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use std::fmt;
/// A representation of byte oriented equivalence classes.
///
/// This is used in an FSM to reduce the size of the transition table. This can
/// have a particularly large impact not only on the total size of an FSM, but
/// also on compile times.
#[derive(Clone, Copy)]
pub struct ByteClasses([u8; 256]);
impl ByteClasses {
/// Creates a new set of equivalence classes where all bytes are mapped to
/// the same class.
pub fn empty() -> ByteClasses {
ByteClasses([0; 256])
}
/// Creates a new set of equivalence classes where each byte belongs to
/// its own equivalence class.
pub fn singletons() -> ByteClasses {
let mut classes = ByteClasses::empty();
for i in 0..256 {
classes.set(i as u8, i as u8);
}
classes
}
/// Set the equivalence class for the given byte.
#[inline]
pub fn set(&mut self, byte: u8, class: u8) {
self.0[byte as usize] = class;
}
/// Get the equivalence class for the given byte.
#[inline]
pub fn get(&self, byte: u8) -> u8 {
// SAFETY: This is safe because all dense transitions have
// exactly 256 elements, so all u8 values are valid indices.
self.0[byte as usize]
}
/// Return the total number of elements in the alphabet represented by
/// these equivalence classes. Equivalently, this returns the total number
/// of equivalence classes.
#[inline]
pub fn alphabet_len(&self) -> usize {
self.0[255] as usize + 1
}
/// Returns true if and only if every byte in this class maps to its own
/// equivalence class. Equivalently, there are 256 equivalence classes
/// and each class contains exactly one byte.
#[inline]
pub fn is_singleton(&self) -> bool {
self.alphabet_len() == 256
}
/// Returns an iterator over a sequence of representative bytes from each
/// equivalence class. Namely, this yields exactly N items, where N is
/// equivalent to the number of equivalence classes. Each item is an
/// arbitrary byte drawn from each equivalence class.
///
/// This is useful when one is determinizing an NFA and the NFA's alphabet
/// hasn't been converted to equivalence classes yet. Picking an arbitrary
/// byte from each equivalence class then permits a full exploration of
/// the NFA instead of using every possible byte value.
pub fn representatives(&self) -> ByteClassRepresentatives<'_> {
ByteClassRepresentatives { classes: self, byte: 0, last_class: None }
}
/// Returns all of the bytes in the given equivalence class.
///
/// The second element in the tuple indicates the number of elements in
/// the array.
fn elements(&self, equiv: u8) -> ([u8; 256], usize) {
let (mut array, mut len) = ([0; 256], 0);
for b in 0..256 {
if self.get(b as u8) == equiv {
array[len] = b as u8;
len += 1;
}
}
(array, len)
}
}
impl fmt::Debug for ByteClasses {
fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
if self.is_singleton() {
write!(f, "ByteClasses({{singletons}})")
} else {
write!(f, "ByteClasses(")?;
for equiv in 0..self.alphabet_len() {
let (members, len) = self.elements(equiv as u8);
write!(f, " {} => {:?}", equiv, &members[..len])?;
}
write!(f, ")")
}
}
}
/// An iterator over representative bytes from each equivalence class.
#[derive(Debug)]
pub struct ByteClassRepresentatives<'a> {
classes: &'a ByteClasses,
byte: usize,
last_class: Option<u8>,
}
impl<'a> Iterator for ByteClassRepresentatives<'a> {
type Item = u8;
fn next(&mut self) -> Option<u8> {
while self.byte < 256 {
let byte = self.byte as u8;
let class = self.classes.get(byte);
self.byte += 1;
if self.last_class != Some(class) {
self.last_class = Some(class);
return Some(byte);
}
}
None
}
}
/// A byte class builder keeps track of an *approximation* of equivalence
/// classes of bytes during NFA construction. That is, every byte in an
/// equivalence class cannot discriminate between a match and a non-match.
///
/// For example, in the literals `abc` and `xyz`, the bytes [\x00-`], [d-w]
/// and [{-\xFF] never discriminate between a match and a non-match, precisely
/// because they never occur in the literals anywhere.
///
/// Note though that this does not necessarily compute the minimal set of
/// equivalence classes. For example, in the literals above, the byte ranges
/// [\x00-`], [d-w] and [{-\xFF] are all treated as distinct equivalence
/// classes even though they could be treated a single class. The reason for
/// this is implementation complexity. In the future, we should endeavor to
/// compute the minimal equivalence classes since they can have a rather large
/// impact on the size of the DFA.
///
/// The representation here is 256 booleans, all initially set to false. Each
/// boolean maps to its corresponding byte based on position. A `true` value
/// indicates the end of an equivalence class, where its corresponding byte
/// and all of the bytes corresponding to all previous contiguous `false`
/// values are in the same equivalence class.
///
/// This particular representation only permits contiguous ranges of bytes to
/// be in the same equivalence class, which means that we can never discover
/// the true minimal set of equivalence classes.
#[derive(Debug)]
pub struct ByteClassBuilder(Vec<bool>);
impl ByteClassBuilder {
/// Create a new builder of byte classes where all bytes are part of the
/// same equivalence class.
pub fn new() -> ByteClassBuilder {
ByteClassBuilder(vec![false; 256])
}
/// Indicate the the range of byte given (inclusive) can discriminate a
/// match between it and all other bytes outside of the range.
pub 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;
}
/// Build byte classes that map all byte values to their corresponding
/// equivalence class. The last mapping indicates the largest equivalence
/// class identifier (which is never bigger than 255).
pub fn build(&self) -> ByteClasses {
let mut classes = ByteClasses::empty();
let mut class = 0u8;
let mut i = 0;
loop {
classes.set(i as u8, class as u8);
if i >= 255 {
break;
}
if self.0[i] {
class = class.checked_add(1).unwrap();
}
i += 1;
}
classes
}
}
#[cfg(test)]
mod tests {
use super::*;
#[test]
fn byte_classes() {
let mut set = ByteClassBuilder::new();
set.set_range(b'a', b'z');
let classes = set.build();
assert_eq!(classes.get(0), 0);
assert_eq!(classes.get(1), 0);
assert_eq!(classes.get(2), 0);
assert_eq!(classes.get(b'a' - 1), 0);
assert_eq!(classes.get(b'a'), 1);
assert_eq!(classes.get(b'm'), 1);
assert_eq!(classes.get(b'z'), 1);
assert_eq!(classes.get(b'z' + 1), 2);
assert_eq!(classes.get(254), 2);
assert_eq!(classes.get(255), 2);
let mut set = ByteClassBuilder::new();
set.set_range(0, 2);
set.set_range(4, 6);
let classes = set.build();
assert_eq!(classes.get(0), 0);
assert_eq!(classes.get(1), 0);
assert_eq!(classes.get(2), 0);
assert_eq!(classes.get(3), 1);
assert_eq!(classes.get(4), 2);
assert_eq!(classes.get(5), 2);
assert_eq!(classes.get(6), 2);
assert_eq!(classes.get(7), 3);
assert_eq!(classes.get(255), 3);
}
#[test]
fn full_byte_classes() {
let mut set = ByteClassBuilder::new();
for i in 0..256u16 {
set.set_range(i as u8, i as u8);
}
assert_eq!(set.build().alphabet_len(), 256);
}
}