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
use std::collections::HashMap;
use std::fmt;
use std::iter;
use std::result;
use std::sync::Arc;

use regex_syntax::hir::{self, Hir};
use regex_syntax::is_word_byte;
use regex_syntax::utf8::{Utf8Range, Utf8Sequence, Utf8Sequences};

use crate::prog::{
    EmptyLook, Inst, InstBytes, InstChar, InstEmptyLook, InstPtr, InstRanges,
    InstSave, InstSplit, Program,
};

use crate::Error;

type Result = result::Result<Patch, Error>;
type ResultOrEmpty = result::Result<Option<Patch>, Error>;

#[derive(Debug)]
struct Patch {
    hole: Hole,
    entry: InstPtr,
}

/// A compiler translates a regular expression AST to a sequence of
/// instructions. The sequence of instructions represents an NFA.
// `Compiler` is only public via the `internal` module, so avoid deriving
// `Debug`.
#[allow(missing_debug_implementations)]
pub struct Compiler {
    insts: Vec<MaybeInst>,
    compiled: Program,
    capture_name_idx: HashMap<String, usize>,
    num_exprs: usize,
    size_limit: usize,
    suffix_cache: SuffixCache,
    utf8_seqs: Option<Utf8Sequences>,
    byte_classes: ByteClassSet,
    // This keeps track of extra bytes allocated while compiling the regex
    // program. Currently, this corresponds to two things. First is the heap
    // memory allocated by Unicode character classes ('InstRanges'). Second is
    // a "fake" amount of memory used by empty sub-expressions, so that enough
    // empty sub-expressions will ultimately trigger the compiler to bail
    // because of a size limit restriction. (That empty sub-expressions don't
    // add to heap memory usage is more-or-less an implementation detail.) In
    // the second case, if we don't bail, then an excessively large repetition
    // on an empty sub-expression can result in the compiler using a very large
    // amount of CPU time.
    extra_inst_bytes: usize,
}

impl Compiler {
    /// Create a new regular expression compiler.
    ///
    /// Various options can be set before calling `compile` on an expression.
    pub fn new() -> Self {
        Compiler {
            insts: vec![],
            compiled: Program::new(),
            capture_name_idx: HashMap::new(),
            num_exprs: 0,
            size_limit: 10 * (1 << 20),
            suffix_cache: SuffixCache::new(1000),
            utf8_seqs: Some(Utf8Sequences::new('\x00', '\x00')),
            byte_classes: ByteClassSet::new(),
            extra_inst_bytes: 0,
        }
    }

    /// The size of the resulting program is limited by size_limit. If
    /// the program approximately exceeds the given size (in bytes), then
    /// compilation will stop and return an error.
    pub fn size_limit(mut self, size_limit: usize) -> Self {
        self.size_limit = size_limit;
        self
    }

    /// If bytes is true, then the program is compiled as a byte based
    /// automaton, which incorporates UTF-8 decoding into the machine. If it's
    /// false, then the automaton is Unicode scalar value based, e.g., an
    /// engine utilizing such an automaton is responsible for UTF-8 decoding.
    ///
    /// The specific invariant is that when returning a byte based machine,
    /// the neither the `Char` nor `Ranges` instructions are produced.
    /// Conversely, when producing a Unicode scalar value machine, the `Bytes`
    /// instruction is never produced.
    ///
    /// Note that `dfa(true)` implies `bytes(true)`.
    pub fn bytes(mut self, yes: bool) -> Self {
        self.compiled.is_bytes = yes;
        self
    }

    /// When disabled, the program compiled may match arbitrary bytes.
    ///
    /// When enabled (the default), all compiled programs exclusively match
    /// valid UTF-8 bytes.
    pub fn only_utf8(mut self, yes: bool) -> Self {
        self.compiled.only_utf8 = yes;
        self
    }

    /// When set, the machine returned is suitable for use in the DFA matching
    /// engine.
    ///
    /// In particular, this ensures that if the regex is not anchored in the
    /// beginning, then a preceding `.*?` is included in the program. (The NFA
    /// based engines handle the preceding `.*?` explicitly, which is difficult
    /// or impossible in the DFA engine.)
    pub fn dfa(mut self, yes: bool) -> Self {
        self.compiled.is_dfa = yes;
        self
    }

    /// When set, the machine returned is suitable for matching text in
    /// reverse. In particular, all concatenations are flipped.
    pub fn reverse(mut self, yes: bool) -> Self {
        self.compiled.is_reverse = yes;
        self
    }

    /// Compile a regular expression given its AST.
    ///
    /// The compiler is guaranteed to succeed unless the program exceeds the
    /// specified size limit. If the size limit is exceeded, then compilation
    /// stops and returns an error.
    pub fn compile(mut self, exprs: &[Hir]) -> result::Result<Program, Error> {
        debug_assert!(!exprs.is_empty());
        self.num_exprs = exprs.len();
        if exprs.len() == 1 {
            self.compile_one(&exprs[0])
        } else {
            self.compile_many(exprs)
        }
    }

    fn compile_one(mut self, expr: &Hir) -> result::Result<Program, Error> {
        // If we're compiling a forward DFA and we aren't anchored, then
        // add a `.*?` before the first capture group.
        // Other matching engines handle this by baking the logic into the
        // matching engine itself.
        let mut dotstar_patch = Patch { hole: Hole::None, entry: 0 };
        self.compiled.is_anchored_start = expr.is_anchored_start();
        self.compiled.is_anchored_end = expr.is_anchored_end();
        if self.compiled.needs_dotstar() {
            dotstar_patch = self.c_dotstar()?;
            self.compiled.start = dotstar_patch.entry;
        }
        self.compiled.captures = vec![None];
        let patch =
            self.c_capture(0, expr)?.unwrap_or_else(|| self.next_inst());
        if self.compiled.needs_dotstar() {
            self.fill(dotstar_patch.hole, patch.entry);
        } else {
            self.compiled.start = patch.entry;
        }
        self.fill_to_next(patch.hole);
        self.compiled.matches = vec![self.insts.len()];
        self.push_compiled(Inst::Match(0));
        self.compile_finish()
    }

    fn compile_many(
        mut self,
        exprs: &[Hir],
    ) -> result::Result<Program, Error> {
        debug_assert!(exprs.len() > 1);

        self.compiled.is_anchored_start =
            exprs.iter().all(|e| e.is_anchored_start());
        self.compiled.is_anchored_end =
            exprs.iter().all(|e| e.is_anchored_end());
        let mut dotstar_patch = Patch { hole: Hole::None, entry: 0 };
        if self.compiled.needs_dotstar() {
            dotstar_patch = self.c_dotstar()?;
            self.compiled.start = dotstar_patch.entry;
        } else {
            self.compiled.start = 0; // first instruction is always split
        }
        self.fill_to_next(dotstar_patch.hole);

        let mut prev_hole = Hole::None;
        for (i, expr) in exprs[0..exprs.len() - 1].iter().enumerate() {
            self.fill_to_next(prev_hole);
            let split = self.push_split_hole();
            let Patch { hole, entry } =
                self.c_capture(0, expr)?.unwrap_or_else(|| self.next_inst());
            self.fill_to_next(hole);
            self.compiled.matches.push(self.insts.len());
            self.push_compiled(Inst::Match(i));
            prev_hole = self.fill_split(split, Some(entry), None);
        }
        let i = exprs.len() - 1;
        let Patch { hole, entry } =
            self.c_capture(0, &exprs[i])?.unwrap_or_else(|| self.next_inst());
        self.fill(prev_hole, entry);
        self.fill_to_next(hole);
        self.compiled.matches.push(self.insts.len());
        self.push_compiled(Inst::Match(i));
        self.compile_finish()
    }

    fn compile_finish(mut self) -> result::Result<Program, Error> {
        self.compiled.insts =
            self.insts.into_iter().map(|inst| inst.unwrap()).collect();
        self.compiled.byte_classes = self.byte_classes.byte_classes();
        self.compiled.capture_name_idx = Arc::new(self.capture_name_idx);
        Ok(self.compiled)
    }

    /// Compile expr into self.insts, returning a patch on success,
    /// or an error if we run out of memory.
    ///
    /// All of the c_* methods of the compiler share the contract outlined
    /// here.
    ///
    /// The main thing that a c_* method does is mutate `self.insts`
    /// to add a list of mostly compiled instructions required to execute
    /// the given expression. `self.insts` contains MaybeInsts rather than
    /// Insts because there is some backpatching required.
    ///
    /// The `Patch` value returned by each c_* method provides metadata
    /// about the compiled instructions emitted to `self.insts`. The
    /// `entry` member of the patch refers to the first instruction
    /// (the entry point), while the `hole` member contains zero or
    /// more offsets to partial instructions that need to be backpatched.
    /// The c_* routine can't know where its list of instructions are going to
    /// jump to after execution, so it is up to the caller to patch
    /// these jumps to point to the right place. So compiling some
    /// expression, e, we would end up with a situation that looked like:
    ///
    /// ```text
    /// self.insts = [ ..., i1, i2, ..., iexit1, ..., iexitn, ...]
    ///                     ^              ^             ^
    ///                     |                \         /
    ///                   entry                \     /
    ///                                         hole
    /// ```
    ///
    /// To compile two expressions, e1 and e2, concatenated together we
    /// would do:
    ///
    /// ```ignore
    /// let patch1 = self.c(e1);
    /// let patch2 = self.c(e2);
    /// ```
    ///
    /// while leaves us with a situation that looks like
    ///
    /// ```text
    /// self.insts = [ ..., i1, ..., iexit1, ..., i2, ..., iexit2 ]
    ///                     ^        ^            ^        ^
    ///                     |        |            |        |
    ///                entry1        hole1   entry2        hole2
    /// ```
    ///
    /// Then to merge the two patches together into one we would backpatch
    /// hole1 with entry2 and return a new patch that enters at entry1
    /// and has hole2 for a hole. In fact, if you look at the c_concat
    /// method you will see that it does exactly this, though it handles
    /// a list of expressions rather than just the two that we use for
    /// an example.
    ///
    /// Ok(None) is returned when an expression is compiled to no
    /// instruction, and so no patch.entry value makes sense.
    fn c(&mut self, expr: &Hir) -> ResultOrEmpty {
        use crate::prog;
        use regex_syntax::hir::HirKind::*;

        self.check_size()?;
        match *expr.kind() {
            Empty => self.c_empty(),
            Literal(hir::Literal::Unicode(c)) => self.c_char(c),
            Literal(hir::Literal::Byte(b)) => {
                assert!(self.compiled.uses_bytes());
                self.c_byte(b)
            }
            Class(hir::Class::Unicode(ref cls)) => self.c_class(cls.ranges()),
            Class(hir::Class::Bytes(ref cls)) => {
                if self.compiled.uses_bytes() {
                    self.c_class_bytes(cls.ranges())
                } else {
                    assert!(cls.is_all_ascii());
                    let mut char_ranges = vec![];
                    for r in cls.iter() {
                        let (s, e) = (r.start() as char, r.end() as char);
                        char_ranges.push(hir::ClassUnicodeRange::new(s, e));
                    }
                    self.c_class(&char_ranges)
                }
            }
            Anchor(hir::Anchor::StartLine) if self.compiled.is_reverse => {
                self.byte_classes.set_range(b'\n', b'\n');
                self.c_empty_look(prog::EmptyLook::EndLine)
            }
            Anchor(hir::Anchor::StartLine) => {
                self.byte_classes.set_range(b'\n', b'\n');
                self.c_empty_look(prog::EmptyLook::StartLine)
            }
            Anchor(hir::Anchor::EndLine) if self.compiled.is_reverse => {
                self.byte_classes.set_range(b'\n', b'\n');
                self.c_empty_look(prog::EmptyLook::StartLine)
            }
            Anchor(hir::Anchor::EndLine) => {
                self.byte_classes.set_range(b'\n', b'\n');
                self.c_empty_look(prog::EmptyLook::EndLine)
            }
            Anchor(hir::Anchor::StartText) if self.compiled.is_reverse => {
                self.c_empty_look(prog::EmptyLook::EndText)
            }
            Anchor(hir::Anchor::StartText) => {
                self.c_empty_look(prog::EmptyLook::StartText)
            }
            Anchor(hir::Anchor::EndText) if self.compiled.is_reverse => {
                self.c_empty_look(prog::EmptyLook::StartText)
            }
            Anchor(hir::Anchor::EndText) => {
                self.c_empty_look(prog::EmptyLook::EndText)
            }
            WordBoundary(hir::WordBoundary::Unicode) => {
                if !cfg!(feature = "unicode-perl") {
                    return Err(Error::Syntax(
                        "Unicode word boundaries are unavailable when \
                         the unicode-perl feature is disabled"
                            .to_string(),
                    ));
                }
                self.compiled.has_unicode_word_boundary = true;
                self.byte_classes.set_word_boundary();
                // We also make sure that all ASCII bytes are in a different
                // class from non-ASCII bytes. Otherwise, it's possible for
                // ASCII bytes to get lumped into the same class as non-ASCII
                // bytes. This in turn may cause the lazy DFA to falsely start
                // when it sees an ASCII byte that maps to a byte class with
                // non-ASCII bytes. This ensures that never happens.
                self.byte_classes.set_range(0, 0x7F);
                self.c_empty_look(prog::EmptyLook::WordBoundary)
            }
            WordBoundary(hir::WordBoundary::UnicodeNegate) => {
                if !cfg!(feature = "unicode-perl") {
                    return Err(Error::Syntax(
                        "Unicode word boundaries are unavailable when \
                         the unicode-perl feature is disabled"
                            .to_string(),
                    ));
                }
                self.compiled.has_unicode_word_boundary = true;
                self.byte_classes.set_word_boundary();
                // See comments above for why we set the ASCII range here.
                self.byte_classes.set_range(0, 0x7F);
                self.c_empty_look(prog::EmptyLook::NotWordBoundary)
            }
            WordBoundary(hir::WordBoundary::Ascii) => {
                self.byte_classes.set_word_boundary();
                self.c_empty_look(prog::EmptyLook::WordBoundaryAscii)
            }
            WordBoundary(hir::WordBoundary::AsciiNegate) => {
                self.byte_classes.set_word_boundary();
                self.c_empty_look(prog::EmptyLook::NotWordBoundaryAscii)
            }
            Group(ref g) => match g.kind {
                hir::GroupKind::NonCapturing => self.c(&g.hir),
                hir::GroupKind::CaptureIndex(index) => {
                    if index as usize >= self.compiled.captures.len() {
                        self.compiled.captures.push(None);
                    }
                    self.c_capture(2 * index as usize, &g.hir)
                }
                hir::GroupKind::CaptureName { index, ref name } => {
                    if index as usize >= self.compiled.captures.len() {
                        let n = name.to_string();
                        self.compiled.captures.push(Some(n.clone()));
                        self.capture_name_idx.insert(n, index as usize);
                    }
                    self.c_capture(2 * index as usize, &g.hir)
                }
            },
            Concat(ref es) => {
                if self.compiled.is_reverse {
                    self.c_concat(es.iter().rev())
                } else {
                    self.c_concat(es)
                }
            }
            Alternation(ref es) => self.c_alternate(&**es),
            Repetition(ref rep) => self.c_repeat(rep),
        }
    }

    fn c_empty(&mut self) -> ResultOrEmpty {
        // See: https://github.com/rust-lang/regex/security/advisories/GHSA-m5pq-gvj9-9vr8
        // See: CVE-2022-24713
        //
        // Since 'empty' sub-expressions don't increase the size of
        // the actual compiled object, we "fake" an increase in its
        // size so that our 'check_size_limit' routine will eventually
        // stop compilation if there are too many empty sub-expressions
        // (e.g., via a large repetition).
        self.extra_inst_bytes += std::mem::size_of::<Inst>();
        Ok(None)
    }

    fn c_capture(&mut self, first_slot: usize, expr: &Hir) -> ResultOrEmpty {
        if self.num_exprs > 1 || self.compiled.is_dfa {
            // Don't ever compile Save instructions for regex sets because
            // they are never used. They are also never used in DFA programs
            // because DFAs can't handle captures.
            self.c(expr)
        } else {
            let entry = self.insts.len();
            let hole = self.push_hole(InstHole::Save { slot: first_slot });
            let patch = self.c(expr)?.unwrap_or_else(|| self.next_inst());
            self.fill(hole, patch.entry);
            self.fill_to_next(patch.hole);
            let hole = self.push_hole(InstHole::Save { slot: first_slot + 1 });
            Ok(Some(Patch { hole, entry }))
        }
    }

    fn c_dotstar(&mut self) -> Result {
        Ok(if !self.compiled.only_utf8() {
            self.c(&Hir::repetition(hir::Repetition {
                kind: hir::RepetitionKind::ZeroOrMore,
                greedy: false,
                hir: Box::new(Hir::any(true)),
            }))?
            .unwrap()
        } else {
            self.c(&Hir::repetition(hir::Repetition {
                kind: hir::RepetitionKind::ZeroOrMore,
                greedy: false,
                hir: Box::new(Hir::any(false)),
            }))?
            .unwrap()
        })
    }

    fn c_char(&mut self, c: char) -> ResultOrEmpty {
        if self.compiled.uses_bytes() {
            if c.is_ascii() {
                let b = c as u8;
                let hole =
                    self.push_hole(InstHole::Bytes { start: b, end: b });
                self.byte_classes.set_range(b, b);
                Ok(Some(Patch { hole, entry: self.insts.len() - 1 }))
            } else {
                self.c_class(&[hir::ClassUnicodeRange::new(c, c)])
            }
        } else {
            let hole = self.push_hole(InstHole::Char { c });
            Ok(Some(Patch { hole, entry: self.insts.len() - 1 }))
        }
    }

    fn c_class(&mut self, ranges: &[hir::ClassUnicodeRange]) -> ResultOrEmpty {
        use std::mem::size_of;

        assert!(!ranges.is_empty());
        if self.compiled.uses_bytes() {
            Ok(Some(CompileClass { c: self, ranges }.compile()?))
        } else {
            let ranges: Vec<(char, char)> =
                ranges.iter().map(|r| (r.start(), r.end())).collect();
            let hole = if ranges.len() == 1 && ranges[0].0 == ranges[0].1 {
                self.push_hole(InstHole::Char { c: ranges[0].0 })
            } else {
                self.extra_inst_bytes +=
                    ranges.len() * (size_of::<char>() * 2);
                self.push_hole(InstHole::Ranges { ranges })
            };
            Ok(Some(Patch { hole, entry: self.insts.len() - 1 }))
        }
    }

    fn c_byte(&mut self, b: u8) -> ResultOrEmpty {
        self.c_class_bytes(&[hir::ClassBytesRange::new(b, b)])
    }

    fn c_class_bytes(
        &mut self,
        ranges: &[hir::ClassBytesRange],
    ) -> ResultOrEmpty {
        debug_assert!(!ranges.is_empty());

        let first_split_entry = self.insts.len();
        let mut holes = vec![];
        let mut prev_hole = Hole::None;
        for r in &ranges[0..ranges.len() - 1] {
            self.fill_to_next(prev_hole);
            let split = self.push_split_hole();
            let next = self.insts.len();
            self.byte_classes.set_range(r.start(), r.end());
            holes.push(self.push_hole(InstHole::Bytes {
                start: r.start(),
                end: r.end(),
            }));
            prev_hole = self.fill_split(split, Some(next), None);
        }
        let next = self.insts.len();
        let r = &ranges[ranges.len() - 1];
        self.byte_classes.set_range(r.start(), r.end());
        holes.push(
            self.push_hole(InstHole::Bytes { start: r.start(), end: r.end() }),
        );
        self.fill(prev_hole, next);
        Ok(Some(Patch { hole: Hole::Many(holes), entry: first_split_entry }))
    }

    fn c_empty_look(&mut self, look: EmptyLook) -> ResultOrEmpty {
        let hole = self.push_hole(InstHole::EmptyLook { look });
        Ok(Some(Patch { hole, entry: self.insts.len() - 1 }))
    }

    fn c_concat<'a, I>(&mut self, exprs: I) -> ResultOrEmpty
    where
        I: IntoIterator<Item = &'a Hir>,
    {
        let mut exprs = exprs.into_iter();
        let Patch { mut hole, entry } = loop {
            match exprs.next() {
                None => return self.c_empty(),
                Some(e) => {
                    if let Some(p) = self.c(e)? {
                        break p;
                    }
                }
            }
        };
        for e in exprs {
            if let Some(p) = self.c(e)? {
                self.fill(hole, p.entry);
                hole = p.hole;
            }
        }
        Ok(Some(Patch { hole, entry }))
    }

    fn c_alternate(&mut self, exprs: &[Hir]) -> ResultOrEmpty {
        debug_assert!(
            exprs.len() >= 2,
            "alternates must have at least 2 exprs"
        );

        // Initial entry point is always the first split.
        let first_split_entry = self.insts.len();

        // Save up all of the holes from each alternate. They will all get
        // patched to point to the same location.
        let mut holes = vec![];

        // true indicates that the hole is a split where we want to fill
        // the second branch.
        let mut prev_hole = (Hole::None, false);
        for e in &exprs[0..exprs.len() - 1] {
            if prev_hole.1 {
                let next = self.insts.len();
                self.fill_split(prev_hole.0, None, Some(next));
            } else {
                self.fill_to_next(prev_hole.0);
            }
            let split = self.push_split_hole();
            if let Some(Patch { hole, entry }) = self.c(e)? {
                holes.push(hole);
                prev_hole = (self.fill_split(split, Some(entry), None), false);
            } else {
                let (split1, split2) = split.dup_one();
                holes.push(split1);
                prev_hole = (split2, true);
            }
        }
        if let Some(Patch { hole, entry }) = self.c(&exprs[exprs.len() - 1])? {
            holes.push(hole);
            if prev_hole.1 {
                self.fill_split(prev_hole.0, None, Some(entry));
            } else {
                self.fill(prev_hole.0, entry);
            }
        } else {
            // We ignore prev_hole.1. When it's true, it means we have two
            // empty branches both pushing prev_hole.0 into holes, so both
            // branches will go to the same place anyway.
            holes.push(prev_hole.0);
        }
        Ok(Some(Patch { hole: Hole::Many(holes), entry: first_split_entry }))
    }

    fn c_repeat(&mut self, rep: &hir::Repetition) -> ResultOrEmpty {
        use regex_syntax::hir::RepetitionKind::*;
        match rep.kind {
            ZeroOrOne => self.c_repeat_zero_or_one(&rep.hir, rep.greedy),
            ZeroOrMore => self.c_repeat_zero_or_more(&rep.hir, rep.greedy),
            OneOrMore => self.c_repeat_one_or_more(&rep.hir, rep.greedy),
            Range(hir::RepetitionRange::Exactly(min_max)) => {
                self.c_repeat_range(&rep.hir, rep.greedy, min_max, min_max)
            }
            Range(hir::RepetitionRange::AtLeast(min)) => {
                self.c_repeat_range_min_or_more(&rep.hir, rep.greedy, min)
            }
            Range(hir::RepetitionRange::Bounded(min, max)) => {
                self.c_repeat_range(&rep.hir, rep.greedy, min, max)
            }
        }
    }

    fn c_repeat_zero_or_one(
        &mut self,
        expr: &Hir,
        greedy: bool,
    ) -> ResultOrEmpty {
        let split_entry = self.insts.len();
        let split = self.push_split_hole();
        let Patch { hole: hole_rep, entry: entry_rep } = match self.c(expr)? {
            Some(p) => p,
            None => return self.pop_split_hole(),
        };
        let split_hole = if greedy {
            self.fill_split(split, Some(entry_rep), None)
        } else {
            self.fill_split(split, None, Some(entry_rep))
        };
        let holes = vec![hole_rep, split_hole];
        Ok(Some(Patch { hole: Hole::Many(holes), entry: split_entry }))
    }

    fn c_repeat_zero_or_more(
        &mut self,
        expr: &Hir,
        greedy: bool,
    ) -> ResultOrEmpty {
        let split_entry = self.insts.len();
        let split = self.push_split_hole();
        let Patch { hole: hole_rep, entry: entry_rep } = match self.c(expr)? {
            Some(p) => p,
            None => return self.pop_split_hole(),
        };

        self.fill(hole_rep, split_entry);
        let split_hole = if greedy {
            self.fill_split(split, Some(entry_rep), None)
        } else {
            self.fill_split(split, None, Some(entry_rep))
        };
        Ok(Some(Patch { hole: split_hole, entry: split_entry }))
    }

    fn c_repeat_one_or_more(
        &mut self,
        expr: &Hir,
        greedy: bool,
    ) -> ResultOrEmpty {
        let Patch { hole: hole_rep, entry: entry_rep } = match self.c(expr)? {
            Some(p) => p,
            None => return Ok(None),
        };
        self.fill_to_next(hole_rep);
        let split = self.push_split_hole();

        let split_hole = if greedy {
            self.fill_split(split, Some(entry_rep), None)
        } else {
            self.fill_split(split, None, Some(entry_rep))
        };
        Ok(Some(Patch { hole: split_hole, entry: entry_rep }))
    }

    fn c_repeat_range_min_or_more(
        &mut self,
        expr: &Hir,
        greedy: bool,
        min: u32,
    ) -> ResultOrEmpty {
        let min = u32_to_usize(min);
        // Using next_inst() is ok, because we can't return it (concat would
        // have to return Some(_) while c_repeat_range_min_or_more returns
        // None).
        let patch_concat = self
            .c_concat(iter::repeat(expr).take(min))?
            .unwrap_or_else(|| self.next_inst());
        if let Some(patch_rep) = self.c_repeat_zero_or_more(expr, greedy)? {
            self.fill(patch_concat.hole, patch_rep.entry);
            Ok(Some(Patch { hole: patch_rep.hole, entry: patch_concat.entry }))
        } else {
            Ok(None)
        }
    }

    fn c_repeat_range(
        &mut self,
        expr: &Hir,
        greedy: bool,
        min: u32,
        max: u32,
    ) -> ResultOrEmpty {
        let (min, max) = (u32_to_usize(min), u32_to_usize(max));
        debug_assert!(min <= max);
        let patch_concat = self.c_concat(iter::repeat(expr).take(min))?;
        if min == max {
            return Ok(patch_concat);
        }
        // Same reasoning as in c_repeat_range_min_or_more (we know that min <
        // max at this point).
        let patch_concat = patch_concat.unwrap_or_else(|| self.next_inst());
        let initial_entry = patch_concat.entry;
        // It is much simpler to compile, e.g., `a{2,5}` as:
        //
        //     aaa?a?a?
        //
        // But you end up with a sequence of instructions like this:
        //
        //     0: 'a'
        //     1: 'a',
        //     2: split(3, 4)
        //     3: 'a'
        //     4: split(5, 6)
        //     5: 'a'
        //     6: split(7, 8)
        //     7: 'a'
        //     8: MATCH
        //
        // This is *incredibly* inefficient because the splits end
        // up forming a chain, which has to be resolved everything a
        // transition is followed.
        let mut holes = vec![];
        let mut prev_hole = patch_concat.hole;
        for _ in min..max {
            self.fill_to_next(prev_hole);
            let split = self.push_split_hole();
            let Patch { hole, entry } = match self.c(expr)? {
                Some(p) => p,
                None => return self.pop_split_hole(),
            };
            prev_hole = hole;
            if greedy {
                holes.push(self.fill_split(split, Some(entry), None));
            } else {
                holes.push(self.fill_split(split, None, Some(entry)));
            }
        }
        holes.push(prev_hole);
        Ok(Some(Patch { hole: Hole::Many(holes), entry: initial_entry }))
    }

    /// Can be used as a default value for the c_* functions when the call to
    /// c_function is followed by inserting at least one instruction that is
    /// always executed after the ones written by the c* function.
    fn next_inst(&self) -> Patch {
        Patch { hole: Hole::None, entry: self.insts.len() }
    }

    fn fill(&mut self, hole: Hole, goto: InstPtr) {
        match hole {
            Hole::None => {}
            Hole::One(pc) => {
                self.insts[pc].fill(goto);
            }
            Hole::Many(holes) => {
                for hole in holes {
                    self.fill(hole, goto);
                }
            }
        }
    }

    fn fill_to_next(&mut self, hole: Hole) {
        let next = self.insts.len();
        self.fill(hole, next);
    }

    fn fill_split(
        &mut self,
        hole: Hole,
        goto1: Option<InstPtr>,
        goto2: Option<InstPtr>,
    ) -> Hole {
        match hole {
            Hole::None => Hole::None,
            Hole::One(pc) => match (goto1, goto2) {
                (Some(goto1), Some(goto2)) => {
                    self.insts[pc].fill_split(goto1, goto2);
                    Hole::None
                }
                (Some(goto1), None) => {
                    self.insts[pc].half_fill_split_goto1(goto1);
                    Hole::One(pc)
                }
                (None, Some(goto2)) => {
                    self.insts[pc].half_fill_split_goto2(goto2);
                    Hole::One(pc)
                }
                (None, None) => unreachable!(
                    "at least one of the split \
                     holes must be filled"
                ),
            },
            Hole::Many(holes) => {
                let mut new_holes = vec![];
                for hole in holes {
                    new_holes.push(self.fill_split(hole, goto1, goto2));
                }
                if new_holes.is_empty() {
                    Hole::None
                } else if new_holes.len() == 1 {
                    new_holes.pop().unwrap()
                } else {
                    Hole::Many(new_holes)
                }
            }
        }
    }

    fn push_compiled(&mut self, inst: Inst) {
        self.insts.push(MaybeInst::Compiled(inst));
    }

    fn push_hole(&mut self, inst: InstHole) -> Hole {
        let hole = self.insts.len();
        self.insts.push(MaybeInst::Uncompiled(inst));
        Hole::One(hole)
    }

    fn push_split_hole(&mut self) -> Hole {
        let hole = self.insts.len();
        self.insts.push(MaybeInst::Split);
        Hole::One(hole)
    }

    fn pop_split_hole(&mut self) -> ResultOrEmpty {
        self.insts.pop();
        Ok(None)
    }

    fn check_size(&self) -> result::Result<(), Error> {
        use std::mem::size_of;

        let size =
            self.extra_inst_bytes + (self.insts.len() * size_of::<Inst>());
        if size > self.size_limit {
            Err(Error::CompiledTooBig(self.size_limit))
        } else {
            Ok(())
        }
    }
}

#[derive(Debug)]
enum Hole {
    None,
    One(InstPtr),
    Many(Vec<Hole>),
}

impl Hole {
    fn dup_one(self) -> (Self, Self) {
        match self {
            Hole::One(pc) => (Hole::One(pc), Hole::One(pc)),
            Hole::None | Hole::Many(_) => {
                unreachable!("must be called on single hole")
            }
        }
    }
}

#[derive(Clone, Debug)]
enum MaybeInst {
    Compiled(Inst),
    Uncompiled(InstHole),
    Split,
    Split1(InstPtr),
    Split2(InstPtr),
}

impl MaybeInst {
    fn fill(&mut self, goto: InstPtr) {
        let maybeinst = match *self {
            MaybeInst::Split => MaybeInst::Split1(goto),
            MaybeInst::Uncompiled(ref inst) => {
                MaybeInst::Compiled(inst.fill(goto))
            }
            MaybeInst::Split1(goto1) => {
                MaybeInst::Compiled(Inst::Split(InstSplit {
                    goto1,
                    goto2: goto,
                }))
            }
            MaybeInst::Split2(goto2) => {
                MaybeInst::Compiled(Inst::Split(InstSplit {
                    goto1: goto,
                    goto2,
                }))
            }
            _ => unreachable!(
                "not all instructions were compiled! \
                 found uncompiled instruction: {:?}",
                self
            ),
        };
        *self = maybeinst;
    }

    fn fill_split(&mut self, goto1: InstPtr, goto2: InstPtr) {
        let filled = match *self {
            MaybeInst::Split => Inst::Split(InstSplit { goto1, goto2 }),
            _ => unreachable!(
                "must be called on Split instruction, \
                 instead it was called on: {:?}",
                self
            ),
        };
        *self = MaybeInst::Compiled(filled);
    }

    fn half_fill_split_goto1(&mut self, goto1: InstPtr) {
        let half_filled = match *self {
            MaybeInst::Split => goto1,
            _ => unreachable!(
                "must be called on Split instruction, \
                 instead it was called on: {:?}",
                self
            ),
        };
        *self = MaybeInst::Split1(half_filled);
    }

    fn half_fill_split_goto2(&mut self, goto2: InstPtr) {
        let half_filled = match *self {
            MaybeInst::Split => goto2,
            _ => unreachable!(
                "must be called on Split instruction, \
                 instead it was called on: {:?}",
                self
            ),
        };
        *self = MaybeInst::Split2(half_filled);
    }

    fn unwrap(self) -> Inst {
        match self {
            MaybeInst::Compiled(inst) => inst,
            _ => unreachable!(
                "must be called on a compiled instruction, \
                 instead it was called on: {:?}",
                self
            ),
        }
    }
}

#[derive(Clone, Debug)]
enum InstHole {
    Save { slot: usize },
    EmptyLook { look: EmptyLook },
    Char { c: char },
    Ranges { ranges: Vec<(char, char)> },
    Bytes { start: u8, end: u8 },
}

impl InstHole {
    fn fill(&self, goto: InstPtr) -> Inst {
        match *self {
            InstHole::Save { slot } => Inst::Save(InstSave { goto, slot }),
            InstHole::EmptyLook { look } => {
                Inst::EmptyLook(InstEmptyLook { goto, look })
            }
            InstHole::Char { c } => Inst::Char(InstChar { goto, c }),
            InstHole::Ranges { ref ranges } => Inst::Ranges(InstRanges {
                goto,
                ranges: ranges.clone().into_boxed_slice(),
            }),
            InstHole::Bytes { start, end } => {
                Inst::Bytes(InstBytes { goto, start, end })
            }
        }
    }
}

struct CompileClass<'a, 'b> {
    c: &'a mut Compiler,
    ranges: &'b [hir::ClassUnicodeRange],
}

impl<'a, 'b> CompileClass<'a, 'b> {
    fn compile(mut self) -> Result {
        let mut holes = vec![];
        let mut initial_entry = None;
        let mut last_split = Hole::None;
        let mut utf8_seqs = self.c.utf8_seqs.take().unwrap();
        self.c.suffix_cache.clear();

        for (i, range) in self.ranges.iter().enumerate() {
            let is_last_range = i + 1 == self.ranges.len();
            utf8_seqs.reset(range.start(), range.end());
            let mut it = (&mut utf8_seqs).peekable();
            loop {
                let utf8_seq = match it.next() {
                    None => break,
                    Some(utf8_seq) => utf8_seq,
                };
                if is_last_range && it.peek().is_none() {
                    let Patch { hole, entry } = self.c_utf8_seq(&utf8_seq)?;
                    holes.push(hole);
                    self.c.fill(last_split, entry);
                    last_split = Hole::None;
                    if initial_entry.is_none() {
                        initial_entry = Some(entry);
                    }
                } else {
                    if initial_entry.is_none() {
                        initial_entry = Some(self.c.insts.len());
                    }
                    self.c.fill_to_next(last_split);
                    last_split = self.c.push_split_hole();
                    let Patch { hole, entry } = self.c_utf8_seq(&utf8_seq)?;
                    holes.push(hole);
                    last_split =
                        self.c.fill_split(last_split, Some(entry), None);
                }
            }
        }
        self.c.utf8_seqs = Some(utf8_seqs);
        Ok(Patch { hole: Hole::Many(holes), entry: initial_entry.unwrap() })
    }

    fn c_utf8_seq(&mut self, seq: &Utf8Sequence) -> Result {
        if self.c.compiled.is_reverse {
            self.c_utf8_seq_(seq)
        } else {
            self.c_utf8_seq_(seq.into_iter().rev())
        }
    }

    fn c_utf8_seq_<'r, I>(&mut self, seq: I) -> Result
    where
        I: IntoIterator<Item = &'r Utf8Range>,
    {
        // The initial instruction for each UTF-8 sequence should be the same.
        let mut from_inst = ::std::usize::MAX;
        let mut last_hole = Hole::None;
        for byte_range in seq {
            let key = SuffixCacheKey {
                from_inst,
                start: byte_range.start,
                end: byte_range.end,
            };
            {
                let pc = self.c.insts.len();
                if let Some(cached_pc) = self.c.suffix_cache.get(key, pc) {
                    from_inst = cached_pc;
                    continue;
                }
            }
            self.c.byte_classes.set_range(byte_range.start, byte_range.end);
            if from_inst == ::std::usize::MAX {
                last_hole = self.c.push_hole(InstHole::Bytes {
                    start: byte_range.start,
                    end: byte_range.end,
                });
            } else {
                self.c.push_compiled(Inst::Bytes(InstBytes {
                    goto: from_inst,
                    start: byte_range.start,
                    end: byte_range.end,
                }));
            }
            from_inst = self.c.insts.len().checked_sub(1).unwrap();
            debug_assert!(from_inst < ::std::usize::MAX);
        }
        debug_assert!(from_inst < ::std::usize::MAX);
        Ok(Patch { hole: last_hole, entry: from_inst })
    }
}

/// `SuffixCache` is a simple bounded hash map for caching suffix entries in
/// UTF-8 automata. For example, consider the Unicode range \u{0}-\u{FFFF}.
/// The set of byte ranges looks like this:
///
/// [0-7F]
/// [C2-DF][80-BF]
/// [E0][A0-BF][80-BF]
/// [E1-EC][80-BF][80-BF]
/// [ED][80-9F][80-BF]
/// [EE-EF][80-BF][80-BF]
///
/// Each line above translates to one alternate in the compiled regex program.
/// However, all but one of the alternates end in the same suffix, which is
/// a waste of an instruction. The suffix cache facilitates reusing them across
/// alternates.
///
/// Note that a HashMap could be trivially used for this, but we don't need its
/// overhead. Some small bounded space (LRU style) is more than enough.
///
/// This uses similar idea to [`SparseSet`](../sparse/struct.SparseSet.html),
/// except it uses hashes as original indices and then compares full keys for
/// validation against `dense` array.
#[derive(Debug)]
struct SuffixCache {
    sparse: Box<[usize]>,
    dense: Vec<SuffixCacheEntry>,
}

#[derive(Clone, Copy, Debug, Default, Eq, Hash, PartialEq)]
struct SuffixCacheEntry {
    key: SuffixCacheKey,
    pc: InstPtr,
}

#[derive(Clone, Copy, Debug, Default, Eq, Hash, PartialEq)]
struct SuffixCacheKey {
    from_inst: InstPtr,
    start: u8,
    end: u8,
}

impl SuffixCache {
    fn new(size: usize) -> Self {
        SuffixCache {
            sparse: vec![0usize; size].into(),
            dense: Vec::with_capacity(size),
        }
    }

    fn get(&mut self, key: SuffixCacheKey, pc: InstPtr) -> Option<InstPtr> {
        let hash = self.hash(&key);
        let pos = &mut self.sparse[hash];
        if let Some(entry) = self.dense.get(*pos) {
            if entry.key == key {
                return Some(entry.pc);
            }
        }
        *pos = self.dense.len();
        self.dense.push(SuffixCacheEntry { key, pc });
        None
    }

    fn clear(&mut self) {
        self.dense.clear();
    }

    fn hash(&self, suffix: &SuffixCacheKey) -> usize {
        // Basic FNV-1a hash as described:
        // https://en.wikipedia.org/wiki/Fowler%E2%80%93Noll%E2%80%93Vo_hash_function
        const FNV_PRIME: u64 = 1_099_511_628_211;
        let mut h = 14_695_981_039_346_656_037;
        h = (h ^ (suffix.from_inst as u64)).wrapping_mul(FNV_PRIME);
        h = (h ^ (suffix.start as u64)).wrapping_mul(FNV_PRIME);
        h = (h ^ (suffix.end as u64)).wrapping_mul(FNV_PRIME);
        (h as usize) % self.sparse.len()
    }
}

struct ByteClassSet([bool; 256]);

impl ByteClassSet {
    fn new() -> Self {
        ByteClassSet([false; 256])
    }

    fn set_range(&mut self, start: u8, end: u8) {
        debug_assert!(start <= end);
        if start > 0 {
            self.0[start as usize - 1] = true;
        }
        self.0[end as usize] = true;
    }

    fn set_word_boundary(&mut self) {
        // We need to mark all ranges of bytes whose pairs result in
        // evaluating \b differently.
        let iswb = is_word_byte;
        let mut b1: u16 = 0;
        let mut b2: u16;
        while b1 <= 255 {
            b2 = b1 + 1;
            while b2 <= 255 && iswb(b1 as u8) == iswb(b2 as u8) {
                b2 += 1;
            }
            self.set_range(b1 as u8, (b2 - 1) as u8);
            b1 = b2;
        }
    }

    fn byte_classes(&self) -> Vec<u8> {
        // N.B. If you're debugging the DFA, it's useful to simply return
        // `(0..256).collect()`, which effectively removes the byte classes
        // and makes the transitions easier to read.
        // (0usize..256).map(|x| x as u8).collect()
        let mut byte_classes = vec![0; 256];
        let mut class = 0u8;
        let mut i = 0;
        loop {
            byte_classes[i] = class as u8;
            if i >= 255 {
                break;
            }
            if self.0[i] {
                class = class.checked_add(1).unwrap();
            }
            i += 1;
        }
        byte_classes
    }
}

impl fmt::Debug for ByteClassSet {
    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
        f.debug_tuple("ByteClassSet").field(&&self.0[..]).finish()
    }
}

fn u32_to_usize(n: u32) -> usize {
    // In case usize is less than 32 bits, we need to guard against overflow.
    // On most platforms this compiles to nothing.
    // TODO Use `std::convert::TryFrom` once it's stable.
    if (n as u64) > (::std::usize::MAX as u64) {
        panic!("BUG: {} is too big to be pointer sized", n)
    }
    n as usize
}

#[cfg(test)]
mod tests {
    use super::ByteClassSet;

    #[test]
    fn byte_classes() {
        let mut set = ByteClassSet::new();
        set.set_range(b'a', b'z');
        let classes = set.byte_classes();
        assert_eq!(classes[0], 0);
        assert_eq!(classes[1], 0);
        assert_eq!(classes[2], 0);
        assert_eq!(classes[b'a' as usize - 1], 0);
        assert_eq!(classes[b'a' as usize], 1);
        assert_eq!(classes[b'm' as usize], 1);
        assert_eq!(classes[b'z' as usize], 1);
        assert_eq!(classes[b'z' as usize + 1], 2);
        assert_eq!(classes[254], 2);
        assert_eq!(classes[255], 2);

        let mut set = ByteClassSet::new();
        set.set_range(0, 2);
        set.set_range(4, 6);
        let classes = set.byte_classes();
        assert_eq!(classes[0], 0);
        assert_eq!(classes[1], 0);
        assert_eq!(classes[2], 0);
        assert_eq!(classes[3], 1);
        assert_eq!(classes[4], 2);
        assert_eq!(classes[5], 2);
        assert_eq!(classes[6], 2);
        assert_eq!(classes[7], 3);
        assert_eq!(classes[255], 3);
    }

    #[test]
    fn full_byte_classes() {
        let mut set = ByteClassSet::new();
        for i in 0..256u16 {
            set.set_range(i as u8, i as u8);
        }
        assert_eq!(set.byte_classes().len(), 256);
    }
}