172
3173
3174
3175
3176
3177
3178
3179
3180
3181
3182
3183
3184
3185
3186
3187
3188
3189
3190
3191
3192
/*!
A DFA that can return spans for matching capturing groups.

This module is the home of a [one-pass DFA](DFA).

This module also contains a [`Builder`] and a [`Config`] for building and
configuring a one-pass DFA.
*/

// A note on naming and credit:
//
// As far as I know, Russ Cox came up with the practical vision and
// implementation of a "one-pass regex engine." He mentions and describes it
// briefly in the third article of his regexp article series:
// https://swtch.com/~rsc/regexp/regexp3.html
//
// Cox's implementation is in RE2, and the implementation below is most
// heavily inspired by RE2's. The key thing they have in common is that
// their transitions are defined over an alphabet of bytes. In contrast,
// Go's regex engine also has a one-pass engine, but its transitions are
// more firmly rooted on Unicode codepoints. The ideas are the same, but the
// implementations are different.
//
// RE2 tends to call this a "one-pass NFA." Here, we call it a "one-pass DFA."
// They're both true in their own ways:
//
// * The "one-pass" criterion is generally a property of the NFA itself. In
// particular, it is said that an NFA is one-pass if, after each byte of input
// during a search, there is at most one "VM thread" remaining to take for the
// next byte of input. That is, there is never any ambiguity as to the path to
// take through the NFA during a search.
//
// * On the other hand, once a one-pass NFA has its representation converted
// to something where a constant number of instructions is used for each byte
// of input, the implementation looks a lot more like a DFA. It's technically
// more powerful than a DFA since it has side effects (storing offsets inside
// of slots activated by a transition), but it is far closer to a DFA than an
// NFA simulation.
//
// Thus, in this crate, we call it a one-pass DFA.

use alloc::{vec, vec::Vec};

use crate::{
    dfa::{remapper::Remapper, DEAD},
    nfa::thompson::{self, NFA},
    util::{
        alphabet::ByteClasses,
        captures::Captures,
        escape::DebugByte,
        int::{Usize, U32, U64, U8},
        look::{Look, LookSet, UnicodeWordBoundaryError},
        primitives::{NonMaxUsize, PatternID, StateID},
        search::{Anchored, Input, Match, MatchError, MatchKind, Span},
        sparse_set::SparseSet,
    },
};

/// The configuration used for building a [one-pass DFA](DFA).
///
/// A one-pass DFA configuration is a simple data object that is typically used
/// with [`Builder::configure`]. It can be cheaply cloned.
///
/// A default configuration can be created either with `Config::new`, or
/// perhaps more conveniently, with [`DFA::config`].
#[derive(Clone, Debug, Default)]
pub struct Config {
    match_kind: Option<MatchKind>,
    starts_for_each_pattern: Option<bool>,
    byte_classes: Option<bool>,
    size_limit: Option<Option<usize>>,
}

impl Config {
    /// Return a new default one-pass DFA configuration.
    pub fn new() -> Config {
        Config::default()
    }

    /// Set the desired match semantics.
    ///
    /// The default is [`MatchKind::LeftmostFirst`], which corresponds to the
    /// match semantics of Perl-like regex engines. That is, when multiple
    /// patterns would match at the same leftmost position, the pattern that
    /// appears first in the concrete syntax is chosen.
    ///
    /// Currently, the only other kind of match semantics supported is
    /// [`MatchKind::All`]. This corresponds to "classical DFA" construction
    /// where all possible matches are visited.
    ///
    /// When it comes to the one-pass DFA, it is rarer for preference order and
    /// "longest match" to actually disagree. Since if they did disagree, then
    /// the regex typically isn't one-pass. For example, searching `Samwise`
    /// for `Sam|Samwise` will report `Sam` for leftmost-first matching and
    /// `Samwise` for "longest match" or "all" matching. However, this regex is
    /// not one-pass if taken literally. The equivalent regex, `Sam(?:|wise)`
    /// is one-pass and `Sam|Samwise` may be optimized to it.
    ///
    /// The other main difference is that "all" match semantics don't support
    /// non-greedy matches. "All" match semantics always try to match as much
    /// as possible.
    pub fn match_kind(mut self, kind: MatchKind) -> Config {
        self.match_kind = Some(kind);
        self
    }

    /// Whether to compile a separate start state for each pattern in the
    /// one-pass DFA.
    ///
    /// When enabled, a separate **anchored** start state is added for each
    /// pattern in the DFA. When this start state is used, then the DFA will
    /// only search for matches for the pattern specified, even if there are
    /// other patterns in the DFA.
    ///
    /// The main downside of this option is that it can potentially increase
    /// the size of the DFA and/or increase the time it takes to build the DFA.
    ///
    /// You might want to enable this option when you want to both search for
    /// anchored matches of any pattern or to search for anchored matches of
    /// one particular pattern while using the same DFA. (Otherwise, you would
    /// need to compile a new DFA for each pattern.)
    ///
    /// By default this is disabled.
    ///
    /// # Example
    ///
    /// This example shows how to build a multi-regex and then search for
    /// matches for a any of the patterns or matches for a specific pattern.
    ///
    /// ```
    /// use regex_automata::{
    ///     dfa::onepass::DFA, Anchored, Input, Match, PatternID,
    /// };
    ///
    /// let re = DFA::builder()
    ///     .configure(DFA::config().starts_for_each_pattern(true))
    ///     .build_many(&["[a-z]+", "[0-9]+"])?;
    /// let (mut cache, mut caps) = (re.create_cache(), re.create_captures());
    /// let haystack = "123abc";
    /// let input = Input::new(haystack).anchored(Anchored::Yes);
    ///
    /// // A normal multi-pattern search will show pattern 1 matches.
    /// re.try_search(&mut cache, &input, &mut caps)?;
    /// assert_eq!(Some(Match::must(1, 0..3)), caps.get_match());
    ///
    /// // If we only want to report pattern 0 matches, then we'll get no
    /// // match here.
    /// let input = input.anchored(Anchored::Pattern(PatternID::must(0)));
    /// re.try_search(&mut cache, &input, &mut caps)?;
    /// assert_eq!(None, caps.get_match());
    ///
    /// # Ok::<(), Box<dyn std::error::Error>>(())
    /// ```
    pub fn starts_for_each_pattern(mut self, yes: bool) -> Config {
        self.starts_for_each_pattern = Some(yes);
        self
    }

    /// Whether to attempt to shrink the size of the DFA's alphabet or not.
    ///
    /// This option is enabled by default and should never be disabled unless
    /// one is debugging a one-pass DFA.
    ///
    /// When enabled, the DFA will use a map from all possible bytes to their
    /// corresponding equivalence class. Each equivalence class represents a
    /// set of bytes that does not discriminate between a match and a non-match
    /// in the DFA. For example, the pattern `[ab]+` has at least two
    /// equivalence classes: a set containing `a` and `b` and a set containing
    /// every byte except for `a` and `b`. `a` and `b` are in the same
    /// equivalence class because they never discriminate between a match and a
    /// non-match.
    ///
    /// The advantage of this map is that the size of the transition table
    /// can be reduced drastically from (approximately) `#states * 256 *
    /// sizeof(StateID)` to `#states * k * sizeof(StateID)` where `k` is the
    /// number of equivalence classes (rounded up to the nearest power of 2).
    /// As a result, total space usage can decrease substantially. Moreover,
    /// since a smaller alphabet is used, DFA compilation becomes faster as
    /// well.
    ///
    /// **WARNING:** This is only useful for debugging DFAs. Disabling this
    /// does not yield any speed advantages. Namely, even when this is
    /// disabled, a byte class map is still used while searching. The only
    /// difference is that every byte will be forced into its own distinct
    /// equivalence class. This is useful for debugging the actual generated
    /// transitions because it lets one see the transitions defined on actual
    /// bytes instead of the equivalence classes.
    pub fn byte_classes(mut self, yes: bool) -> Config {
        self.byte_classes = Some(yes);
        self
    }

    /// Set a size limit on the total heap used by a one-pass DFA.
    ///
    /// This size limit is expressed in bytes and is applied during
    /// construction of a one-pass DFA. If the DFA's heap usage exceeds
    /// this configured limit, then construction is stopped and an error is
    /// returned.
    ///
    /// The default is no limit.
    ///
    /// # Example
    ///
    /// This example shows a one-pass DFA that fails to build because of
    /// a configured size limit. This particular example also serves as a
    /// cautionary tale demonstrating just how big DFAs with large Unicode
    /// character classes can get.
    ///
    /// ```
    /// # if cfg!(miri) { return Ok(()); } // miri takes too long
    /// use regex_automata::{dfa::onepass::DFA, Match};
    ///
    /// // 6MB isn't enough!
    /// DFA::builder()
    ///     .configure(DFA::config().size_limit(Some(6_000_000)))
    ///     .build(r"\w{20}")
    ///     .unwrap_err();
    ///
    /// // ... but 7MB probably is!
    /// // (Note that DFA sizes aren't necessarily stable between releases.)
    /// let re = DFA::builder()
    ///     .configure(DFA::config().size_limit(Some(7_000_000)))
    ///     .build(r"\w{20}")?;
    /// let (mut cache, mut caps) = (re.create_cache(), re.create_captures());
    /// let haystack = "A".repeat(20);
    /// re.captures(&mut cache, &haystack, &mut caps);
    /// assert_eq!(Some(Match::must(0, 0..20)), caps.get_match());
    ///
    /// # Ok::<(), Box<dyn std::error::Error>>(())
    /// ```
    ///
    /// While one needs a little more than 3MB to represent `\w{20}`, it
    /// turns out that you only need a little more than 4KB to represent
    /// `(?-u:\w{20})`. So only use Unicode if you need it!
    pub fn size_limit(mut self, limit: Option<usize>) -> Config {
        self.size_limit = Some(limit);
        self
    }

    /// Returns the match semantics set in this configuration.
    pub fn get_match_kind(&self) -> MatchKind {
        self.match_kind.unwrap_or(MatchKind::LeftmostFirst)
    }

    /// Returns whether this configuration has enabled anchored starting states
    /// for every pattern in the DFA.
    pub fn get_starts_for_each_pattern(&self) -> bool {
        self.starts_for_each_pattern.unwrap_or(false)
    }

    /// Returns whether this configuration has enabled byte classes or not.
    /// This is typically a debugging oriented option, as disabling it confers
    /// no speed benefit.
    pub fn get_byte_classes(&self) -> bool {
        self.byte_classes.unwrap_or(true)
    }

    /// Returns the DFA size limit of this configuration if one was set.
    /// The size limit is total number of bytes on the heap that a DFA is
    /// permitted to use. If the DFA exceeds this limit during construction,
    /// then construction is stopped and an error is returned.
    pub fn get_size_limit(&self) -> Option<usize> {
        self.size_limit.unwrap_or(None)
    }

    /// Overwrite the default configuration such that the options in `o` are
    /// always used. If an option in `o` is not set, then the corresponding
    /// option in `self` is used. If it's not set in `self` either, then it
    /// remains not set.
    pub(crate) fn overwrite(&self, o: Config) -> Config {
        Config {
            match_kind: o.match_kind.or(self.match_kind),
            starts_for_each_pattern: o
                .starts_for_each_pattern
                .or(self.starts_for_each_pattern),
            byte_classes: o.byte_classes.or(self.byte_classes),
            size_limit: o.size_limit.or(self.size_limit),
        }
    }
}

/// A builder for a [one-pass DFA](DFA).
///
/// This builder permits configuring options for the syntax of a pattern, the
/// NFA construction and the DFA construction. This builder is different from a
/// general purpose regex builder in that it permits fine grain configuration
/// of the construction process. The trade off for this is complexity, and
/// the possibility of setting a configuration that might not make sense. For
/// example, there are two different UTF-8 modes:
///
/// * [`syntax::Config::utf8`](crate::util::syntax::Config::utf8) controls
/// whether the pattern itself can contain sub-expressions that match invalid
/// UTF-8.
/// * [`thompson::Config::utf8`] controls whether empty matches that split a
/// Unicode codepoint are reported or not.
///
/// Generally speaking, callers will want to either enable all of these or
/// disable all of these.
///
/// # Example
///
/// This example shows how to disable UTF-8 mode in the syntax and the NFA.
/// This is generally what you want for matching on arbitrary bytes.
///
/// ```
/// # if cfg!(miri) { return Ok(()); } // miri takes too long
/// use regex_automata::{
///     dfa::onepass::DFA,
///     nfa::thompson,
///     util::syntax,
///     Match,
/// };
///
/// let re = DFA::builder()
///     .syntax(syntax::Config::new().utf8(false))
///     .thompson(thompson::Config::new().utf8(false))
///     .build(r"foo(?-u:[^b])ar.*")?;
/// let (mut cache, mut caps) = (re.create_cache(), re.create_captures());
///
/// let haystack = b"foo\xFFarzz\xE2\x98\xFF\n";
/// re.captures(&mut cache, haystack, &mut caps);
/// // Notice that `(?-u:[^b])` matches invalid UTF-8,
/// // but the subsequent `.*` does not! Disabling UTF-8
/// // on the syntax permits this.
/// //
/// // N.B. This example does not show the impact of
/// // disabling UTF-8 mode on a one-pass DFA Config,
/// //  since that only impacts regexes that can
/// // produce matches of length 0.
/// assert_eq!(Some(Match::must(0, 0..8)), caps.get_match());
///
/// # Ok::<(), Box<dyn std::error::Error>>(())
/// ```
#[derive(Clone, Debug)]
pub struct Builder {
    config: Config,
    #[cfg(feature = "syntax")]
    thompson: thompson::Compiler,
}

impl Builder {
    /// Create a new one-pass DFA builder with the default configuration.
    pub fn new() -> Builder {
        Builder {
            config: Config::default(),
            #[cfg(feature = "syntax")]
            thompson: thompson::Compiler::new(),
        }
    }

    /// Build a one-pass DFA from the given pattern.
    ///
    /// If there was a problem parsing or compiling the pattern, then an error
    /// is returned.
    #[cfg(feature = "syntax")]
    pub fn build(&self, pattern: &str) -> Result<DFA, BuildError> {
        self.build_many(&[pattern])
    }

    /// Build a one-pass DFA from the given patterns.
    ///
    /// When matches are returned, the pattern ID corresponds to the index of
    /// the pattern in the slice given.
    #[cfg(feature = "syntax")]
    pub fn build_many<P: AsRef<str>>(
        &self,
        patterns: &[P],
    ) -> Result<DFA, BuildError> {
        let nfa =
            self.thompson.build_many(patterns).map_err(BuildError::nfa)?;
        self.build_from_nfa(nfa)
    }

    /// Build a DFA from the given NFA.
    ///
    /// # Example
    ///
    /// This example shows how to build a DFA if you already have an NFA in
    /// hand.
    ///
    /// ```
    /// use regex_automata::{dfa::onepass::DFA, nfa::thompson::NFA, Match};
    ///
    /// // This shows how to set non-default options for building an NFA.
    /// let nfa = NFA::compiler()
    ///     .configure(NFA::config().shrink(true))
    ///     .build(r"[a-z0-9]+")?;
    /// let re = DFA::builder().build_from_nfa(nfa)?;
    /// let (mut cache, mut caps) = (re.create_cache(), re.create_captures());
    /// re.captures(&mut cache, "foo123bar", &mut caps);
    /// assert_eq!(Some(Match::must(0, 0..9)), caps.get_match());
    ///
    /// # Ok::<(), Box<dyn std::error::Error>>(())
    /// ```
    pub fn build_from_nfa(&self, nfa: NFA) -> Result<DFA, BuildError> {
        // Why take ownership if we're just going to pass a reference to the
        // NFA to our internal builder? Well, the first thing to note is that
        // an NFA uses reference counting internally, so either choice is going
        // to be cheap. So there isn't much cost either way.
        //
        // The real reason is that a one-pass DFA, semantically, shares
        // ownership of an NFA. This is unlike other DFAs that don't share
        // ownership of an NFA at all, primarily because they want to be
        // self-contained in order to support cheap (de)serialization.
        //
        // But then why pass a '&nfa' below if we want to share ownership?
        // Well, it turns out that using a '&NFA' in our internal builder
        // separates its lifetime from the DFA we're building, and this turns
        // out to make code a bit more composable. e.g., We can iterate over
        // things inside the NFA while borrowing the builder as mutable because
        // we know the NFA cannot be mutated. So TL;DR --- this weirdness is
        // "because borrow checker."
        InternalBuilder::new(self.config.clone(), &nfa).build()
    }

    /// Apply the given one-pass DFA configuration options to this builder.
    pub fn configure(&mut self, config: Config) -> &mut Builder {
        self.config = self.config.overwrite(config);
        self
    }

    /// Set the syntax configuration for this builder using
    /// [`syntax::Config`](crate::util::syntax::Config).
    ///
    /// This permits setting things like case insensitivity, Unicode and multi
    /// line mode.
    ///
    /// These settings only apply when constructing a one-pass DFA directly
    /// from a pattern.
    #[cfg(feature = "syntax")]
    pub fn syntax(
        &mut self,
        config: crate::util::syntax::Config,
    ) -> &mut Builder {
        self.thompson.syntax(config);
        self
    }

    /// Set the Thompson NFA configuration for this builder using
    /// [`nfa::thompson::Config`](crate::nfa::thompson::Config).
    ///
    /// This permits setting things like whether additional time should be
    /// spent shrinking the size of the NFA.
    ///
    /// These settings only apply when constructing a DFA directly from a
    /// pattern.
    #[cfg(feature = "syntax")]
    pub fn thompson(&mut self, config: thompson::Config) -> &mut Builder {
        self.thompson.configure(config);
        self
    }
}

/// An internal builder for encapsulating the state necessary to build a
/// one-pass DFA. Typical use is just `InternalBuilder::new(..).build()`.
///
/// There is no separate pass for determining whether the NFA is one-pass or
/// not. We just try to build the DFA. If during construction we discover that
/// it is not one-pass, we bail out. This is likely to lead to some undesirable
/// expense in some cases, so it might make sense to try an identify common
/// patterns in the NFA that make it definitively not one-pass. That way, we
/// can avoid ever trying to build a one-pass DFA in the first place. For
/// example, '\w*\s' is not one-pass, and since '\w' is Unicode-aware by
/// default, it's probably not a trivial cost to try and build a one-pass DFA
/// for it and then fail.
///
/// Note that some (immutable) fields are duplicated here. For example, the
/// 'nfa' and 'classes' fields are both in the 'DFA'. They are the same thing,
/// but we duplicate them because it makes composition easier below. Otherwise,
/// since the borrow checker can't see through method calls, the mutable borrow
/// we use to mutate the DFA winds up preventing borrowing from any other part
/// of the DFA, even though we aren't mutating those parts. We only do this
/// because the duplication is cheap.
#[derive(Debug)]
struct InternalBuilder<'a> {
    /// The DFA we're building.
    dfa: DFA,
    /// An unordered collection of NFA state IDs that we haven't yet tried to
    /// build into a DFA state yet.
    ///
    /// This collection does not ultimately wind up including every NFA state
    /// ID. Instead, each ID represents a "start" state for a sub-graph of the
    /// NFA. The set of NFA states we then use to build a DFA state consists
    /// of that "start" state and all states reachable from it via epsilon
    /// transitions.
    uncompiled_nfa_ids: Vec<StateID>,
    /// A map from NFA state ID to DFA state ID. This is useful for easily
    /// determining whether an NFA state has been used as a "starting" point
    /// to build a DFA state yet. If it hasn't, then it is mapped to DEAD,
    /// and since DEAD is specially added and never corresponds to any NFA
    /// state, it follows that a mapping to DEAD implies the NFA state has
    /// no corresponding DFA state yet.
    nfa_to_dfa_id: Vec<StateID>,
    /// A stack used to traverse the NFA states that make up a single DFA
    /// state. Traversal occurs until the stack is empty, and we only push to
    /// the stack when the state ID isn't in 'seen'. Actually, even more than
    /// that, if we try to push something on to this stack that is already in
    /// 'seen', then we bail out on construction completely, since it implies
    /// that the NFA is not one-pass.
    stack: Vec<(StateID, Epsilons)>,
    /// The set of NFA states that we've visited via 'stack'.
    seen: SparseSet,
    /// Whether a match NFA state has been observed while constructing a
    /// one-pass DFA state. Once a match state is seen, assuming we are using
    /// leftmost-first match semantics, then we don't add any more transitions
    /// to the DFA state we're building.
    matched: bool,
    /// The config passed to the builder.
    ///
    /// This is duplicated in dfa.config.
    config: Config,
    /// The NFA we're building a one-pass DFA from.
    ///
    /// This is duplicated in dfa.nfa.
    nfa: &'a NFA,
    /// The equivalence classes that make up the alphabet for this DFA>
    ///
    /// This is duplicated in dfa.classes.
    classes: ByteClasses,
}

impl<'a> InternalBuilder<'a> {
    /// Create a new builder with an initial empty DFA.
    fn new(config: Config, nfa: &'a NFA) -> InternalBuilder<'a> {
        let classes = if !config.get_byte_classes() {
            // A one-pass DFA will always use the equivalence class map, but
            // enabling this option is useful for debugging. Namely, this will
            // cause all transitions to be defined over their actual bytes
            // instead of an opaque equivalence class identifier. The former is
            // much easier to grok as a human.
            ByteClasses::singletons()
        } else {
            nfa.byte_classes().clone()
        };
        // Normally a DFA alphabet includes the EOI symbol, but we don't need
        // that in the one-pass DFA since we handle look-around explicitly
        // without encoding it into the DFA. Thus, we don't need to delay
        // matches by 1 byte. However, we reuse the space that *would* be used
        // by the EOI transition by putting match information there (like which
        // pattern matches and which look-around assertions need to hold). So
        // this means our real alphabet length is 1 fewer than what the byte
        // classes report, since we don't use EOI.
        let alphabet_len = classes.alphabet_len().checked_sub(1).unwrap();
        let stride2 = classes.stride2();
        let dfa = DFA {
            config: config.clone(),
            nfa: nfa.clone(),
            table: vec![],
            starts: vec![],
            // Since one-pass DFAs have a smaller state ID max than
            // StateID::MAX, it follows that StateID::MAX is a valid initial
            // value for min_match_id since no state ID can ever be greater
            // than it. In the case of a one-pass DFA with no match states, the
            // min_match_id will keep this sentinel value.
            min_match_id: StateID::MAX,
            classes: classes.clone(),
            alphabet_len,
            stride2,
            pateps_offset: alphabet_len,
            // OK because PatternID::MAX*2 is guaranteed not to overflow.
            explicit_slot_start: nfa.pattern_len().checked_mul(2).unwrap(),
        };
        InternalBuilder {
            dfa,
            uncompiled_nfa_ids: vec![],
            nfa_to_dfa_id: vec![DEAD; nfa.states().len()],
            stack: vec![],
            seen: SparseSet::new(nfa.states().len()),
            matched: false,
            config,
            nfa,
            classes,
        }
    }

    /// Build the DFA from the NFA given to this builder. If the NFA is not
    /// one-pass, then return an error. An error may also be returned if a
    /// particular limit is exceeded. (Some limits, like the total heap memory
    /// used, are configurable. Others, like the total patterns or slots, are
    /// hard-coded based on representational limitations.)
    fn build(mut self) -> Result<DFA, BuildError> {
        self.nfa.look_set_any().available().map_err(BuildError::word)?;
        for look in self.nfa.look_set_any().iter() {
            // This is a future incompatibility check where if we add any
            // more look-around assertions, then the one-pass DFA either
            // needs to reject them (what we do here) or it needs to have its
            // Transition representation modified to be capable of storing the
            // new assertions.
            if look.as_repr() > Look::WordUnicodeNegate.as_repr() {
                return Err(BuildError::unsupported_look(look));
            }
        }
        if self.nfa.pattern_len().as_u64() > PatternEpsilons::PATTERN_ID_LIMIT
        {
            return Err(BuildError::too_many_patterns(
                PatternEpsilons::PATTERN_ID_LIMIT,
            ));
        }
        if self.nfa.group_info().explicit_slot_len() > Slots::LIMIT {
            return Err(BuildError::not_one_pass(
                "too many explicit capturing groups (max is 16)",
            ));
        }
        assert_eq!(DEAD, self.add_empty_state()?);

        // This is where the explicit slots start. We care about this because
        // we only need to track explicit slots. The implicit slots---two for
        // each pattern---are tracked as part of the search routine itself.
        let explicit_slot_start = self.nfa.pattern_len() * 2;
        self.add_start_state(None, self.nfa.start_anchored())?;
        if self.config.get_starts_for_each_pattern() {
            for pid in self.nfa.patterns() {
                self.add_start_state(
                    Some(pid),
                    self.nfa.start_pattern(pid).unwrap(),
                )?;
            }
        }
        // NOTE: One wonders what the effects of treating 'uncompiled_nfa_ids'
        // as a stack are. It is really an unordered *set* of NFA state IDs.
        // If it, for example, in practice led to discovering whether a regex
        // was or wasn't one-pass later than if we processed NFA state IDs in
        // ascending order, then that would make this routine more costly in
        // the somewhat common case of a regex that isn't one-pass.
        while let Some(nfa_id) = self.uncompiled_nfa_ids.pop() {
            let dfa_id = self.nfa_to_dfa_id[nfa_id];
            // Once we see a match, we keep going, but don't add any new
            // transitions. Normally we'd just stop, but we have to keep
            // going in order to verify that our regex is actually one-pass.
            self.matched = false;
            // The NFA states we've already explored for this DFA state.
            self.seen.clear();
            // The NFA states to explore via epsilon transitions. If we ever
            // try to push an NFA state that we've already seen, then the NFA
            // is not one-pass because it implies there are multiple epsilon
            // transition paths that lead to the same NFA state. In other
            // words, there is ambiguity.
            self.stack_push(nfa_id, Epsilons::empty())?;
            while let Some((id, epsilons)) = self.stack.pop() {
                match *self.nfa.state(id) {
                    thompson::State::ByteRange { ref trans } => {
                        self.compile_transition(dfa_id, trans, epsilons)?;
                    }
                    thompson::State::Sparse(ref sparse) => {
                        for trans in sparse.transitions.iter() {
                            self.compile_transition(dfa_id, trans, epsilons)?;
                        }
                    }
                    thompson::State::Dense(ref dense) => {
                        for trans in dense.iter() {
                            self.compile_transition(dfa_id, &trans, epsilons)?;
                        }
                    }
                    thompson::State::Look { look, next } => {
                        let looks = epsilons.looks().insert(look);
                        self.stack_push(next, epsilons.set_looks(looks))?;
                    }
                    thompson::State::Union { ref alternates } => {
                        for &sid in alternates.iter().rev() {
                            self.stack_push(sid, epsilons)?;
                        }
                    }
                    thompson::State::BinaryUnion { alt1, alt2 } => {
                        self.stack_push(alt2, epsilons)?;
                        self.stack_push(alt1, epsilons)?;
                    }
                    thompson::State::Capture { next, slot, .. } => {
                        let slot = slot.as_usize();
                        let epsilons = if slot < explicit_slot_start {
                            // If this is an implicit slot, we don't care
                            // about it, since we handle implicit slots in
                            // the search routine. We can get away with that
                            // because there are 2 implicit slots for every
                            // pattern.
                            epsilons
                        } else {
                            // Offset our explicit slots so that they start
                            // at index 0.
                            let offset = slot - explicit_slot_start;
                            epsilons.set_slots(epsilons.slots().insert(offset))
                        };
                        self.stack_push(next, epsilons)?;
                    }
                    thompson::State::Fail => {
                        continue;
                    }
                    thompson::State::Match { pattern_id } => {
                        // If we found two different paths to a match state
                        // for the same DFA state, then we have ambiguity.
                        // Thus, it's not one-pass.
                        if self.matched {
                            return Err(BuildError::not_one_pass(
                                "multiple epsilon transitions to match state",
                            ));
                        }
                        self.matched = true;
                        // Shove the matching pattern ID and the 'epsilons'
                        // into the current DFA state's pattern epsilons. The
                        // 'epsilons' includes the slots we need to capture
                        // before reporting the match and also the conditional
                        // epsilon transitions we need to check before we can
                        // report a match.
                        self.dfa.set_pattern_epsilons(
                            dfa_id,
                            PatternEpsilons::empty()
                                .set_pattern_id(pattern_id)
                                .set_epsilons(epsilons),
                        );
                        // N.B. It is tempting to just bail out here when
                        // compiling a leftmost-first DFA, since we will never
                        // compile any more transitions in that case. But we
                        // actually need to keep going in order to verify that
                        // we actually have a one-pass regex. e.g., We might
                        // see more Match states (e.g., for other patterns)
                        // that imply that we don't have a one-pass regex.
                        // So instead, we mark that we've found a match and
                        // continue on. When we go to compile a new DFA state,
                        // we just skip that part. But otherwise check that the
                        // one-pass property is upheld.
                    }
                }
            }
        }
        self.shuffle_states();
        Ok(self.dfa)
    }

    /// Shuffle all match states to the end of the transition table and set
    /// 'min_match_id' to the ID of the first such match state.
    ///
    /// The point of this is to make it extremely cheap to determine whether
    /// a state is a match state or not. We need to check on this on every
    /// transition during a search, so it being cheap is important. This
    /// permits us to check it by simply comparing two state identifiers, as
    /// opposed to looking for the pattern ID in the state's `PatternEpsilons`.
    /// (Which requires a memory load and some light arithmetic.)
    fn shuffle_states(&mut self) {
        let mut remapper = Remapper::new(&self.dfa);
        let mut next_dest = self.dfa.last_state_id();
        for i in (0..self.dfa.state_len()).rev() {
            let id = StateID::must(i);
            let is_match =
                self.dfa.pattern_epsilons(id).pattern_id().is_some();
            if !is_match {
                continue;
            }
            remapper.swap(&mut self.dfa, next_dest, id);
            self.dfa.min_match_id = next_dest;
            next_dest = self.dfa.prev_state_id(next_dest).expect(
                "match states should be a proper subset of all states",
            );
        }
        remapper.remap(&mut self.dfa);
    }

    /// Compile the given NFA transition into the DFA state given.
    ///
    /// 'Epsilons' corresponds to any conditional epsilon transitions that need
    /// to be satisfied to follow this transition, and any slots that need to
    /// be saved if the transition is followed.
    ///
    /// If this transition indicates that the NFA is not one-pass, then
    /// this returns an error. (This occurs, for example, if the DFA state
    /// already has a transition defined for the same input symbols as the
    /// given transition, *and* the result of the old and new transitions is
    /// different.)
    fn compile_transition(
        &mut self,
        dfa_id: StateID,
        trans: &thompson::Transition,
        epsilons: Epsilons,
    ) -> Result<(), BuildError> {
        let next_dfa_id = self.add_dfa_state_for_nfa_state(trans.next)?;
        for byte in self
            .classes
            .representatives(trans.start..=trans.end)
            .filter_map(|r| r.as_u8())
        {
            let oldtrans = self.dfa.transition(dfa_id, byte);
            let newtrans =
                Transition::new(self.matched, next_dfa_id, epsilons);
            // If the old transition points to the DEAD state, then we know
            // 'byte' has not been mapped to any transition for this DFA state
            // yet. So set it unconditionally. Otherwise, we require that the
            // old and new transitions are equivalent. Otherwise, there is
            // ambiguity and thus the regex is not one-pass.
            if oldtrans.state_id() == DEAD {
                self.dfa.set_transition(dfa_id, byte, newtrans);
            } else if oldtrans != newtrans {
                return Err(BuildError::not_one_pass(
                    "conflicting transition",
                ));
            }
        }
        Ok(())
    }

    /// Add a start state to the DFA corresponding to the given NFA starting
    /// state ID.
    ///
    /// If adding a state would blow any limits (configured or hard-coded),
    /// then an error is returned.
    ///
    /// If the starting state is an anchored state for a particular pattern,
    /// then callers must provide the pattern ID for that starting state.
    /// Callers must also ensure that the first starting state added is the
    /// start state for all patterns, and then each anchored starting state for
    /// each pattern (if necessary) added in order. Otherwise, this panics.
    fn add_start_state(
        &mut self,
        pid: Option<PatternID>,
        nfa_id: StateID,
    ) -> Result<StateID, BuildError> {
        match pid {
            // With no pid, this should be the start state for all patterns
            // and thus be the first one.
            None => assert!(self.dfa.starts.is_empty()),
            // With a pid, we want it to be at self.dfa.starts[pid+1].
            Some(pid) => assert!(self.dfa.starts.len() == pid.one_more()),
        }
        let dfa_id = self.add_dfa_state_for_nfa_state(nfa_id)?;
        self.dfa.starts.push(dfa_id);
        Ok(dfa_id)
    }

    /// Add a new DFA state corresponding to the given NFA state. If adding a
    /// state would blow any limits (configured or hard-coded), then an error
    /// is returned. If a DFA state already exists for the given NFA state,
    /// then that DFA state's ID is returned and no new states are added.
    ///
    /// It is not expected that this routine is called for every NFA state.
    /// Instead, an NFA state ID will usually correspond to the "start" state
    /// for a sub-graph of the NFA, where all states in the sub-graph are
    /// reachable via epsilon transitions (conditional or unconditional). That
    /// sub-graph of NFA states is ultimately what produces a single DFA state.
    fn add_dfa_state_for_nfa_state(
        &mut self,
        nfa_id: StateID,
    ) -> Result<StateID, BuildError> {
        // If we've already built a DFA state for the given NFA state, then
        // just return that. We definitely do not want to have more than one
        // DFA state in existence for the same NFA state, since all but one of
        // them will likely become unreachable. And at least some of them are
        // likely to wind up being incomplete.
        let existing_dfa_id = self.nfa_to_dfa_id[nfa_id];
        if existing_dfa_id != DEAD {
            return Ok(existing_dfa_id);
        }
        // If we don't have any DFA state yet, add it and then add the given
        // NFA state to the list of states to explore.
        let dfa_id = self.add_empty_state()?;
        self.nfa_to_dfa_id[nfa_id] = dfa_id;
        self.uncompiled_nfa_ids.push(nfa_id);
        Ok(dfa_id)
    }

    /// Unconditionally add a new empty DFA state. If adding it would exceed
    /// any limits (configured or hard-coded), then an error is returned. The
    /// ID of the new state is returned on success.
    ///
    /// The added state is *not* a match state.
    fn add_empty_state(&mut self) -> Result<StateID, BuildError> {
        let state_limit = Transition::STATE_ID_LIMIT;
        // Note that unlike dense and lazy DFAs, we specifically do NOT
        // premultiply our state IDs here. The reason is that we want to pack
        // our state IDs into 64-bit transitions with other info, so the fewer
        // the bits we use for state IDs the better. If we premultiply, then
        // our state ID space shrinks. We justify this by the assumption that
        // a one-pass DFA is just already doing a fair bit more work than a
        // normal DFA anyway, so an extra multiplication to compute a state
        // transition doesn't seem like a huge deal.
        let next_id = self.dfa.table.len() >> self.dfa.stride2();
        let id = StateID::new(next_id)
            .map_err(|_| BuildError::too_many_states(state_limit))?;
        if id.as_u64() > Transition::STATE_ID_LIMIT {
            return Err(BuildError::too_many_states(state_limit));
        }
        self.dfa
            .table
            .extend(core::iter::repeat(Transition(0)).take(self.dfa.stride()));
        // The default empty value for 'PatternEpsilons' is sadly not all
        // zeroes. Instead, a special sentinel is used to indicate that there
        // is no pattern. So we need to explicitly set the pattern epsilons to
        // the correct "empty" PatternEpsilons.
        self.dfa.set_pattern_epsilons(id, PatternEpsilons::empty());
        if let Some(size_limit) = self.config.get_size_limit() {
            if self.dfa.memory_usage() > size_limit {
                return Err(BuildError::exceeded_size_limit(size_limit));
            }
        }
        Ok(id)
    }

    /// Push the given NFA state ID and its corresponding epsilons (slots and
    /// conditional epsilon transitions) on to a stack for use in a depth first
    /// traversal of a sub-graph of the NFA.
    ///
    /// If the given NFA state ID has already been pushed on to the stack, then
    /// it indicates the regex is not one-pass and this correspondingly returns
    /// an error.
    fn stack_push(
        &mut self,
        nfa_id: StateID,
        epsilons: Epsilons,
    ) -> Result<(), BuildError> {
        // If we already have seen a match and we are compiling a leftmost
        // first DFA, then we shouldn't add any more states to look at. This is
        // effectively how preference order and non-greediness is implemented.
        // if !self.config.get_match_kind().continue_past_first_match()
        // && self.matched
        // {
        // return Ok(());
        // }
        if !self.seen.insert(nfa_id) {
            return Err(BuildError::not_one_pass(
                "multiple epsilon transitions to same state",
            ));
        }
        self.stack.push((nfa_id, epsilons));
        Ok(())
    }
}

/// A one-pass DFA for executing a subset of anchored regex searches while
/// resolving capturing groups.
///
/// A one-pass DFA can be built from an NFA that is one-pass. An NFA is
/// one-pass when there is never any ambiguity about how to continue a search.
/// For example, `a*a` is not one-pass becuase during a search, it's not
/// possible to know whether to continue matching the `a*` or to move on to
/// the single `a`. However, `a*b` is one-pass, because for every byte in the
/// input, it's always clear when to move on from `a*` to `b`.
///
/// # Only anchored searches are supported
///
/// In this crate, especially for DFAs, unanchored searches are implemented by
/// treating the pattern as if it had a `(?s-u:.)*?` prefix. While the prefix
/// is one-pass on its own, adding anything after it, e.g., `(?s-u:.)*?a` will
/// make the overall pattern not one-pass. Why? Because the `(?s-u:.)` matches
/// any byte, and there is therefore ambiguity as to when the prefix should
/// stop matching and something else should start matching.
///
/// Therefore, one-pass DFAs do not support unanchored searches. In addition
/// to many regexes simply not being one-pass, it implies that one-pass DFAs
/// have limited utility. With that said, when a one-pass DFA can be used, it
/// can potentially provide a dramatic speed up over alternatives like the
/// [`BoundedBacktracker`](crate::nfa::thompson::backtrack::BoundedBacktracker)
/// and the [`PikeVM`](crate::nfa::thompson::pikevm::PikeVM). In particular,
/// a one-pass DFA is the only DFA capable of reporting the spans of matching
/// capturing groups.
///
/// To clarify, when we say that unanchored searches are not supported, what
/// that actually means is:
///
/// * The high level routines, [`DFA::is_match`] and [`DFA::captures`], always
/// do anchored searches.
/// * Since iterators are most useful in the context of unanchored searches,
/// there is no `DFA::captures_iter` method.
/// * For lower level routines like [`DFA::try_search`], an error will be
/// returned if the given [`Input`] is configured to do an unanchored search or
/// search for an invalid pattern ID. (Note that an [`Input`] is configured to
/// do an unanchored search by default, so just giving a `Input::new` is
/// guaranteed to return an error.)
///
/// # Other limitations
///
/// In addition to the [configurable heap limit](Config::size_limit) and
/// the requirement that a regex pattern be one-pass, there are some other
/// limitations:
///
/// * There is an internal limit on the total number of explicit capturing
/// groups that appear across all patterns. It is somewhat small and there is
/// no way to configure it. If your pattern(s) exceed this limit, then building
/// a one-pass DFA will fail.
/// * If the number of patterns exceeds an internal unconfigurable limit, then
/// building a one-pass DFA will fail. This limit is quite large and you're
/// unlikely to hit it.
/// * If the total number of states exceeds an internal unconfigurable limit,
/// then building a one-pass DFA will fail. This limit is quite large and
/// you're unlikely to hit it.
///
/// # Other examples of regexes that aren't one-pass
///
/// One particularly unfortunate example is that enabling Unicode can cause
/// regexes that were one-pass to no longer be one-pass. Consider the regex
/// `(?-u)\w*\s` for example. It is one-pass because there is exactly no
/// overlap between the ASCII definitions of `\w` and `\s`. But `\w*\s`
/// (i.e., with Unicode enabled) is *not* one-pass because `\w` and `\s` get
/// translated to UTF-8 automatons. And while the *codepoints* in `\w` and `\s`
/// do not overlap, the underlying UTF-8 encodings do. Indeed, because of the
/// overlap between UTF-8 automata, the use of Unicode character classes will
/// tend to vastly increase the likelihood of a regex not being one-pass.
///
/// # How does one know if a regex is one-pass or not?
///
/// At the time of writing, the only way to know is to try and build a one-pass
/// DFA. The one-pass property is checked while constructing the DFA.
///
/// This does mean that you might potentially waste some CPU cycles and memory
/// by optimistically trying to build a one-pass DFA. But this is currently the
/// only way. In the future, building a one-pass DFA might be able to use some
/// heuristics to detect common violations of the one-pass property and bail
/// more quickly.
///
/// # Resource usage
///
/// Unlike a general DFA, a one-pass DFA has stricter bounds on its resource
/// usage. Namely, construction of a one-pass DFA has a time and space
/// complexity of `O(n)`, where `n ~ nfa.states().len()`. (A general DFA's time
/// and space complexity is `O(2^n)`.) This smaller time bound is achieved
/// because there is at most one DFA state created for each NFA state. If
/// additional DFA states would be required, then the pattern is not one-pass
/// and construction will fail.
///
/// Note though that currently, this DFA uses a fully dense representation.
/// This means that while its space complexity is no worse than an NFA, it may
/// in practice use more memory because of higher constant factors. The reason
/// for this trade off is two-fold. Firstly, a dense representation makes the
/// search faster. Secondly, the bigger an NFA, the more unlikely it is to be
/// one-pass. Therefore, most one-pass DFAs are usually pretty small.
///
/// # Example
///
/// This example shows that the one-pass DFA implements Unicode word boundaries
/// correctly while simultaneously reporting spans for capturing groups that
/// participate in a match. (This is the only DFA that implements full support
/// for Unicode word boundaries.)
///
/// ```
/// # if cfg!(miri) { return Ok(()); } // miri takes too long
/// use regex_automata::{dfa::onepass::DFA, Match, Span};
///
/// let re = DFA::new(r"\b(?P<first>\w+)[[:space:]]+(?P<last>\w+)\b")?;
/// let (mut cache, mut caps) = (re.create_cache(), re.create_captures());
///
/// re.captures(&mut cache, "Шерлок Холмс", &mut caps);
/// assert_eq!(Some(Match::must(0, 0..23)), caps.get_match());
/// assert_eq!(Some(Span::from(0..12)), caps.get_group_by_name("first"));
/// assert_eq!(Some(Span::from(13..23)), caps.get_group_by_name("last"));
/// # Ok::<(), Box<dyn std::error::Error>>(())
/// ```
///
/// # Example: iteration
///
/// Unlike other regex engines in this crate, this one does not provide
/// iterator search functions. This is because a one-pass DFA only supports
/// anchored searches, and so iterator functions are generally not applicable.
///
/// However, if you know that all of your matches are
/// directly adjacent, then an iterator can be used. The
/// [`util::iter::Searcher`](crate::util::iter::Searcher) type can be used for
/// this purpose:
///
/// ```
/// # if cfg!(miri) { return Ok(()); } // miri takes too long
/// use regex_automata::{
///     dfa::onepass::DFA,
///     util::iter::Searcher,
///     Anchored, Input, Span,
/// };
///
/// let re = DFA::new(r"\w(\d)\w")?;
/// let (mut cache, caps) = (re.create_cache(), re.create_captures());
/// let input = Input::new("a1zb2yc3x").anchored(Anchored::Yes);
///
/// let mut it = Searcher::new(input).into_captures_iter(caps, |input, caps| {
///     Ok(re.try_search(&mut cache, input, caps)?)
/// }).infallible();
/// let caps0 = it.next().unwrap();
/// assert_eq!(Some(Span::from(1..2)), caps0.get_group(1));
///
/// # Ok::<(), Box<dyn std::error::Error>>(())
/// ```
#[derive(Clone)]
pub struct DFA {
    /// The configuration provided by the caller.
    config: Config,
    /// The NFA used to build this DFA.
    ///
    /// NOTE: We probably don't need to store the NFA here, but we use enough
    /// bits from it that it's convenient to do so. And there really isn't much
    /// cost to doing so either, since an NFA is reference counted internally.
    nfa: NFA,
    /// The transition table. Given a state ID 's' and a byte of haystack 'b',
    /// the next state is `table[sid + classes[byte]]`.
    ///
    /// The stride of this table (i.e., the number of columns) is always
    /// a power of 2, even if the alphabet length is smaller. This makes
    /// converting between state IDs and state indices very cheap.
    ///
    /// Note that the stride always includes room for one extra "transition"
    /// that isn't actually a transition. It is a 'PatternEpsilons' that is
    /// used for match states only. Because of this, the maximum number of
    /// active columns in the transition table is 257, which means the maximum
    /// stride is 512 (the next power of 2 greater than or equal to 257).
    table: Vec<Transition>,
    /// The DFA state IDs of the starting states.
    ///
    /// `starts[0]` is always present and corresponds to the starting state
    /// when searching for matches of any pattern in the DFA.
    ///
    /// `starts[i]` where i>0 corresponds to the starting state for the pattern
    /// ID 'i-1'. These starting states are optional.
    starts: Vec<StateID>,
    /// Every state ID >= this value corresponds to a match state.
    ///
    /// This is what a search uses to detect whether a state is a match state
    /// or not. It requires only a simple comparison instead of bit-unpacking
    /// the PatternEpsilons from every state.
    min_match_id: StateID,
    /// The alphabet of this DFA, split into equivalence classes. Bytes in the
    /// same equivalence class can never discriminate between a match and a
    /// non-match.
    classes: ByteClasses,
    /// The number of elements in each state in the transition table. This may
    /// be less than the stride, since the stride is always a power of 2 and
    /// the alphabet length can be anything up to and including 256.
    alphabet_len: usize,
    /// The number of columns in the transition table, expressed as a power of
    /// 2.
    stride2: usize,
    /// The offset at which the PatternEpsilons for a match state is stored in
    /// the transition table.
    ///
    /// PERF: One wonders whether it would be better to put this in a separate
    /// allocation, since only match states have a non-empty PatternEpsilons
    /// and the number of match states tends be dwarfed by the number of
    /// non-match states. So this would save '8*len(non_match_states)' for each
    /// DFA. The question is whether moving this to a different allocation will
    /// lead to a perf hit during searches. You might think dealing with match
    /// states is rare, but some regexes spend a lot of time in match states
    /// gobbling up input. But... match state handling is already somewhat
    /// expensive, so maybe this wouldn't do much? Either way, it's worth
    /// experimenting.
    pateps_offset: usize,
    /// The first explicit slot index. This refers to the first slot appearing
    /// immediately after the last implicit slot. It is always 'patterns.len()
    /// * 2'.
    ///
    /// We record this because we only store the explicit slots in our DFA
    /// transition table that need to be saved. Implicit slots are handled
    /// automatically as part of the search.
    explicit_slot_start: usize,
}

impl DFA {
    /// Parse the given regular expression using the default configuration and
    /// return the corresponding one-pass DFA.
    ///
    /// If you want a non-default configuration, then use the [`Builder`] to
    /// set your own configuration.
    ///
    /// # Example
    ///
    /// ```
    /// use regex_automata::{dfa::onepass::DFA, Match};
    ///
    /// let re = DFA::new("foo[0-9]+bar")?;
    /// let (mut cache, mut caps) = (re.create_cache(), re.create_captures());
    ///
    /// re.captures(&mut cache, "foo12345barzzz", &mut caps);
    /// assert_eq!(Some(Match::must(0, 0..11)), caps.get_match());
    /// # Ok::<(), Box<dyn std::error::Error>>(())
    /// ```
    #[cfg(feature = "syntax")]
    #[inline]
    pub fn new(pattern: &str) -> Result<DFA, BuildError> {
        DFA::builder().build(pattern)
    }

    /// Like `new`, but parses multiple patterns into a single "multi regex."
    /// This similarly uses the default regex configuration.
    ///
    /// # Example
    ///
    /// ```
    /// use regex_automata::{dfa::onepass::DFA, Match};
    ///
    /// let re = DFA::new_many(&["[a-z]+", "[0-9]+"])?;
    /// let (mut cache, mut caps) = (re.create_cache(), re.create_captures());
    ///
    /// re.captures(&mut cache, "abc123", &mut caps);
    /// assert_eq!(Some(Match::must(0, 0..3)), caps.get_match());
    ///
    /// re.captures(&mut cache, "123abc", &mut caps);
    /// assert_eq!(Some(Match::must(1, 0..3)), caps.get_match());
    ///
    /// # Ok::<(), Box<dyn std::error::Error>>(())
    /// ```
    #[cfg(feature = "syntax")]
    #[inline]
    pub fn new_many<P: AsRef<str>>(patterns: &[P]) -> Result<DFA, BuildError> {
        DFA::builder().build_many(patterns)
    }

    /// Like `new`, but builds a one-pass DFA directly from an NFA. This is
    /// useful if you already have an NFA, or even if you hand-assembled the
    /// NFA.
    ///
    /// # Example
    ///
    /// This shows how to hand assemble a regular expression via its HIR,
    /// compile an NFA from it and build a one-pass DFA from the NFA.
    ///
    /// ```
    /// use regex_automata::{
    ///     dfa::onepass::DFA,
    ///     nfa::thompson::NFA,
    ///     Match,
    /// };
    /// use regex_syntax::hir::{Hir, Class, ClassBytes, ClassBytesRange};
    ///
    /// let hir = Hir::class(Class::Bytes(ClassBytes::new(vec![
    ///     ClassBytesRange::new(b'0', b'9'),
    ///     ClassBytesRange::new(b'A', b'Z'),
    ///     ClassBytesRange::new(b'_', b'_'),
    ///     ClassBytesRange::new(b'a', b'z'),
    /// ])));
    ///
    /// let config = NFA::config().nfa_size_limit(Some(1_000));
    /// let nfa = NFA::compiler().configure(config).build_from_hir(&hir)?;
    ///
    /// let re = DFA::new_from_nfa(nfa)?;
    /// let (mut cache, mut caps) = (re.create_cache(), re.create_captures());
    /// let expected = Some(Match::must(0, 0..1));
    /// re.captures(&mut cache, "A", &mut caps);
    /// assert_eq!(expected, caps.get_match());
    ///
    /// # Ok::<(), Box<dyn std::error::Error>>(())
    /// ```
    pub fn new_from_nfa(nfa: NFA) -> Result<DFA, BuildError> {
        DFA::builder().build_from_nfa(nfa)
    }

    /// Create a new one-pass DFA that matches every input.
    ///
    /// # Example
    ///
    /// ```
    /// use regex_automata::{dfa::onepass::DFA, Match};
    ///
    /// let dfa = DFA::always_match()?;
    /// let mut cache = dfa.create_cache();
    /// let mut caps = dfa.create_captures();
    ///
    /// let expected = Match::must(0, 0..0);
    /// dfa.captures(&mut cache, "", &mut caps);
    /// assert_eq!(Some(expected), caps.get_match());
    /// dfa.captures(&mut cache, "foo", &mut caps);
    /// assert_eq!(Some(expected), caps.get_match());
    /// # Ok::<(), Box<dyn std::error::Error>>(())
    /// ```
    pub fn always_match() -> Result<DFA, BuildError> {
        let nfa = thompson::NFA::always_match();
        Builder::new().build_from_nfa(nfa)
    }

    /// Create a new one-pass DFA that never matches any input.
    ///
    /// # Example
    ///
    /// ```
    /// use regex_automata::dfa::onepass::DFA;
    ///
    /// let dfa = DFA::never_match()?;
    /// let mut cache = dfa.create_cache();
    /// let mut caps = dfa.create_captures();
    ///
    /// dfa.captures(&mut cache, "", &mut caps);
    /// assert_eq!(None, caps.get_match());
    /// dfa.captures(&mut cache, "foo", &mut caps);
    /// assert_eq!(None, caps.get_match());
    /// # Ok::<(), Box<dyn std::error::Error>>(())
    /// ```
    pub fn never_match() -> Result<DFA, BuildError> {
        let nfa = thompson::NFA::never_match();
        Builder::new().build_from_nfa(nfa)
    }

    /// Return a default configuration for a DFA.
    ///
    /// This is a convenience routine to avoid needing to import the `Config`
    /// type when customizing the construction of a DFA.
    ///
    /// # Example
    ///
    /// This example shows how to change the match semantics of this DFA from
    /// its default "leftmost first" to "all." When using "all," non-greediness
    /// doesn't apply and neither does preference order matching. Instead, the
    /// longest match possible is always returned. (Although, by construction,
    /// it's impossible for a one-pass DFA to have a different answer for
    /// "preference order" vs "longest match.")
    ///
    /// ```
    /// use regex_automata::{dfa::onepass::DFA, Match, MatchKind};
    ///
    /// let re = DFA::builder()
    ///     .configure(DFA::config().match_kind(MatchKind::All))
    ///     .build(r"(abc)+?")?;
    /// let mut cache = re.create_cache();
    /// let mut caps = re.create_captures();
    ///
    /// re.captures(&mut cache, "abcabc", &mut caps);
    /// // Normally, the non-greedy repetition would give us a 0..3 match.
    /// assert_eq!(Some(Match::must(0, 0..6)), caps.get_match());
    /// # Ok::<(), Box<dyn std::error::Error>>(())
    /// ```
    #[inline]
    pub fn config() -> Config {
        Config::new()
    }

    /// Return a builder for configuring the construction of a DFA.
    ///
    /// This is a convenience routine to avoid needing to import the
    /// [`Builder`] type in common cases.
    ///
    /// # Example
    ///
    /// This example shows how to use the builder to disable UTF-8 mode.
    ///
    /// ```
    /// # if cfg!(miri) { return Ok(()); } // miri takes too long
    /// use regex_automata::{
    ///     dfa::onepass::DFA,
    ///     nfa::thompson,
    ///     util::syntax,
    ///     Match,
    /// };
    ///
    /// let re = DFA::builder()
    ///     .syntax(syntax::Config::new().utf8(false))
    ///     .thompson(thompson::Config::new().utf8(false))
    ///     .build(r"foo(?-u:[^b])ar.*")?;
    /// let (mut cache, mut caps) = (re.create_cache(), re.create_captures());
    ///
    /// let haystack = b"foo\xFFarzz\xE2\x98\xFF\n";
    /// let expected = Some(Match::must(0, 0..8));
    /// re.captures(&mut cache, haystack, &mut caps);
    /// assert_eq!(expected, caps.get_match());
    ///
    /// # Ok::<(), Box<dyn std::error::Error>>(())
    /// ```
    #[inline]
    pub fn builder() -> Builder {
        Builder::new()
    }

    /// Create a new empty set of capturing groups that is guaranteed to be
    /// valid for the search APIs on this DFA.
    ///
    /// A `Captures` value created for a specific DFA cannot be used with any
    /// other DFA.
    ///
    /// This is a convenience function for [`Captures::all`]. See the
    /// [`Captures`] documentation for an explanation of its alternative
    /// constructors that permit the DFA to do less work during a search, and
    /// thus might make it faster.
    #[inline]
    pub fn create_captures(&self) -> Captures {
        Captures::all(self.nfa.group_info().clone())
    }

    /// Create a new cache for this DFA.
    ///
    /// The cache returned should only be used for searches for this
    /// DFA. If you want to reuse the cache for another DFA, then you
    /// must call [`Cache::reset`] with that DFA (or, equivalently,
    /// [`DFA::reset_cache`]).
    #[inline]
    pub fn create_cache(&self) -> Cache {
        Cache::new(self)
    }

    /// Reset the given cache such that it can be used for searching with the
    /// this DFA (and only this DFA).
    ///
    /// A cache reset permits reusing memory already allocated in this cache
    /// with a different DFA.
    ///
    /// # Example
    ///
    /// This shows how to re-purpose a cache for use with a different DFA.
    ///
    /// ```
    /// # if cfg!(miri) { return Ok(()); } // miri takes too long
    /// use regex_automata::{dfa::onepass::DFA, Match};
    ///
    /// let re1 = DFA::new(r"\w")?;
    /// let re2 = DFA::new(r"\W")?;
    /// let mut caps1 = re1.create_captures();
    /// let mut caps2 = re2.create_captures();
    ///
    /// let mut cache = re1.create_cache();
    /// assert_eq!(
    ///     Some(Match::must(0, 0..2)),
    ///     { re1.captures(&mut cache, "Δ", &mut caps1); caps1.get_match() },
    /// );
    ///
    /// // Using 'cache' with re2 is not allowed. It may result in panics or
    /// // incorrect results. In order to re-purpose the cache, we must reset
    /// // it with the one-pass DFA we'd like to use it with.
    /// //
    /// // Similarly, after this reset, using the cache with 're1' is also not
    /// // allowed.
    /// re2.reset_cache(&mut cache);
    /// assert_eq!(
    ///     Some(Match::must(0, 0..3)),
    ///     { re2.captures(&mut cache, "☃", &mut caps2); caps2.get_match() },
    /// );
    ///
    /// # Ok::<(), Box<dyn std::error::Error>>(())
    /// ```
    #[inline]
    pub fn reset_cache(&self, cache: &mut Cache) {
        cache.reset(self);
    }

    /// Return the config for this one-pass DFA.
    #[inline]
    pub fn get_config(&self) -> &Config {
        &self.config
    }

    /// Returns a reference to the underlying NFA.
    #[inline]
    pub fn get_nfa(&self) -> &NFA {
        &self.nfa
    }

    /// Returns the total number of patterns compiled into this DFA.
    ///
    /// In the case of a DFA that contains no patterns, this returns `0`.
    #[inline]
    pub fn pattern_len(&self) -> usize {
        self.get_nfa().pattern_len()
    }

    /// Returns the total number of states in this one-pass DFA.
    ///
    /// Note that unlike dense or sparse DFAs, a one-pass DFA does not expose
    /// a low level DFA API. Therefore, this routine has little use other than
    /// being informational.
    #[inline]
    pub fn state_len(&self) -> usize {
        self.table.len() >> self.stride2()
    }

    /// Returns the total number of elements in the alphabet for this DFA.
    ///
    /// That is, this returns the total number of transitions that each
    /// state in this DFA must have. The maximum alphabet size is 256, which
    /// corresponds to each possible byte value.
    ///
    /// The alphabet size may be less than 256 though, and unless
    /// [`Config::byte_classes`] is disabled, it is typically must less than
    /// 256. Namely, bytes are grouped into equivalence classes such that no
    /// two bytes in the same class can distinguish a match from a non-match.
    /// For example, in the regex `^[a-z]+$`, the ASCII bytes `a-z` could
    /// all be in the same equivalence class. This leads to a massive space
    /// savings.
    ///
    /// Note though that the alphabet length does _not_ necessarily equal the
    /// total stride space taken up by a single DFA state in the transition
    /// table. Namely, for performance reasons, the stride is always the
    /// smallest power of two that is greater than or equal to the alphabet
    /// length. For this reason, [`DFA::stride`] or [`DFA::stride2`] are
    /// often more useful. The alphabet length is typically useful only for
    /// informational purposes.
    ///
    /// Note also that unlike dense or sparse DFAs, a one-pass DFA does
    /// not have a special end-of-input (EOI) transition. This is because
    /// a one-pass DFA handles look-around assertions explicitly (like the
    /// [`PikeVM`](crate::nfa::thompson::pikevm::PikeVM)) and does not build
    /// them into the transitions of the DFA.
    #[inline]
    pub fn alphabet_len(&self) -> usize {
        self.alphabet_len
    }

    /// Returns the total stride for every state in this DFA, expressed as the
    /// exponent of a power of 2. The stride is the amount of space each state
    /// takes up in the transition table, expressed as a number of transitions.
    /// (Unused transitions map to dead states.)
    ///
    /// The stride of a DFA is always equivalent to the smallest power of
    /// 2 that is greater than or equal to the DFA's alphabet length. This
    /// definition uses extra space, but possibly permits faster translation
    /// between state identifiers and their corresponding offsets in this DFA's
    /// transition table.
    ///
    /// For example, if the DFA's stride is 16 transitions, then its `stride2`
    /// is `4` since `2^4 = 16`.
    ///
    /// The minimum `stride2` value is `1` (corresponding to a stride of `2`)
    /// while the maximum `stride2` value is `9` (corresponding to a stride
    /// of `512`). The maximum in theory should be `8`, but because of some
    /// implementation quirks that may be relaxed in the future, it is one more
    /// than `8`. (Do note that a maximal stride is incredibly rare, as it
    /// would imply that there is almost no redundant in the regex pattern.)
    ///
    /// Note that unlike dense or sparse DFAs, a one-pass DFA does not expose
    /// a low level DFA API. Therefore, this routine has little use other than
    /// being informational.
    #[inline]
    pub fn stride2(&self) -> usize {
        self.stride2
    }

    /// Returns the total stride for every state in this DFA. This corresponds
    /// to the total number of transitions used by each state in this DFA's
    /// transition table.
    ///
    /// Please see [`DFA::stride2`] for more information. In particular, this
    /// returns the stride as the number of transitions, where as `stride2`
    /// returns it as the exponent of a power of 2.
    ///
    /// Note that unlike dense or sparse DFAs, a one-pass DFA does not expose
    /// a low level DFA API. Therefore, this routine has little use other than
    /// being informational.
    #[inline]
    pub fn stride(&self) -> usize {
        1 << self.stride2()
    }

    /// Returns the memory usage, in bytes, of this DFA.
    ///
    /// The memory usage is computed based on the number of bytes used to
    /// represent this DFA.
    ///
    /// This does **not** include the stack size used up by this DFA. To
    /// compute that, use `std::mem::size_of::<onepass::DFA>()`.
    #[inline]
    pub fn memory_usage(&self) -> usize {
        use core::mem::size_of;

        self.table.len() * size_of::<Transition>()
            + self.starts.len() * size_of::<StateID>()
    }
}

impl DFA {
    /// Executes an anchored leftmost forward search, and returns true if and
    /// only if this one-pass DFA matches the given haystack.
    ///
    /// This routine may short circuit if it knows that scanning future
    /// input will never lead to a different result. In particular, if the
    /// underlying DFA enters a match state, then this routine will return
    /// `true` immediately without inspecting any future input. (Consider how
    /// this might make a difference given the regex `a+` on the haystack
    /// `aaaaaaaaaaaaaaa`. This routine can stop after it sees the first `a`,
    /// but routines like `find` need to continue searching because `+` is
    /// greedy by default.)
    ///
    /// The given `Input` is forcefully set to use [`Anchored::Yes`] if the
    /// given configuration was [`Anchored::No`] (which is the default).
    ///
    /// # Panics
    ///
    /// This routine panics if the search could not complete. This can occur
    /// in the following circumstances:
    ///
    /// * When the provided `Input` configuration is not supported. For
    /// example, by providing an unsupported anchor mode. Concretely,
    /// this occurs when using [`Anchored::Pattern`] without enabling
    /// [`Config::starts_for_each_pattern`].
    ///
    /// When a search panics, callers cannot know whether a match exists or
    /// not.
    ///
    /// Use [`DFA::try_search`] if you want to handle these panics as error
    /// values instead.
    ///
    /// # Example
    ///
    /// This shows basic usage:
    ///
    /// ```
    /// use regex_automata::dfa::onepass::DFA;
    ///
    /// let re = DFA::new("foo[0-9]+bar")?;
    /// let mut cache = re.create_cache();
    ///
    /// assert!(re.is_match(&mut cache, "foo12345bar"));
    /// assert!(!re.is_match(&mut cache, "foobar"));
    /// # Ok::<(), Box<dyn std::error::Error>>(())
    /// ```
    ///
    /// # Example: consistency with search APIs
    ///
    /// `is_match` is guaranteed to return `true` whenever `captures` returns
    /// a match. This includes searches that are executed entirely within a
    /// codepoint:
    ///
    /// ```
    /// use regex_automata::{dfa::onepass::DFA, Input};
    ///
    /// let re = DFA::new("a*")?;
    /// let mut cache = re.create_cache();
    ///
    /// assert!(!re.is_match(&mut cache, Input::new("☃").span(1..2)));
    /// # Ok::<(), Box<dyn std::error::Error>>(())
    /// ```
    ///
    /// Notice that when UTF-8 mode is disabled, then the above reports a
    /// match because the restriction against zero-width matches that split a
    /// codepoint has been lifted:
    ///
    /// ```
    /// use regex_automata::{dfa::onepass::DFA, nfa::thompson::NFA, Input};
    ///
    /// let re = DFA::builder()
    ///     .thompson(NFA::config().utf8(false))
    ///     .build("a*")?;
    /// let mut cache = re.create_cache();
    ///
    /// assert!(re.is_match(&mut cache, Input::new("☃").span(1..2)));
    /// # Ok::<(), Box<dyn std::error::Error>>(())
    /// ```
    #[inline]
    pub fn is_match<'h, I: Into<Input<'h>>>(
        &self,
        cache: &mut Cache,
        input: I,
    ) -> bool {
        let mut input = input.into().earliest(true);
        if matches!(input.get_anchored(), Anchored::No) {
            input.set_anchored(Anchored::Yes);
        }
        self.try_search_slots(cache, &input, &mut []).unwrap().is_some()
    }

    /// Executes an anchored leftmost forward search, and returns a `Match` if
    /// and only if this one-pass DFA matches the given haystack.
    ///
    /// This routine only includes the overall match span. To get access to the
    /// individual spans of each capturing group, use [`DFA::captures`].
    ///
    /// The given `Input` is forcefully set to use [`Anchored::Yes`] if the
    /// given configuration was [`Anchored::No`] (which is the default).
    ///
    /// # Panics
    ///
    /// This routine panics if the search could not complete. This can occur
    /// in the following circumstances:
    ///
    /// * When the provided `Input` configuration is not supported. For
    /// example, by providing an unsupported anchor mode. Concretely,
    /// this occurs when using [`Anchored::Pattern`] without enabling
    /// [`Config::starts_for_each_pattern`].
    ///
    /// When a search panics, callers cannot know whether a match exists or
    /// not.
    ///
    /// Use [`DFA::try_search`] if you want to handle these panics as error
    /// values instead.
    ///
    /// # Example
    ///
    /// Leftmost first match semantics corresponds to the match with the
    /// smallest starting offset, but where the end offset is determined by
    /// preferring earlier branches in the original regular expression. For
    /// example, `Sam|Samwise` will match `Sam` in `Samwise`, but `Samwise|Sam`
    /// will match `Samwise` in `Samwise`.
    ///
    /// Generally speaking, the "leftmost first" match is how most backtracking
    /// regular expressions tend to work. This is in contrast to POSIX-style
    /// regular expressions that yield "leftmost longest" matches. Namely,
    /// both `Sam|Samwise` and `Samwise|Sam` match `Samwise` when using
    /// leftmost longest semantics. (This crate does not currently support
    /// leftmost longest semantics.)
    ///
    /// ```
    /// use regex_automata::{dfa::onepass::DFA, Match};
    ///
    /// let re = DFA::new("foo[0-9]+")?;
    /// let mut cache = re.create_cache();
    /// let expected = Match::must(0, 0..8);
    /// assert_eq!(Some(expected), re.find(&mut cache, "foo12345"));
    ///
    /// // Even though a match is found after reading the first byte (`a`),
    /// // the leftmost first match semantics demand that we find the earliest
    /// // match that prefers earlier parts of the pattern over later parts.
    /// let re = DFA::new("abc|a")?;
    /// let mut cache = re.create_cache();
    /// let expected = Match::must(0, 0..3);
    /// assert_eq!(Some(expected), re.find(&mut cache, "abc"));
    ///
    /// # Ok::<(), Box<dyn std::error::Error>>(())
    /// ```
    #[inline]
    pub fn find<'h, I: Into<Input<'h>>>(
        &self,
        cache: &mut Cache,
        input: I,
    ) -> Option<Match> {
        let mut input = input.into();
        if matches!(input.get_anchored(), Anchored::No) {
            input.set_anchored(Anchored::Yes);
        }
        if self.get_nfa().pattern_len() == 1 {
            let mut slots = [None, None];
            let pid =
                self.try_search_slots(cache, &input, &mut slots).unwrap()?;
            let start = slots[0].unwrap().get();
            let end = slots[1].unwrap().get();
            return Some(Match::new(pid, Span { start, end }));
        }
        let ginfo = self.get_nfa().group_info();
        let slots_len = ginfo.implicit_slot_len();
        let mut slots = vec![None; slots_len];
        let pid = self.try_search_slots(cache, &input, &mut slots).unwrap()?;
        let start = slots[pid.as_usize() * 2].unwrap().get();
        let end = slots[pid.as_usize() * 2 + 1].unwrap().get();
        Some(Match::new(pid, Span { start, end }))
    }

    /// Executes an anchored leftmost forward search and writes the spans
    /// of capturing groups that participated in a match into the provided
    /// [`Captures`] value. If no match was found, then [`Captures::is_match`]
    /// is guaranteed to return `false`.
    ///
    /// The given `Input` is forcefully set to use [`Anchored::Yes`] if the
    /// given configuration was [`Anchored::No`] (which is the default).
    ///
    /// # Panics
    ///
    /// This routine panics if the search could not complete. This can occur
    /// in the following circumstances:
    ///
    /// * When the provided `Input` configuration is not supported. For
    /// example, by providing an unsupported anchor mode. Concretely,
    /// this occurs when using [`Anchored::Pattern`] without enabling
    /// [`Config::starts_for_each_pattern`].
    ///
    /// When a search panics, callers cannot know whether a match exists or
    /// not.
    ///
    /// Use [`DFA::try_search`] if you want to handle these panics as error
    /// values instead.
    ///
    /// # Example
    ///
    /// This shows a simple example of a one-pass regex that extracts
    /// capturing group spans.
    ///
    /// ```
    /// use regex_automata::{dfa::onepass::DFA, Match, Span};
    ///
    /// let re = DFA::new(
    ///     // Notice that we use ASCII here. The corresponding Unicode regex
    ///     // is sadly not one-pass.
    ///     "(?P<first>[[:alpha:]]+)[[:space:]]+(?P<last>[[:alpha:]]+)",
    /// )?;
    /// let (mut cache, mut caps) = (re.create_cache(), re.create_captures());
    ///
    /// re.captures(&mut cache, "Bruce Springsteen", &mut caps);
    /// assert_eq!(Some(Match::must(0, 0..17)), caps.get_match());
    /// assert_eq!(Some(Span::from(0..5)), caps.get_group(1));
    /// assert_eq!(Some(Span::from(6..17)), caps.get_group_by_name("last"));
    ///
    /// # Ok::<(), Box<dyn std::error::Error>>(())
    /// ```
    #[inline]
    pub fn captures<'h, I: Into<Input<'h>>>(
        &self,
        cache: &mut Cache,
        input: I,
        caps: &mut Captures,
    ) {
        let mut input = input.into();
        if matches!(input.get_anchored(), Anchored::No) {
            input.set_anchored(Anchored::Yes);
        }
        self.try_search(cache, &input, caps).unwrap();
    }

    /// Executes an anchored leftmost forward search and writes the spans
    /// of capturing groups that participated in a match into the provided
    /// [`Captures`] value. If no match was found, then [`Captures::is_match`]
    /// is guaranteed to return `false`.
    ///
    /// The differences with [`DFA::captures`] are:
    ///
    /// 1. This returns an error instead of panicking if the search fails.
    /// 2. Accepts an `&Input` instead of a `Into<Input>`. This permits reusing
    /// the same input for multiple searches, which _may_ be important for
    /// latency.
    /// 3. This does not automatically change the [`Anchored`] mode from `No`
    /// to `Yes`. Instead, if [`Input::anchored`] is `Anchored::No`, then an
    /// error is returned.
    ///
    /// # Errors
    ///
    /// This routine errors if the search could not complete. This can occur
    /// in the following circumstances:
    ///
    /// * When the provided `Input` configuration is not supported. For
    /// example, by providing an unsupported anchor mode. Concretely,
    /// this occurs when using [`Anchored::Pattern`] without enabling
    /// [`Config::starts_for_each_pattern`].
    ///
    /// When a search returns an error, callers cannot know whether a match
    /// exists or not.
    ///
    /// # Example: specific pattern search
    ///
    /// This example shows how to build a multi-regex that permits searching
    /// for specific patterns. Note that this is somewhat less useful than
    /// in other regex engines, since a one-pass DFA by definition has no
    /// ambiguity about which pattern can match at a position. That is, if it
    /// were possible for two different patterns to match at the same starting
    /// position, then the multi-regex would not be one-pass and construction
    /// would have failed.
    ///
    /// Nevertheless, this can still be useful if you only care about matches
    /// for a specific pattern, and want the DFA to report "no match" even if
    /// some other pattern would have matched.
    ///
    /// Note that in order to make use of this functionality,
    /// [`Config::starts_for_each_pattern`] must be enabled. It is disabled
    /// by default since it may result in higher memory usage.
    ///
    /// ```
    /// use regex_automata::{
    ///     dfa::onepass::DFA, Anchored, Input, Match, PatternID,
    /// };
    ///
    /// let re = DFA::builder()
    ///     .configure(DFA::config().starts_for_each_pattern(true))
    ///     .build_many(&["[a-z]+", "[0-9]+"])?;
    /// let (mut cache, mut caps) = (re.create_cache(), re.create_captures());
    /// let haystack = "123abc";
    /// let input = Input::new(haystack).anchored(Anchored::Yes);
    ///
    /// // A normal multi-pattern search will show pattern 1 matches.
    /// re.try_search(&mut cache, &input, &mut caps)?;
    /// assert_eq!(Some(Match::must(1, 0..3)), caps.get_match());
    ///
    /// // If we only want to report pattern 0 matches, then we'll get no
    /// // match here.
    /// let input = input.anchored(Anchored::Pattern(PatternID::must(0)));
    /// re.try_search(&mut cache, &input, &mut caps)?;
    /// assert_eq!(None, caps.get_match());
    ///
    /// # Ok::<(), Box<dyn std::error::Error>>(())
    /// ```
    ///
    /// # Example: specifying the bounds of a search
    ///
    /// This example shows how providing the bounds of a search can produce
    /// different results than simply sub-slicing the haystack.
    ///
    /// ```
    /// # if cfg!(miri) { return Ok(()); } // miri takes too long
    /// use regex_automata::{dfa::onepass::DFA, Anchored, Input, Match};
    ///
    /// // one-pass DFAs fully support Unicode word boundaries!
    /// // A sad joke is that a Unicode aware regex like \w+\s is not one-pass.
    /// // :-(
    /// let re = DFA::new(r"\b[0-9]{3}\b")?;
    /// let (mut cache, mut caps) = (re.create_cache(), re.create_captures());
    /// let haystack = "foo123bar";
    ///
    /// // Since we sub-slice the haystack, the search doesn't know about
    /// // the larger context and assumes that `123` is surrounded by word
    /// // boundaries. And of course, the match position is reported relative
    /// // to the sub-slice as well, which means we get `0..3` instead of
    /// // `3..6`.
    /// let expected = Some(Match::must(0, 0..3));
    /// let input = Input::new(&haystack[3..6]).anchored(Anchored::Yes);
    /// re.try_search(&mut cache, &input, &mut caps)?;
    /// assert_eq!(expected, caps.get_match());
    ///
    /// // But if we provide the bounds of the search within the context of the
    /// // entire haystack, then the search can take the surrounding context
    /// // into account. (And if we did find a match, it would be reported
    /// // as a valid offset into `haystack` instead of its sub-slice.)
    /// let expected = None;
    /// let input = Input::new(haystack).range(3..6).anchored(Anchored::Yes);
    /// re.try_search(&mut cache, &input, &mut caps)?;
    /// assert_eq!(expected, caps.get_match());
    ///
    /// # Ok::<(), Box<dyn std::error::Error>>(())
    /// ```
    #[inline]
    pub fn try_search(
        &self,
        cache: &mut Cache,
        input: &Input<'_>,
        caps: &mut Captures,
    ) -> Result<(), MatchError> {
        let pid = self.try_search_slots(cache, input, caps.slots_mut())?;
        caps.set_pattern(pid);
        Ok(())
    }

    /// Executes an anchored leftmost forward search and writes the spans
    /// of capturing groups that participated in a match into the provided
    /// `slots`, and returns the matching pattern ID. The contents of the
    /// slots for patterns other than the matching pattern are unspecified. If
    /// no match was found, then `None` is returned and the contents of all
    /// `slots` is unspecified.
    ///
    /// This is like [`DFA::try_search`], but it accepts a raw slots slice
    /// instead of a `Captures` value. This is useful in contexts where you
    /// don't want or need to allocate a `Captures`.
    ///
    /// It is legal to pass _any_ number of slots to this routine. If the regex
    /// engine would otherwise write a slot offset that doesn't fit in the
    /// provided slice, then it is simply skipped. In general though, there are
    /// usually three slice lengths you might want to use:
    ///
    /// * An empty slice, if you only care about which pattern matched.
    /// * A slice with
    /// [`pattern_len() * 2`](crate::dfa::onepass::DFA::pattern_len)
    /// slots, if you only care about the overall match spans for each matching
    /// pattern.
    /// * A slice with
    /// [`slot_len()`](crate::util::captures::GroupInfo::slot_len) slots, which
    /// permits recording match offsets for every capturing group in every
    /// pattern.
    ///
    /// # Errors
    ///
    /// This routine errors if the search could not complete. This can occur
    /// in the following circumstances:
    ///
    /// * When the provided `Input` configuration is not supported. For
    /// example, by providing an unsupported anchor mode. Concretely,
    /// this occurs when using [`Anchored::Pattern`] without enabling
    /// [`Config::starts_for_each_pattern`].
    ///
    /// When a search returns an error, callers cannot know whether a match
    /// exists or not.
    ///
    /// # Example
    ///
    /// This example shows how to find the overall match offsets in a
    /// multi-pattern search without allocating a `Captures` value. Indeed, we
    /// can put our slots right on the stack.
    ///
    /// ```
    /// use regex_automata::{dfa::onepass::DFA, Anchored, Input, PatternID};
    ///
    /// let re = DFA::new_many(&[
    ///     r"[a-zA-Z]+",
    ///     r"[0-9]+",
    /// ])?;
    /// let mut cache = re.create_cache();
    /// let input = Input::new("123").anchored(Anchored::Yes);
    ///
    /// // We only care about the overall match offsets here, so we just
    /// // allocate two slots for each pattern. Each slot records the start
    /// // and end of the match.
    /// let mut slots = [None; 4];
    /// let pid = re.try_search_slots(&mut cache, &input, &mut slots)?;
    /// assert_eq!(Some(PatternID::must(1)), pid);
    ///
    /// // The overall match offsets are always at 'pid * 2' and 'pid * 2 + 1'.
    /// // See 'GroupInfo' for more details on the mapping between groups and
    /// // slot indices.
    /// let slot_start = pid.unwrap().as_usize() * 2;
    /// let slot_end = slot_start + 1;
    /// assert_eq!(Some(0), slots[slot_start].map(|s| s.get()));
    /// assert_eq!(Some(3), slots[slot_end].map(|s| s.get()));
    ///
    /// # Ok::<(), Box<dyn std::error::Error>>(())
    /// ```
    #[inline]
    pub fn try_search_slots(
        &self,
        cache: &mut Cache,
        input: &Input<'_>,
        slots: &mut [Option<NonMaxUsize>],
    ) -> Result<Option<PatternID>, MatchError> {
        let utf8empty = self.get_nfa().has_empty() && self.get_nfa().is_utf8();
        if !utf8empty {
            return self.try_search_slots_imp(cache, input, slots);
        }
        // See PikeVM::try_search_slots for why we do this.
        let min = self.get_nfa().group_info().implicit_slot_len();
        if slots.len() >= min {
            return self.try_search_slots_imp(cache, input, slots);
        }
        if self.get_nfa().pattern_len() == 1 {
            let mut enough = [None, None];
            let got = self.try_search_slots_imp(cache, input, &mut enough)?;
            // This is OK because we know `enough_slots` is strictly bigger
            // than `slots`, otherwise this special case isn't reached.
            slots.copy_from_slice(&enough[..slots.len()]);
            return Ok(got);
        }
        let mut enough = vec![None; min];
        let got = self.try_search_slots_imp(cache, input, &mut enough)?;
        // This is OK because we know `enough_slots` is strictly bigger than
        // `slots`, otherwise this special case isn't reached.
        slots.copy_from_slice(&enough[..slots.len()]);
        Ok(got)
    }

    #[inline(never)]
    fn try_search_slots_imp(
        &self,
        cache: &mut Cache,
        input: &Input<'_>,
        slots: &mut [Option<NonMaxUsize>],
    ) -> Result<Option<PatternID>, MatchError> {
        let utf8empty = self.get_nfa().has_empty() && self.get_nfa().is_utf8();
        match self.search_imp(cache, input, slots)? {
            None => return Ok(None),
            Some(pid) if !utf8empty => return Ok(Some(pid)),
            Some(pid) => {
                // These slot indices are always correct because we know our
                // 'pid' is valid and thus we know that the slot indices for it
                // are valid.
                let slot_start = pid.as_usize().wrapping_mul(2);
                let slot_end = slot_start.wrapping_add(1);
                // OK because we know we have a match and we know our caller
                // provided slots are big enough (which we make true above if
                // the caller didn't). Namely, we're only here when 'utf8empty'
                // is true, and when that's true, we require slots for every
                // pattern.
                let start = slots[slot_start].unwrap().get();
                let end = slots[slot_end].unwrap().get();
                // If our match splits a codepoint, then we cannot report is
                // as a match. And since one-pass DFAs only support anchored
                // searches, we don't try to skip ahead to find the next match.
                // We can just quit with nothing.
                if start == end && !input.is_char_boundary(start) {
                    return Ok(None);
                }
                Ok(Some(pid))
            }
        }
    }
}

impl DFA {
    fn search_imp(
        &self,
        cache: &mut Cache,
        input: &Input<'_>,
        slots: &mut [Option<NonMaxUsize>],
    ) -> Result<Option<PatternID>, MatchError> {
        // PERF: Some ideas. I ran out of steam after my initial impl to try
        // many of these.
        //
        // 1) Try doing more state shuffling. Right now, all we do is push
        // match states to the end of the transition table so that we can do
        // 'if sid >= self.min_match_id' to know whether we're in a match
        // state or not. But what about doing something like dense DFAs and
        // pushing dead, match and states with captures/looks all toward the
        // beginning of the transition table. Then we could do 'if sid <=
        // self.max_special_id', in which case, we need to do some special
        // handling of some sort. Otherwise, we get the happy path, just
        // like in a DFA search. The main argument against this is that the
        // one-pass DFA is likely to be used most often with capturing groups
        // and if capturing groups are common, then this might wind up being a
        // pessimization.
        //
        // 2) Consider moving 'PatternEpsilons' out of the transition table.
        // It is only needed for match states and usually a small minority of
        // states are match states. Therefore, we're using an extra 'u64' for
        // most states.
        //
        // 3) I played around with the match state handling and it seems like
        // there is probably a lot left on the table for improvement. The
        // key tension is that the 'find_match' routine is a giant mess, but
        // splitting it out into a non-inlineable function is a non-starter
        // because the match state might consume input, so 'find_match' COULD
        // be called quite a lot, and a function call at that point would trash
        // perf. In theory, we could detect whether a match state consumes
        // input and then specialize our search routine based on that. In that
        // case, maybe an extra function call is OK, but even then, it might be
        // too much of a latency hit. Another idea is to just try and figure
        // out how to reduce the code size of 'find_match'. RE2 has a trick
        // here where the match handling isn't done if we know the next byte of
        // input yields a match too. Maybe we adopt that?
        //
        // This just might be a tricky DFA to optimize.

        if input.is_done() {
            return Ok(None);
        }
        // We unfortunately have a bit of book-keeping to do to set things
        // up. We do have to setup our cache and clear all of our slots. In
        // particular, clearing the slots is necessary for the case where we
        // report a match, but one of the capturing groups didn't participate
        // in the match but had a span set from a previous search. That would
        // be bad. In theory, we could avoid all this slot clearing if we knew
        // that every slot was always activated for every match. Then we would
        // know they would always be overwritten when a match is found.
        let explicit_slots_len = core::cmp::min(
            Slots::LIMIT,
            slots.len().saturating_sub(self.explicit_slot_start),
        );
        cache.setup_search(explicit_slots_len);
        for slot in cache.explicit_slots() {
            *slot = None;
        }
        for slot in slots.iter_mut() {
            *slot = None;
        }
        // We set the starting slots for every pattern up front. This does
        // increase our latency somewhat, but it avoids having to do it every
        // time we see a match state (which could be many times in a single
        // search if the match state consumes input).
        for pid in self.nfa.patterns() {
            let i = pid.as_usize() * 2;
            if i >= slots.len() {
                break;
            }
            slots[i] = NonMaxUsize::new(input.start());
        }
        let mut pid = None;
        let mut next_sid = match input.get_anchored() {
            Anchored::Yes => self.start(),
            Anchored::Pattern(pid) => self.start_pattern(pid)?,
            Anchored::No => {
                // If the regex is itself always anchored, then we're fine,
                // even if the search is configured to be unanchored.
                if !self.nfa.is_always_start_anchored() {
                    return Err(MatchError::unsupported_anchored(
                        Anchored::No,
                    ));
                }
                self.start()
            }
        };
        let leftmost_first =
            matches!(self.config.get_match_kind(), MatchKind::LeftmostFirst);
        for at in input.start()..input.end() {
            let sid = next_sid;
            let trans = self.transition(sid, input.haystack()[at]);
            next_sid = trans.state_id();
            let epsilons = trans.epsilons();
            if sid >= self.min_match_id {
                if self.find_match(cache, input, at, sid, slots, &mut pid) {
                    if input.get_earliest()
                        || (leftmost_first && trans.match_wins())
                    {
                        return Ok(pid);
                    }
                }
            }
            if sid == DEAD
                || (!epsilons.looks().is_empty()
                    && !self.nfa.look_matcher().matches_set_inline(
                        epsilons.looks(),
                        input.haystack(),
                        at,
                    ))
            {
                return Ok(pid);
            }
            epsilons.slots().apply(at, cache.explicit_slots());
        }
        if next_sid >= self.min_match_id {
            self.find_match(
                cache,
                input,
                input.end(),
                next_sid,
                slots,
                &mut pid,
            );
        }
        Ok(pid)
    }

    /// Assumes 'sid' is a match state and looks for whether a match can
    /// be reported. If so, appropriate offsets are written to 'slots' and
    /// 'matched_pid' is set to the matching pattern ID.
    ///
    /// Even when 'sid' is a match state, it's possible that a match won't
    /// be reported. For example, when the conditional epsilon transitions
    /// leading to the match state aren't satisfied at the given position in
    /// the haystack.
    #[cfg_attr(feature = "perf-inline", inline(always))]
    fn find_match(
        &self,
        cache: &mut Cache,
        input: &Input<'_>,
        at: usize,
        sid: StateID,
        slots: &mut [Option<NonMaxUsize>],
        matched_pid: &mut Option<PatternID>,
    ) -> bool {
        debug_assert!(sid >= self.min_match_id);
        let pateps = self.pattern_epsilons(sid);
        let epsilons = pateps.epsilons();
        if !epsilons.looks().is_empty()
            && !self.nfa.look_matcher().matches_set_inline(
                epsilons.looks(),
                input.haystack(),
                at,
            )
        {
            return false;
        }
        let pid = pateps.pattern_id_unchecked();
        // This calculation is always correct because we know our 'pid' is
        // valid and thus we know that the slot indices for it are valid.
        let slot_end = pid.as_usize().wrapping_mul(2).wrapping_add(1);
        // Set the implicit 'end' slot for the matching pattern. (The 'start'
        // slot was set at the beginning of the search.)
        if slot_end < slots.len() {
            slots[slot_end] = NonMaxUsize::new(at);
        }
        // If the caller provided enough room, copy the previously recorded
        // explicit slots from our scratch space to the caller provided slots.
        // We *also* need to set any explicit slots that are active as part of
        // the path to the match state.
        if self.explicit_slot_start < slots.len() {
            // NOTE: The 'cache.explicit_slots()' slice is setup at the
            // beginning of every search such that it is guaranteed to return a
            // slice of length equivalent to 'slots[explicit_slot_start..]'.
            slots[self.explicit_slot_start..]
                .copy_from_slice(cache.explicit_slots());
            epsilons.slots().apply(at, &mut slots[self.explicit_slot_start..]);
        }
        *matched_pid = Some(pid);
        true
    }
}

impl DFA {
    /// Returns the anchored start state for matching any pattern in this DFA.
    fn start(&self) -> StateID {
        self.starts[0]
    }

    /// Returns the anchored start state for matching the given pattern. If
    /// 'starts_for_each_pattern'
    /// was not enabled, then this returns an error. If the given pattern is
    /// not in this DFA, then `Ok(None)` is returned.
    fn start_pattern(&self, pid: PatternID) -> Result<StateID, MatchError> {
        if !self.config.get_starts_for_each_pattern() {
            return Err(MatchError::unsupported_anchored(Anchored::Pattern(
                pid,
            )));
        }
        // 'starts' always has non-zero length. The first entry is always the
        // anchored starting state for all patterns, and the following entries
        // are optional and correspond to the anchored starting states for
        // patterns at pid+1. Thus, starts.len()-1 corresponds to the total
        // number of patterns that one can explicitly search for. (And it may
        // be zero.)
        Ok(self.starts.get(pid.one_more()).copied().unwrap_or(DEAD))
    }

    /// Returns the transition from the given state ID and byte of input. The
    /// transition includes the next state ID, the slots that should be saved
    /// and any conditional epsilon transitions that must be satisfied in order
    /// to take this transition.
    fn transition(&self, sid: StateID, byte: u8) -> Transition {
        let offset = sid.as_usize() << self.stride2();
        let class = self.classes.get(byte).as_usize();
        self.table[offset + class]
    }

    /// Set the transition from the given state ID and byte of input to the
    /// transition given.
    fn set_transition(&mut self, sid: StateID, byte: u8, to: Transition) {
        let offset = sid.as_usize() << self.stride2();
        let class = self.classes.get(byte).as_usize();
        self.table[offset + class] = to;
    }

    /// Return an iterator of "sparse" transitions for the given state ID.
    /// "sparse" in this context means that consecutive transitions that are
    /// equivalent are returned as one group, and transitions to the DEAD state
    /// are ignored.
    ///
    /// This winds up being useful for debug printing, since it's much terser
    /// to display runs of equivalent transitions than the transition for every
    /// possible byte value. Indeed, in practice, it's very common for runs
    /// of equivalent transitions to appear.
    fn sparse_transitions(&self, sid: StateID) -> SparseTransitionIter<'_> {
        let start = sid.as_usize() << self.stride2();
        let end = start + self.alphabet_len();
        SparseTransitionIter {
            it: self.table[start..end].iter().enumerate(),
            cur: None,
        }
    }

    /// Return the pattern epsilons for the given state ID.
    ///
    /// If the given state ID does not correspond to a match state ID, then the
    /// pattern epsilons returned is empty.
    fn pattern_epsilons(&self, sid: StateID) -> PatternEpsilons {
        let offset = sid.as_usize() << self.stride2();
        PatternEpsilons(self.table[offset + self.pateps_offset].0)
    }

    /// Set the pattern epsilons for the given state ID.
    fn set_pattern_epsilons(&mut self, sid: StateID, pateps: PatternEpsilons) {
        let offset = sid.as_usize() << self.stride2();
        self.table[offset + self.pateps_offset] = Transition(pateps.0);
    }

    /// Returns the state ID prior to the one given. This returns None if the
    /// given ID is the first DFA state.
    fn prev_state_id(&self, id: StateID) -> Option<StateID> {
        if id == DEAD {
            None
        } else {
            // CORRECTNESS: Since 'id' is not the first state, subtracting 1
            // is always valid.
            Some(StateID::new_unchecked(id.as_usize().checked_sub(1).unwrap()))
        }
    }

    /// Returns the state ID of the last state in this DFA's transition table.
    /// "last" in this context means the last state to appear in memory, i.e.,
    /// the one with the greatest ID.
    fn last_state_id(&self) -> StateID {
        // CORRECTNESS: A DFA table is always non-empty since it always at
        // least contains a DEAD state. Since every state has the same stride,
        // we can just compute what the "next" state ID would have been and
        // then subtract 1 from it.
        StateID::new_unchecked(
            (self.table.len() >> self.stride2()).checked_sub(1).unwrap(),
        )
    }

    /// Move the transitions from 'id1' to 'id2' and vice versa.
    ///
    /// WARNING: This does not update the rest of the transition table to have
    /// transitions to 'id1' changed to 'id2' and vice versa. This merely moves
    /// the states in memory.
    pub(super) fn swap_states(&mut self, id1: StateID, id2: StateID) {
        let o1 = id1.as_usize() << self.stride2();
        let o2 = id2.as_usize() << self.stride2();
        for b in 0..self.stride() {
            self.table.swap(o1 + b, o2 + b);
        }
    }

    /// Map all state IDs in this DFA (transition table + start states)
    /// according to the closure given.
    pub(super) fn remap(&mut self, map: impl Fn(StateID) -> StateID) {
        for i in 0..self.state_len() {
            let offset = i << self.stride2();
            for b in 0..self.alphabet_len() {
                let next = self.table[offset + b].state_id();
                self.table[offset + b].set_state_id(map(next));
            }
        }
        for i in 0..self.starts.len() {
            self.starts[i] = map(self.starts[i]);
        }
    }
}

impl core::fmt::Debug for DFA {
    fn fmt(&self, f: &mut core::fmt::Formatter) -> core::fmt::Result {
        fn debug_state_transitions(
            f: &mut core::fmt::Formatter,
            dfa: &DFA,
            sid: StateID,
        ) -> core::fmt::Result {
            for (i, (start, end, trans)) in
                dfa.sparse_transitions(sid).enumerate()
            {
                let next = trans.state_id();
                if i > 0 {
                    write!(f, ", ")?;
                }
                if start == end {
                    write!(
                        f,
                        "{:?} => {:?}",
                        DebugByte(start),
                        next.as_usize(),
                    )?;
                } else {
                    write!(
                        f,
                        "{:?}-{:?} => {:?}",
                        DebugByte(start),
                        DebugByte(end),
                        next.as_usize(),
                    )?;
                }
                if trans.match_wins() {
                    write!(f, " (MW)")?;
                }
                if !trans.epsilons().is_empty() {
                    write!(f, " ({:?})", trans.epsilons())?;
                }
            }
            Ok(())
        }

        writeln!(f, "onepass::DFA(")?;
        for index in 0..self.state_len() {
            let sid = StateID::must(index);
            let pateps = self.pattern_epsilons(sid);
            if sid == DEAD {
                write!(f, "D ")?;
            } else if pateps.pattern_id().is_some() {
                write!(f, "* ")?;
            } else {
                write!(f, "  ")?;
            }
            write!(f, "{:06?}", sid.as_usize())?;
            if !pateps.is_empty() {
                write!(f, " ({:?})", pateps)?;
            }
            write!(f, ": ")?;
            debug_state_transitions(f, self, sid)?;
            write!(f, "\n")?;
        }
        writeln!(f, "")?;
        for (i, &sid) in self.starts.iter().enumerate() {
            if i == 0 {
                writeln!(f, "START(ALL): {:?}", sid.as_usize())?;
            } else {
                writeln!(
                    f,
                    "START(pattern: {:?}): {:?}",
                    i - 1,
                    sid.as_usize(),
                )?;
            }
        }
        writeln!(f, "state length: {:?}", self.state_len())?;
        writeln!(f, "pattern length: {:?}", self.pattern_len())?;
        writeln!(f, ")")?;
        Ok(())
    }
}

/// An iterator over groups of consecutive equivalent transitions in a single
/// state.
#[derive(Debug)]
struct SparseTransitionIter<'a> {
    it: core::iter::Enumerate<core::slice::Iter<'a, Transition>>,
    cur: Option<(u8, u8, Transition)>,
}

impl<'a> Iterator for SparseTransitionIter<'a> {
    type Item = (u8, u8, Transition);

    fn next(&mut self) -> Option<(u8, u8, Transition)> {
        while let Some((b, &trans)) = self.it.next() {
            // Fine because we'll never have more than u8::MAX transitions in
            // one state.
            let b = b.as_u8();
            let (prev_start, prev_end, prev_trans) = match self.cur {
                Some(t) => t,
                None => {
                    self.cur = Some((b, b, trans));
                    continue;
                }
            };
            if prev_trans == trans {
                self.cur = Some((prev_start, b, prev_trans));
            } else {
                self.cur = Some((b, b, trans));
                if prev_trans.state_id() != DEAD {
                    return Some((prev_start, prev_end, prev_trans));
                }
            }
        }
        if let Some((start, end, trans)) = self.cur.take() {
            if trans.state_id() != DEAD {
                return Some((start, end, trans));
            }
        }
        None
    }
}

/// A cache represents mutable state that a one-pass [`DFA`] requires during a
/// search.
///
/// For a given one-pass DFA, its corresponding cache may be created either via
/// [`DFA::create_cache`], or via [`Cache::new`]. They are equivalent in every
/// way, except the former does not require explicitly importing `Cache`.
///
/// A particular `Cache` is coupled with the one-pass DFA from which it was
/// created. It may only be used with that one-pass DFA. A cache and its
/// allocations may be re-purposed via [`Cache::reset`], in which case, it can
/// only be used with the new one-pass DFA (and not the old one).
#[derive(Clone, Debug)]
pub struct Cache {
    /// Scratch space used to store slots during a search. Basically, we use
    /// the caller provided slots to store slots known when a match occurs.
    /// But after a match occurs, we might continue a search but ultimately
    /// fail to extend the match. When continuing the search, we need some
    /// place to store candidate capture offsets without overwriting the slot
    /// offsets recorded for the most recently seen match.
    explicit_slots: Vec<Option<NonMaxUsize>>,
    /// The number of slots in the caller-provided 'Captures' value for the
    /// current search. This is always at most 'explicit_slots.len()', but
    /// might be less than it, if the caller provided fewer slots to fill.
    explicit_slot_len: usize,
}

impl Cache {
    /// Create a new [`onepass::DFA`](DFA) cache.
    ///
    /// A potentially more convenient routine to create a cache is
    /// [`DFA::create_cache`], as it does not require also importing the
    /// `Cache` type.
    ///
    /// If you want to reuse the returned `Cache` with some other one-pass DFA,
    /// then you must call [`Cache::reset`] with the desired one-pass DFA.
    pub fn new(re: &DFA) -> Cache {
        let mut cache = Cache { explicit_slots: vec![], explicit_slot_len: 0 };
        cache.reset(re);
        cache
    }

    /// Reset this cache such that it can be used for searching with a
    /// different [`onepass::DFA`](DFA).
    ///
    /// A cache reset permits reusing memory already allocated in this cache
    /// with a different one-pass DFA.
    ///
    /// # Example
    ///
    /// This shows how to re-purpose a cache for use with a different one-pass
    /// DFA.
    ///
    /// ```
    /// # if cfg!(miri) { return Ok(()); } // miri takes too long
    /// use regex_automata::{dfa::onepass::DFA, Match};
    ///
    /// let re1 = DFA::new(r"\w")?;
    /// let re2 = DFA::new(r"\W")?;
    /// let mut caps1 = re1.create_captures();
    /// let mut caps2 = re2.create_captures();
    ///
    /// let mut cache = re1.create_cache();
    /// assert_eq!(
    ///     Some(Match::must(0, 0..2)),
    ///     { re1.captures(&mut cache, "Δ", &mut caps1); caps1.get_match() },
    /// );
    ///
    /// // Using 'cache' with re2 is not allowed. It may result in panics or
    /// // incorrect results. In order to re-purpose the cache, we must reset
    /// // it with the one-pass DFA we'd like to use it with.
    /// //
    /// // Similarly, after this reset, using the cache with 're1' is also not
    /// // allowed.
    /// re2.reset_cache(&mut cache);
    /// assert_eq!(
    ///     Some(Match::must(0, 0..3)),
    ///     { re2.captures(&mut cache, "☃", &mut caps2); caps2.get_match() },
    /// );
    ///
    /// # Ok::<(), Box<dyn std::error::Error>>(())
    /// ```
    pub fn reset(&mut self, re: &DFA) {
        let explicit_slot_len = re.get_nfa().group_info().explicit_slot_len();
        self.explicit_slots.resize(explicit_slot_len, None);
        self.explicit_slot_len = explicit_slot_len;
    }

    /// Returns the heap memory usage, in bytes, of this cache.
    ///
    /// This does **not** include the stack size used up by this cache. To
    /// compute that, use `std::mem::size_of::<Cache>()`.
    pub fn memory_usage(&self) -> usize {
        self.explicit_slots.len() * core::mem::size_of::<Option<NonMaxUsize>>()
    }

    fn explicit_slots(&mut self) -> &mut [Option<NonMaxUsize>] {
        &mut self.explicit_slots[..self.explicit_slot_len]
    }

    fn setup_search(&mut self, explicit_slot_len: usize) {
        self.explicit_slot_len = explicit_slot_len;
    }
}

/// Represents a single transition in a one-pass DFA.
///
/// The high 21 bits corresponds to the state ID. The bit following corresponds
/// to the special "match wins" flag. The remaining low 42 bits corresponds to
/// the transition epsilons, which contains the slots that should be saved when
/// this transition is followed and the conditional epsilon transitions that
/// must be satisfied in order to follow this transition.
#[derive(Clone, Copy, Eq, PartialEq)]
struct Transition(u64);

impl Transition {
    const STATE_ID_BITS: u64 = 21;
    const STATE_ID_SHIFT: u64 = 64 - Transition::STATE_ID_BITS;
    const STATE_ID_LIMIT: u64 = 1 << Transition::STATE_ID_BITS;
    const MATCH_WINS_SHIFT: u64 = 64 - (Transition::STATE_ID_BITS + 1);
    const INFO_MASK: u64 = 0x000003FF_FFFFFFFF;

    /// Return a new transition to the given state ID with the given epsilons.
    fn new(match_wins: bool, sid: StateID, epsilons: Epsilons) -> Transition {
        let match_wins =
            if match_wins { 1 << Transition::MATCH_WINS_SHIFT } else { 0 };
        let sid = sid.as_u64() << Transition::STATE_ID_SHIFT;
        Transition(sid | match_wins | epsilons.0)
    }

    /// Returns true if and only if this transition points to the DEAD state.
    fn is_dead(self) -> bool {
        self.state_id() == DEAD
    }

    /// Return whether this transition has a "match wins" property.
    ///
    /// When a transition has this property, it means that if a match has been
    /// found and the search uses leftmost-first semantics, then that match
    /// should be returned immediately instead of continuing on.
    ///
    /// The "match wins" name comes from RE2, which uses a pretty much
    /// identical mechanism for implementing leftmost-first semantics.
    fn match_wins(&self) -> bool {
        (self.0 >> Transition::MATCH_WINS_SHIFT & 1) == 1
    }

    /// Return the "next" state ID that this transition points to.
    fn state_id(&self) -> StateID {
        // OK because a Transition has a valid StateID in its upper bits by
        // construction. The cast to usize is also correct, even on 16-bit
        // targets because, again, we know the upper bits is a valid StateID,
        // which can never overflow usize on any supported target.
        StateID::new_unchecked(
            (self.0 >> Transition::STATE_ID_SHIFT).as_usize(),
        )
    }

    /// Set the "next" state ID in this transition.
    fn set_state_id(&mut self, sid: StateID) {
        *self = Transition::new(self.match_wins(), sid, self.epsilons());
    }

    /// Return the epsilons embedded in this transition.
    fn epsilons(&self) -> Epsilons {
        Epsilons(self.0 & Transition::INFO_MASK)
    }
}

impl core::fmt::Debug for Transition {
    fn fmt(&self, f: &mut core::fmt::Formatter) -> core::fmt::Result {
        if self.is_dead() {
            return write!(f, "0");
        }
        write!(f, "{}", self.state_id().as_usize())?;
        if self.match_wins() {
            write!(f, "-MW")?;
        }
        if !self.epsilons().is_empty() {
            write!(f, "-{:?}", self.epsilons())?;
        }
        Ok(())
    }
}

/// A representation of a match state's pattern ID along with the epsilons for
/// when a match occurs.
///
/// A match state in a one-pass DFA, unlike in a more general DFA, has exactly
/// one pattern ID. If it had more, then the original NFA would not have been
/// one-pass.
///
/// The "epsilons" part of this corresponds to what was found in the epsilon
/// transitions between the transition taken in the last byte of input and the
/// ultimate match state. This might include saving slots and/or conditional
/// epsilon transitions that must be satisfied before one can report the match.
///
/// Technically, every state has room for a 'PatternEpsilons', but it is only
/// ever non-empty for match states.
#[derive(Clone, Copy)]
struct PatternEpsilons(u64);

impl PatternEpsilons {
    const PATTERN_ID_BITS: u64 = 22;
    const PATTERN_ID_SHIFT: u64 = 64 - PatternEpsilons::PATTERN_ID_BITS;
    // A sentinel value indicating that this is not a match state. We don't
    // use 0 since 0 is a valid pattern ID.
    const PATTERN_ID_NONE: u64 = 0x00000000_003FFFFF;
    const PATTERN_ID_LIMIT: u64 = PatternEpsilons::PATTERN_ID_NONE;
    const PATTERN_ID_MASK: u64 = 0xFFFFFC00_00000000;
    const EPSILONS_MASK: u64 = 0x000003FF_FFFFFFFF;

    /// Return a new empty pattern epsilons that has no pattern ID and has no
    /// epsilons. This is suitable for non-match states.
    fn empty() -> PatternEpsilons {
        PatternEpsilons(
            PatternEpsilons::PATTERN_ID_NONE
                << PatternEpsilons::PATTERN_ID_SHIFT,
        )
    }

    /// Whether this pattern epsilons is empty or not. It's empty when it has
    /// no pattern ID and an empty epsilons.
    fn is_empty(self) -> bool {
        self.pattern_id().is_none() && self.epsilons().is_empty()
    }

    /// Return the pattern ID in this pattern epsilons if one exists.
    fn pattern_id(self) -> Option<PatternID> {
        let pid = self.0 >> PatternEpsilons::PATTERN_ID_SHIFT;
        if pid == PatternEpsilons::PATTERN_ID_LIMIT {
            None
        } else {
            Some(PatternID::new_unchecked(pid.as_usize()))
        }
    }

    /// Returns the pattern ID without checking whether it's valid. If this is
    /// called and there is no pattern ID in this `PatternEpsilons`, then this
    /// will likely produce an incorrect result or possibly even a panic or
    /// an overflow. But safety will not be violated.
    ///
    /// This is useful when you know a particular state is a match state. If
    /// it's a match state, then it must have a pattern ID.
    fn pattern_id_unchecked(self) -> PatternID {
        let pid = self.0 >> PatternEpsilons::PATTERN_ID_SHIFT;
        PatternID::new_unchecked(pid.as_usize())
    }

    /// Return a new pattern epsilons with the given pattern ID, but the same
    /// epsilons.
    fn set_pattern_id(self, pid: PatternID) -> PatternEpsilons {
        PatternEpsilons(
            (pid.as_u64() << PatternEpsilons::PATTERN_ID_SHIFT)
                | (self.0 & PatternEpsilons::EPSILONS_MASK),
        )
    }

    /// Return the epsilons part of this pattern epsilons.
    fn epsilons(self) -> Epsilons {
        Epsilons(self.0 & PatternEpsilons::EPSILONS_MASK)
    }

    /// Return a new pattern epsilons with the given epsilons, but the same
    /// pattern ID.
    fn set_epsilons(self, epsilons: Epsilons) -> PatternEpsilons {
        PatternEpsilons(
            (self.0 & PatternEpsilons::PATTERN_ID_MASK)
                | (u64::from(epsilons.0) & PatternEpsilons::EPSILONS_MASK),
        )
    }
}

impl core::fmt::Debug for PatternEpsilons {
    fn fmt(&self, f: &mut core::fmt::Formatter) -> core::fmt::Result {
        if self.is_empty() {
            return write!(f, "N/A");
        }
        if let Some(pid) = self.pattern_id() {
            write!(f, "{}", pid.as_usize())?;
        }
        if !self.epsilons().is_empty() {
            if self.pattern_id().is_some() {
                write!(f, "/")?;
            }
            write!(f, "{:?}", self.epsilons())?;
        }
        Ok(())
    }
}

/// Epsilons represents all of the NFA epsilons transitions that went into a
/// single transition in a single DFA state. In this case, it only represents
/// the epsilon transitions that have some kind of non-consuming side effect:
/// either the transition requires storing the current position of the search
/// into a slot, or the transition is conditional and requires the current
/// position in the input to satisfy an assertion before the transition may be
/// taken.
///
/// This folds the cumulative effect of a group of NFA states (all connected
/// by epsilon transitions) down into a single set of bits. While these bits
/// can represent all possible conditional epsilon transitions, it only permits
/// storing up to a somewhat small number of slots.
///
/// Epsilons is represented as a 42-bit integer. For example, it is packed into
/// the lower 42 bits of a `Transition`. (Where the high 22 bits contains a
/// `StateID` and a special "match wins" property.)
#[derive(Clone, Copy)]
struct Epsilons(u64);

impl Epsilons {
    const SLOT_MASK: u64 = 0x000003FF_FFFFFC00;
    const SLOT_SHIFT: u64 = 10;
    const LOOK_MASK: u64 = 0x00000000_000003FF;

    /// Create a new empty epsilons. It has no slots and no assertions that
    /// need to be satisfied.
    fn empty() -> Epsilons {
        Epsilons(0)
    }

    /// Returns true if this epsilons contains no slots and no assertions.
    fn is_empty(self) -> bool {
        self.0 == 0
    }

    /// Returns the slot epsilon transitions.
    fn slots(self) -> Slots {
        Slots((self.0 >> Epsilons::SLOT_SHIFT).low_u32())
    }

    /// Set the slot epsilon transitions.
    fn set_slots(self, slots: Slots) -> Epsilons {
        Epsilons(
            (u64::from(slots.0) << Epsilons::SLOT_SHIFT)
                | (self.0 & Epsilons::LOOK_MASK),
        )
    }

    /// Return the set of look-around assertions in these epsilon transitions.
    fn looks(self) -> LookSet {
        LookSet { bits: (self.0 & Epsilons::LOOK_MASK).low_u32() }
    }

    /// Set the look-around assertions on these epsilon transitions.
    fn set_looks(self, look_set: LookSet) -> Epsilons {
        Epsilons(
            (self.0 & Epsilons::SLOT_MASK)
                | (u64::from(look_set.bits) & Epsilons::LOOK_MASK),
        )
    }
}

impl core::fmt::Debug for Epsilons {
    fn fmt(&self, f: &mut core::fmt::Formatter) -> core::fmt::Result {
        let mut wrote = false;
        if !self.slots().is_empty() {
            write!(f, "{:?}", self.slots())?;
            wrote = true;
        }
        if !self.looks().is_empty() {
            if wrote {
                write!(f, "/")?;
            }
            write!(f, "{:?}", self.looks())?;
            wrote = true;
        }
        if !wrote {
            write!(f, "N/A")?;
        }
        Ok(())
    }
}

/// The set of epsilon transitions indicating that the current position in a
/// search should be saved to a slot.
///
/// This *only* represents explicit slots. So for example, the pattern
/// `[a-z]+([0-9]+)([a-z]+)` has:
///
/// * 3 capturing groups, thus 6 slots.
/// * 1 implicit capturing group, thus 2 implicit slots.
/// * 2 explicit capturing groups, thus 4 explicit slots.
///
/// While implicit slots are represented by epsilon transitions in an NFA, we
/// do not explicitly represent them here. Instead, implicit slots are assumed
/// to be present and handled automatically in the search code. Therefore,
/// that means we only need to represent explicit slots in our epsilon
/// transitions.
///
/// Its representation is a bit set. The bit 'i' is set if and only if there
/// exists an explicit slot at index 'c', where 'c = (#patterns * 2) + i'. That
/// is, the bit 'i' corresponds to the first explicit slot and the first
/// explicit slot appears immediately following the last implicit slot. (If
/// this is confusing, see `GroupInfo` for more details on how slots works.)
///
/// A single `Slots` represents all the active slots in a sub-graph of an NFA,
/// where all the states are connected by epsilon transitions. In effect, when
/// traversing the one-pass DFA during a search, all slots set in a particular
/// transition must be captured by recording the current search position.
///
/// The API of `Slots` requires the caller to handle the explicit slot offset.
/// That is, a `Slots` doesn't know where the explicit slots start for a
/// particular NFA. Thus, if the callers see's the bit 'i' is set, then they
/// need to do the arithmetic above to find 'c', which is the real actual slot
/// index in the corresponding NFA.
#[derive(Clone, Copy)]
struct Slots(u32);

impl Slots {
    const LIMIT: usize = 32;

    /// Insert the slot at the given bit index.
    fn insert(self, slot: usize) -> Slots {
        debug_assert!(slot < Slots::LIMIT);
        Slots(self.0 | (1 << slot.as_u32()))
    }

    /// Remove the slot at the given bit index.
    fn remove(self, slot: usize) -> Slots {
        debug_assert!(slot < Slots::LIMIT);
        Slots(self.0 & !(1 << slot.as_u32()))
    }

    /// Returns true if and only if this set contains no slots.
    fn is_empty(self) -> bool {
        self.0 == 0
    }

    /// Returns an iterator over all of the set bits in this set.
    fn iter(self) -> SlotsIter {
        SlotsIter { slots: self }
    }

    /// For the position `at` in the current haystack, copy it to
    /// `caller_explicit_slots` for all slots that are in this set.
    ///
    /// Callers may pass a slice of any length. Slots in this set bigger than
    /// the length of the given explicit slots are simply skipped.
    ///
    /// The slice *must* correspond only to the explicit slots and the first
    /// element of the slice must always correspond to the first explicit slot
    /// in the corresponding NFA.
    fn apply(
        self,
        at: usize,
        caller_explicit_slots: &mut [Option<NonMaxUsize>],
    ) {
        if self.is_empty() {
            return;
        }
        let at = NonMaxUsize::new(at);
        for slot in self.iter() {
            if slot >= caller_explicit_slots.len() {
                break;
            }
            caller_explicit_slots[slot] = at;
        }
    }
}

impl core::fmt::Debug for Slots {
    fn fmt(&self, f: &mut core::fmt::Formatter) -> core::fmt::Result {
        write!(f, "S")?;
        for slot in self.iter() {
            write!(f, "-{:?}", slot)?;
        }
        Ok(())
    }
}

/// An iterator over all of the bits set in a slot set.
///
/// This returns the bit index that is set, so callers may need to offset it
/// to get the actual NFA slot index.
#[derive(Debug)]
struct SlotsIter {
    slots: Slots,
}

impl Iterator for SlotsIter {
    type Item = usize;

    fn next(&mut self) -> Option<usize> {
        // Number of zeroes here is always <= u8::MAX, and so fits in a usize.
        let slot = self.slots.0.trailing_zeros().as_usize();
        if slot >= Slots::LIMIT {
            return None;
        }
        self.slots = self.slots.remove(slot);
        Some(slot)
    }
}

/// An error that occurred during the construction of a one-pass DFA.
///
/// This error does not provide many introspection capabilities. There are
/// generally only two things you can do with it:
///
/// * Obtain a human readable message via its `std::fmt::Display` impl.
/// * Access an underlying [`thompson::BuildError`] type from its `source`
/// method via the `std::error::Error` trait. This error only occurs when using
/// convenience routines for building a one-pass DFA directly from a pattern
/// string.
///
/// When the `std` feature is enabled, this implements the `std::error::Error`
/// trait.
#[derive(Clone, Debug)]
pub struct BuildError {
    kind: BuildErrorKind,
}

/// The kind of error that occurred during the construction of a one-pass DFA.
#[derive(Clone, Debug)]
enum BuildErrorKind {
    NFA(crate::nfa::thompson::BuildError),
    Word(UnicodeWordBoundaryError),
    TooManyStates { limit: u64 },
    TooManyPatterns { limit: u64 },
    UnsupportedLook { look: Look },
    ExceededSizeLimit { limit: usize },
    NotOnePass { msg: &'static str },
}

impl BuildError {
    fn nfa(err: crate::nfa::thompson::BuildError) -> BuildError {
        BuildError { kind: BuildErrorKind::NFA(err) }
    }

    fn word(err: UnicodeWordBoundaryError) -> BuildError {
        BuildError { kind: BuildErrorKind::Word(err) }
    }

    fn too_many_states(limit: u64) -> BuildError {
        BuildError { kind: BuildErrorKind::TooManyStates { limit } }
    }

    fn too_many_patterns(limit: u64) -> BuildError {
        BuildError { kind: BuildErrorKind::TooManyPatterns { limit } }
    }

    fn unsupported_look(look: Look) -> BuildError {
        BuildError { kind: BuildErrorKind::UnsupportedLook { look } }
    }

    fn exceeded_size_limit(limit: usize) -> BuildError {
        BuildError { kind: BuildErrorKind::ExceededSizeLimit { limit } }
    }

    fn not_one_pass(msg: &'static str) -> BuildError {
        BuildError { kind: BuildErrorKind::NotOnePass { msg } }
    }
}

#[cfg(feature = "std")]
impl std::error::Error for BuildError {
    fn source(&self) -> Option<&(dyn std::error::Error + 'static)> {
        use self::BuildErrorKind::*;

        match self.kind {
            NFA(ref err) => Some(err),
            Word(ref err) => Some(err),
            _ => None,
        }
    }
}

impl core::fmt::Display for BuildError {
    fn fmt(&self, f: &mut core::fmt::Formatter<'_>) -> core::fmt::Result {
        use self::BuildErrorKind::*;

        match self.kind {
            NFA(_) => write!(f, "error building NFA"),
            Word(_) => write!(f, "NFA contains Unicode word boundary"),
            TooManyStates { limit } => write!(
                f,
                "one-pass DFA exceeded a limit of {:?} for number of states",
                limit,
            ),
            TooManyPatterns { limit } => write!(
                f,
                "one-pass DFA exceeded a limit of {:?} for number of patterns",
                limit,
            ),
            UnsupportedLook { look } => write!(
                f,
                "one-pass DFA does not support the {:?} assertion",
                look,
            ),
            ExceededSizeLimit { limit } => write!(
                f,
                "one-pass DFA exceeded size limit of {:?} during building",
                limit,
            ),
            NotOnePass { msg } => write!(
                f,
                "one-pass DFA could not be built because \
                 pattern is not one-pass: {}",
                msg,
            ),
        }
    }
}

#[cfg(all(test, feature = "syntax"))]
mod tests {
    use alloc::string::ToString;

    use super::*;

    #[test]
    fn fail_conflicting_transition() {
        let predicate = |err: &str| err.contains("conflicting transition");

        let err = DFA::new(r"a*[ab]").unwrap_err().to_string();
        assert!(predicate(&err), "{}", err);
    }

    #[test]
    fn fail_multiple_epsilon() {
        let predicate = |err: &str| {
            err.contains("multiple epsilon transitions to same state")
        };

        let err = DFA::new(r"(^|$)a").unwrap_err().to_string();
        assert!(predicate(&err), "{}", err);
    }

    #[test]
    fn fail_multiple_match() {
        let predicate = |err: &str| {
            err.contains("multiple epsilon transitions to match state")
        };

        let err = DFA::new_many(&[r"^", r"$"]).unwrap_err().to_string();
        assert!(predicate(&err), "{}", err);
    }

    // This test is meant to build a one-pass regex with the maximum number of
    // possible slots.
    //
    // NOTE: Remember that the slot limit only applies to explicit capturing
    // groups. Any number of implicit capturing groups is supported (up to the
    // maximum number of supported patterns), since implicit groups are handled
    // by the search loop itself.
    #[test]
    fn max_slots() {
        // One too many...
        let pat = r"(a)(b)(c)(d)(e)(f)(g)(h)(i)(j)(k)(l)(m)(n)(o)(p)(q)";
        assert!(DFA::new(pat).is_err());
        // Just right.
        let pat = r"(a)(b)(c)(d)(e)(f)(g)(h)(i)(j)(k)(l)(m)(n)(o)(p)";
        assert!(DFA::new(pat).is_ok());
    }

    // This test ensures that the one-pass DFA works with all look-around
    // assertions that we expect it to work with.
    //
    // The utility of this test is that each one-pass transition has a small
    // amount of space to store look-around assertions. Currently, there is
    // logic in the one-pass constructor to ensure there aren't more than ten
    // possible assertions. And indeed, there are only ten possible assertions
    // (at time of writing), so this is okay. But conceivably, more assertions
    // could be added. So we check that things at least work with what we
    // expect them to work with.
    #[test]
    fn assertions() {
        // haystack anchors
        assert!(DFA::new(r"^").is_ok());
        assert!(DFA::new(r"$").is_ok());

        // line anchors
        assert!(DFA::new(r"(?m)^").is_ok());
        assert!(DFA::new(r"(?m)$").is_ok());
        assert!(DFA::new(r"(?Rm)^").is_ok());
        assert!(DFA::new(r"(?Rm)$").is_ok());

        // word boundaries
        if cfg!(feature = "unicode-word-boundary") {
            assert!(DFA::new(r"\b").is_ok());
            assert!(DFA::new(r"\B").is_ok());
        }
        assert!(DFA::new(r"(?-u)\b").is_ok());
        assert!(DFA::new(r"(?-u)\B").is_ok());
    }

    #[cfg(not(miri))] // takes too long on miri
    #[test]
    fn is_one_pass() {
        use crate::util::syntax;

        assert!(DFA::new(r"a*b").is_ok());
        if cfg!(feature = "unicode-perl") {
            assert!(DFA::new(r"\w").is_ok());
        }
        assert!(DFA::new(r"(?-u)\w*\s").is_ok());
        assert!(DFA::new(r"(?s:.)*?").is_ok());
        assert!(DFA::builder()
            .syntax(syntax::Config::new().utf8(false))
            .build(r"(?s-u:.)*?")
            .is_ok());
    }

    #[test]
    fn is_not_one_pass() {
        assert!(DFA::new(r"a*a").is_err());
        assert!(DFA::new(r"(?s-u:.)*?").is_err());
        assert!(DFA::new(r"(?s:.)*?a").is_err());
    }

    #[cfg(not(miri))]
    #[test]
    fn is_not_one_pass_bigger() {
        assert!(DFA::new(r"\w*\s").is_err());
    }
}