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use std::cmp;
use std::collections::{BTreeMap, BTreeSet, VecDeque};
use std::iter::repeat_with;
use std::ops::RangeInclusive;
use regex_syntax::hir::{Class, Hir, HirKind, Literal, RepetitionKind, RepetitionRange};
#[derive(Eq, PartialEq, Clone)]
pub struct Range(pub RangeInclusive<char>);
impl PartialOrd for Range {
fn partial_cmp(&self, other: &Self) -> Option<cmp::Ordering> {
Some(self.cmp(other))
}
}
impl Ord for Range {
fn cmp(&self, other: &Self) -> cmp::Ordering {
let start_order = self.0.start().cmp(other.0.start());
start_order.then_with(|| self.0.end().cmp(other.0.end()))
}
}
/// A nondeterministic finite automaton.
/// By convention, always starts in state 0.
#[derive(Default)]
pub struct NFA {
pub state_count: usize,
pub transition_table: BTreeMap<usize, BTreeMap<Option<Range>, BTreeSet<usize>>>,
pub accept_states: BTreeSet<usize>,
}
impl From<&Hir> for NFA {
fn from(hir: &Hir) -> Self {
match hir.kind() {
HirKind::Empty => {
// Conventionally, this matches only the empty string,
// but the regex crate has implicit `.*?` at the front and `.*`
// at the back of every regex.
// TODO figure out what to do with that
let mut result = NFA::default();
result.state_count = 1;
result.accept_states.insert(0);
result
}
HirKind::Literal(literal) => match literal {
Literal::Unicode(ch) => {
let ch = *ch;
let mut result = NFA::default();
result.state_count = 2;
result.accept_states.insert(1);
let range = Range(ch..=ch);
result
.transition_table
.entry(0)
.or_default()
.entry(Some(range))
.or_default()
.insert(1);
result
}
l => todo!("{:?}", l),
},
HirKind::Class(class) => match class {
Class::Unicode(class) => {
let mut result = NFA::default();
result.state_count = 2;
result.accept_states.insert(1);
for range in class.iter() {
let range = Range(range.start()..=range.end());
result
.transition_table
.entry(0)
.or_default()
.entry(Some(range))
.or_default()
.insert(1);
}
result
}
c => todo!("{:?}", c),
},
HirKind::Anchor(_) => {
// TODO actually fucking implement this
let mut result = NFA::default();
result.state_count = 1;
result.accept_states.insert(0);
result
}
HirKind::Repetition(rep) => match &rep.kind {
RepetitionKind::Range(range) => match range {
RepetitionRange::Exactly(n) => repeat_with(|| NFA::from(rep.hir.as_ref()))
.take(*n as usize)
.reduce(NFA::concat)
.expect("exactly 0 of something"),
r => todo!("{:?}", r),
},
k => todo!("{:?}", k),
},
HirKind::Concat(elements) => elements
.iter()
.map(NFA::from)
.reduce(NFA::concat)
.expect("empty HirKind::Concat"),
k => todo!("{:?}", k),
}
}
}
impl NFA {
fn concat(mut self, other: NFA) -> NFA {
// absorb the other automaton's states
let other_state_offset = self.state_count;
self.state_count += other.state_count;
self.transition_table
.extend(
other
.transition_table
.into_iter()
.map(|(state, table_for_state)| {
(
state + other_state_offset,
table_for_state
.into_iter()
.map(|(char_range, next_states)| {
(
char_range,
next_states
.into_iter()
.map(|s| s + other_state_offset)
.collect(),
)
})
.collect(),
)
}),
);
// for all the accept states of this machine...
for &self_accept in &self.accept_states {
// add an ε-transition from that state to the start of the other machine
self.transition_table
.entry(self_accept)
.or_default()
.entry(None)
.or_default()
.insert(other_state_offset);
}
// steal the other machine's accept states
self.accept_states = other.accept_states;
self
}
pub fn epsilon_closure(&self, state: usize) -> BTreeSet<usize> {
let mut set = BTreeSet::new();
set.insert(state);
self.epsilon_closure_all(set)
}
pub fn epsilon_closure_all(&self, states: impl IntoIterator<Item = usize>) -> BTreeSet<usize> {
let mut result = BTreeSet::new();
let mut queue: VecDeque<usize> = VecDeque::new();
queue.extend(states);
while let Some(next_state) = queue.pop_front() {
result.insert(next_state);
let states_after_that = self
.transition_table
.get(&next_state)
.and_then(|state_table| state_table.get(&None))
.map(|next_states| next_states.iter())
.into_iter()
.flatten()
.copied()
.filter(|s| !result.contains(s));
queue.extend(states_after_that);
}
result
}
}
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