Adding upstream version 124.0.1.upstream/124.0.1

Signed-off-by: Daniel Baumann <daniel.baumann@progress-linux.org>

1 files changed, 802 insertions, 0 deletions

diff --git a/third_party/rust/regex-automata/src/hybrid/search.rs b/third_party/rust/regex-automata/src/hybrid/search.rs
new file mode 100644
index 0000000000..f232836854
--- /dev/null
+++ b/third_party/rust/regex-automata/src/hybrid/search.rs
@@ -0,0 +1,802 @@
+use crate::{
+    hybrid::{
+        dfa::{Cache, OverlappingState, DFA},
+        id::LazyStateID,
+    },
+    util::{
+        prefilter::Prefilter,
+        search::{HalfMatch, Input, MatchError, Span},
+    },
+};
+
+#[inline(never)]
+pub(crate) fn find_fwd(
+    dfa: &DFA,
+    cache: &mut Cache,
+    input: &Input<'_>,
+) -> Result<Option<HalfMatch>, MatchError> {
+    if input.is_done() {
+        return Ok(None);
+    }
+    let pre = if input.get_anchored().is_anchored() {
+        None
+    } else {
+        dfa.get_config().get_prefilter()
+    };
+    // So what we do here is specialize four different versions of 'find_fwd':
+    // one for each of the combinations for 'has prefilter' and 'is earliest
+    // search'. The reason for doing this is that both of these things require
+    // branches and special handling in some code that can be very hot,
+    // and shaving off as much as we can when we don't need it tends to be
+    // beneficial in ad hoc benchmarks. To see these differences, you often
+    // need a query with a high match count. In other words, specializing these
+    // four routines *tends* to help latency more than throughput.
+    if pre.is_some() {
+        if input.get_earliest() {
+            find_fwd_imp(dfa, cache, input, pre, true)
+        } else {
+            find_fwd_imp(dfa, cache, input, pre, false)
+        }
+    } else {
+        if input.get_earliest() {
+            find_fwd_imp(dfa, cache, input, None, true)
+        } else {
+            find_fwd_imp(dfa, cache, input, None, false)
+        }
+    }
+}
+
+#[cfg_attr(feature = "perf-inline", inline(always))]
+fn find_fwd_imp(
+    dfa: &DFA,
+    cache: &mut Cache,
+    input: &Input<'_>,
+    pre: Option<&'_ Prefilter>,
+    earliest: bool,
+) -> Result<Option<HalfMatch>, MatchError> {
+    // See 'prefilter_restart' docs for explanation.
+    let universal_start = dfa.get_nfa().look_set_prefix_any().is_empty();
+    let mut mat = None;
+    let mut sid = init_fwd(dfa, cache, input)?;
+    let mut at = input.start();
+    // This could just be a closure, but then I think it would be unsound
+    // because it would need to be safe to invoke. This way, the lack of safety
+    // is clearer in the code below.
+    macro_rules! next_unchecked {
+        ($sid:expr, $at:expr) => {{
+            let byte = *input.haystack().get_unchecked($at);
+            dfa.next_state_untagged_unchecked(cache, $sid, byte)
+        }};
+    }
+
+    if let Some(ref pre) = pre {
+        let span = Span::from(at..input.end());
+        match pre.find(input.haystack(), span) {
+            None => return Ok(mat),
+            Some(ref span) => {
+                at = span.start;
+                if !universal_start {
+                    sid = prefilter_restart(dfa, cache, &input, at)?;
+                }
+            }
+        }
+    }
+    cache.search_start(at);
+    while at < input.end() {
+        if sid.is_tagged() {
+            cache.search_update(at);
+            sid = dfa
+                .next_state(cache, sid, input.haystack()[at])
+                .map_err(|_| gave_up(at))?;
+        } else {
+            // SAFETY: There are two safety invariants we need to uphold
+            // here in the loops below: that 'sid' and 'prev_sid' are valid
+            // state IDs for this DFA, and that 'at' is a valid index into
+            // 'haystack'. For the former, we rely on the invariant that
+            // next_state* and start_state_forward always returns a valid state
+            // ID (given a valid state ID in the former case), and that we are
+            // only at this place in the code if 'sid' is untagged. Moreover,
+            // every call to next_state_untagged_unchecked below is guarded by
+            // a check that sid is untagged. For the latter safety invariant,
+            // we always guard unchecked access with a check that 'at' is less
+            // than 'end', where 'end <= haystack.len()'. In the unrolled loop
+            // below, we ensure that 'at' is always in bounds.
+            //
+            // PERF: For justification of omitting bounds checks, it gives us a
+            // ~10% bump in search time. This was used for a benchmark:
+            //
+            //     regex-cli find hybrid dfa @bigfile '(?m)^.+$' -UBb
+            //
+            // PERF: For justification for the loop unrolling, we use a few
+            // different tests:
+            //
+            //     regex-cli find hybrid dfa @$bigfile '\w{50}' -UBb
+            //     regex-cli find hybrid dfa @$bigfile '(?m)^.+$' -UBb
+            //     regex-cli find hybrid dfa @$bigfile 'ZQZQZQZQ' -UBb
+            //
+            // And there are three different configurations:
+            //
+            //     nounroll: this entire 'else' block vanishes and we just
+            //               always use 'dfa.next_state(..)'.
+            //      unroll1: just the outer loop below
+            //      unroll2: just the inner loop below
+            //      unroll3: both the outer and inner loops below
+            //
+            // This results in a matrix of timings for each of the above
+            // regexes with each of the above unrolling configurations:
+            //
+            //              '\w{50}'   '(?m)^.+$'   'ZQZQZQZQ'
+            //   nounroll   1.51s      2.34s        1.51s
+            //    unroll1   1.53s      2.32s        1.56s
+            //    unroll2   2.22s      1.50s        0.61s
+            //    unroll3   1.67s      1.45s        0.61s
+            //
+            // Ideally we'd be able to find a configuration that yields the
+            // best time for all regexes, but alas we settle for unroll3 that
+            // gives us *almost* the best for '\w{50}' and the best for the
+            // other two regexes.
+            //
+            // So what exactly is going on here? The first unrolling (grouping
+            // together runs of untagged transitions) specifically targets
+            // our choice of representation. The second unrolling (grouping
+            // together runs of self-transitions) specifically targets a common
+            // DFA topology. Let's dig in a little bit by looking at our
+            // regexes:
+            //
+            // '\w{50}': This regex spends a lot of time outside of the DFA's
+            // start state matching some part of the '\w' repetition. This
+            // means that it's a bit of a worst case for loop unrolling that
+            // targets self-transitions since the self-transitions in '\w{50}'
+            // are not particularly active for this haystack. However, the
+            // first unrolling (grouping together untagged transitions)
+            // does apply quite well here since very few transitions hit
+            // match/dead/quit/unknown states. It is however worth mentioning
+            // that if start states are configured to be tagged (which you
+            // typically want to do if you have a prefilter), then this regex
+            // actually slows way down because it is constantly ping-ponging
+            // out of the unrolled loop and into the handling of a tagged start
+            // state below. But when start states aren't tagged, the unrolled
+            // loop stays hot. (This is why it's imperative that start state
+            // tagging be disabled when there isn't a prefilter!)
+            //
+            // '(?m)^.+$': There are two important aspects of this regex: 1)
+            // on this haystack, its match count is very high, much higher
+            // than the other two regex and 2) it spends the vast majority
+            // of its time matching '.+'. Since Unicode mode is disabled,
+            // this corresponds to repeatedly following self transitions for
+            // the vast majority of the input. This does benefit from the
+            // untagged unrolling since most of the transitions will be to
+            // untagged states, but the untagged unrolling does more work than
+            // what is actually required. Namely, it has to keep track of the
+            // previous and next state IDs, which I guess requires a bit more
+            // shuffling. This is supported by the fact that nounroll+unroll1
+            // are both slower than unroll2+unroll3, where the latter has a
+            // loop unrolling that specifically targets self-transitions.
+            //
+            // 'ZQZQZQZQ': This one is very similar to '(?m)^.+$' because it
+            // spends the vast majority of its time in self-transitions for
+            // the (implicit) unanchored prefix. The main difference with
+            // '(?m)^.+$' is that it has a much lower match count. So there
+            // isn't much time spent in the overhead of reporting matches. This
+            // is the primary explainer in the perf difference here. We include
+            // this regex and the former to make sure we have comparison points
+            // with high and low match counts.
+            //
+            // NOTE: I used 'OpenSubtitles2018.raw.sample.en' for 'bigfile'.
+            //
+            // NOTE: In a follow-up, it turns out that the "inner" loop
+            // mentioned above was a pretty big pessimization in some other
+            // cases. Namely, it resulted in too much ping-ponging into and out
+            // of the loop, which resulted in nearly ~2x regressions in search
+            // time when compared to the originally lazy DFA in the regex crate.
+            // So I've removed the second loop unrolling that targets the
+            // self-transition case.
+            let mut prev_sid = sid;
+            while at < input.end() {
+                prev_sid = unsafe { next_unchecked!(sid, at) };
+                if prev_sid.is_tagged() || at + 3 >= input.end() {
+                    core::mem::swap(&mut prev_sid, &mut sid);
+                    break;
+                }
+                at += 1;
+
+                sid = unsafe { next_unchecked!(prev_sid, at) };
+                if sid.is_tagged() {
+                    break;
+                }
+                at += 1;
+
+                prev_sid = unsafe { next_unchecked!(sid, at) };
+                if prev_sid.is_tagged() {
+                    core::mem::swap(&mut prev_sid, &mut sid);
+                    break;
+                }
+                at += 1;
+
+                sid = unsafe { next_unchecked!(prev_sid, at) };
+                if sid.is_tagged() {
+                    break;
+                }
+                at += 1;
+            }
+            // If we quit out of the code above with an unknown state ID at
+            // any point, then we need to re-compute that transition using
+            // 'next_state', which will do NFA powerset construction for us.
+            if sid.is_unknown() {
+                cache.search_update(at);
+                sid = dfa
+                    .next_state(cache, prev_sid, input.haystack()[at])
+                    .map_err(|_| gave_up(at))?;
+            }
+        }
+        if sid.is_tagged() {
+            if sid.is_start() {
+                if let Some(ref pre) = pre {
+                    let span = Span::from(at..input.end());
+                    match pre.find(input.haystack(), span) {
+                        None => {
+                            cache.search_finish(span.end);
+                            return Ok(mat);
+                        }
+                        Some(ref span) => {
+                            // We want to skip any update to 'at' below
+                            // at the end of this iteration and just
+                            // jump immediately back to the next state
+                            // transition at the leading position of the
+                            // candidate match.
+                            //
+                            // ... but only if we actually made progress
+                            // with our prefilter, otherwise if the start
+                            // state has a self-loop, we can get stuck.
+                            if span.start > at {
+                                at = span.start;
+                                if !universal_start {
+                                    sid = prefilter_restart(
+                                        dfa, cache, &input, at,
+                                    )?;
+                                }
+                                continue;
+                            }
+                        }
+                    }
+                }
+            } else if sid.is_match() {
+                let pattern = dfa.match_pattern(cache, sid, 0);
+                // Since slice ranges are inclusive at the beginning and
+                // exclusive at the end, and since forward searches report
+                // the end, we can return 'at' as-is. This only works because
+                // matches are delayed by 1 byte. So by the time we observe a
+                // match, 'at' has already been set to 1 byte past the actual
+                // match location, which is precisely the exclusive ending
+                // bound of the match.
+                mat = Some(HalfMatch::new(pattern, at));
+                if earliest {
+                    cache.search_finish(at);
+                    return Ok(mat);
+                }
+            } else if sid.is_dead() {
+                cache.search_finish(at);
+                return Ok(mat);
+            } else if sid.is_quit() {
+                cache.search_finish(at);
+                return Err(MatchError::quit(input.haystack()[at], at));
+            } else {
+                debug_assert!(sid.is_unknown());
+                unreachable!("sid being unknown is a bug");
+            }
+        }
+        at += 1;
+    }
+    eoi_fwd(dfa, cache, input, &mut sid, &mut mat)?;
+    cache.search_finish(input.end());
+    Ok(mat)
+}
+
+#[inline(never)]
+pub(crate) fn find_rev(
+    dfa: &DFA,
+    cache: &mut Cache,
+    input: &Input<'_>,
+) -> Result<Option<HalfMatch>, MatchError> {
+    if input.is_done() {
+        return Ok(None);
+    }
+    if input.get_earliest() {
+        find_rev_imp(dfa, cache, input, true)
+    } else {
+        find_rev_imp(dfa, cache, input, false)
+    }
+}
+
+#[cfg_attr(feature = "perf-inline", inline(always))]
+fn find_rev_imp(
+    dfa: &DFA,
+    cache: &mut Cache,
+    input: &Input<'_>,
+    earliest: bool,
+) -> Result<Option<HalfMatch>, MatchError> {
+    let mut mat = None;
+    let mut sid = init_rev(dfa, cache, input)?;
+    // In reverse search, the loop below can't handle the case of searching an
+    // empty slice. Ideally we could write something congruent to the forward
+    // search, i.e., 'while at >= start', but 'start' might be 0. Since we use
+    // an unsigned offset, 'at >= 0' is trivially always true. We could avoid
+    // this extra case handling by using a signed offset, but Rust makes it
+    // annoying to do. So... We just handle the empty case separately.
+    if input.start() == input.end() {
+        eoi_rev(dfa, cache, input, &mut sid, &mut mat)?;
+        return Ok(mat);
+    }
+
+    let mut at = input.end() - 1;
+    macro_rules! next_unchecked {
+        ($sid:expr, $at:expr) => {{
+            let byte = *input.haystack().get_unchecked($at);
+            dfa.next_state_untagged_unchecked(cache, $sid, byte)
+        }};
+    }
+    cache.search_start(at);
+    loop {
+        if sid.is_tagged() {
+            cache.search_update(at);
+            sid = dfa
+                .next_state(cache, sid, input.haystack()[at])
+                .map_err(|_| gave_up(at))?;
+        } else {
+            // SAFETY: See comments in 'find_fwd' for a safety argument.
+            //
+            // PERF: The comments in 'find_fwd' also provide a justification
+            // from a performance perspective as to 1) why we elide bounds
+            // checks and 2) why we do a specialized version of unrolling
+            // below. The reverse search does have a slightly different
+            // consideration in that most reverse searches tend to be
+            // anchored and on shorter haystacks. However, this still makes a
+            // difference. Take this command for example:
+            //
+            //     regex-cli find hybrid regex @$bigfile '(?m)^.+$' -UBb
+            //
+            // (Notice that we use 'find hybrid regex', not 'find hybrid dfa'
+            // like in the justification for the forward direction. The 'regex'
+            // sub-command will find start-of-match and thus run the reverse
+            // direction.)
+            //
+            // Without unrolling below, the above command takes around 3.76s.
+            // But with the unrolling below, we get down to 2.55s. If we keep
+            // the unrolling but add in bounds checks, then we get 2.86s.
+            //
+            // NOTE: I used 'OpenSubtitles2018.raw.sample.en' for 'bigfile'.
+            let mut prev_sid = sid;
+            while at >= input.start() {
+                prev_sid = unsafe { next_unchecked!(sid, at) };
+                if prev_sid.is_tagged()
+                    || at <= input.start().saturating_add(3)
+                {
+                    core::mem::swap(&mut prev_sid, &mut sid);
+                    break;
+                }
+                at -= 1;
+
+                sid = unsafe { next_unchecked!(prev_sid, at) };
+                if sid.is_tagged() {
+                    break;
+                }
+                at -= 1;
+
+                prev_sid = unsafe { next_unchecked!(sid, at) };
+                if prev_sid.is_tagged() {
+                    core::mem::swap(&mut prev_sid, &mut sid);
+                    break;
+                }
+                at -= 1;
+
+                sid = unsafe { next_unchecked!(prev_sid, at) };
+                if sid.is_tagged() {
+                    break;
+                }
+                at -= 1;
+            }
+            // If we quit out of the code above with an unknown state ID at
+            // any point, then we need to re-compute that transition using
+            // 'next_state', which will do NFA powerset construction for us.
+            if sid.is_unknown() {
+                cache.search_update(at);
+                sid = dfa
+                    .next_state(cache, prev_sid, input.haystack()[at])
+                    .map_err(|_| gave_up(at))?;
+            }
+        }
+        if sid.is_tagged() {
+            if sid.is_start() {
+                // do nothing
+            } else if sid.is_match() {
+                let pattern = dfa.match_pattern(cache, sid, 0);
+                // Since reverse searches report the beginning of a match
+                // and the beginning is inclusive (not exclusive like the
+                // end of a match), we add 1 to make it inclusive.
+                mat = Some(HalfMatch::new(pattern, at + 1));
+                if earliest {
+                    cache.search_finish(at);
+                    return Ok(mat);
+                }
+            } else if sid.is_dead() {
+                cache.search_finish(at);
+                return Ok(mat);
+            } else if sid.is_quit() {
+                cache.search_finish(at);
+                return Err(MatchError::quit(input.haystack()[at], at));
+            } else {
+                debug_assert!(sid.is_unknown());
+                unreachable!("sid being unknown is a bug");
+            }
+        }
+        if at == input.start() {
+            break;
+        }
+        at -= 1;
+    }
+    cache.search_finish(input.start());
+    eoi_rev(dfa, cache, input, &mut sid, &mut mat)?;
+    Ok(mat)
+}
+
+#[inline(never)]
+pub(crate) fn find_overlapping_fwd(
+    dfa: &DFA,
+    cache: &mut Cache,
+    input: &Input<'_>,
+    state: &mut OverlappingState,
+) -> Result<(), MatchError> {
+    state.mat = None;
+    if input.is_done() {
+        return Ok(());
+    }
+    let pre = if input.get_anchored().is_anchored() {
+        None
+    } else {
+        dfa.get_config().get_prefilter()
+    };
+    if pre.is_some() {
+        find_overlapping_fwd_imp(dfa, cache, input, pre, state)
+    } else {
+        find_overlapping_fwd_imp(dfa, cache, input, None, state)
+    }
+}
+
+#[cfg_attr(feature = "perf-inline", inline(always))]
+fn find_overlapping_fwd_imp(
+    dfa: &DFA,
+    cache: &mut Cache,
+    input: &Input<'_>,
+    pre: Option<&'_ Prefilter>,
+    state: &mut OverlappingState,
+) -> Result<(), MatchError> {
+    // See 'prefilter_restart' docs for explanation.
+    let universal_start = dfa.get_nfa().look_set_prefix_any().is_empty();
+    let mut sid = match state.id {
+        None => {
+            state.at = input.start();
+            init_fwd(dfa, cache, input)?
+        }
+        Some(sid) => {
+            if let Some(match_index) = state.next_match_index {
+                let match_len = dfa.match_len(cache, sid);
+                if match_index < match_len {
+                    state.next_match_index = Some(match_index + 1);
+                    let pattern = dfa.match_pattern(cache, sid, match_index);
+                    state.mat = Some(HalfMatch::new(pattern, state.at));
+                    return Ok(());
+                }
+            }
+            // Once we've reported all matches at a given position, we need to
+            // advance the search to the next position.
+            state.at += 1;
+            if state.at > input.end() {
+                return Ok(());
+            }
+            sid
+        }
+    };
+
+    // NOTE: We don't optimize the crap out of this routine primarily because
+    // it seems like most overlapping searches will have higher match counts,
+    // and thus, throughput is perhaps not as important. But if you have a use
+    // case for something faster, feel free to file an issue.
+    cache.search_start(state.at);
+    while state.at < input.end() {
+        sid = dfa
+            .next_state(cache, sid, input.haystack()[state.at])
+            .map_err(|_| gave_up(state.at))?;
+        if sid.is_tagged() {
+            state.id = Some(sid);
+            if sid.is_start() {
+                if let Some(ref pre) = pre {
+                    let span = Span::from(state.at..input.end());
+                    match pre.find(input.haystack(), span) {
+                        None => return Ok(()),
+                        Some(ref span) => {
+                            if span.start > state.at {
+                                state.at = span.start;
+                                if !universal_start {
+                                    sid = prefilter_restart(
+                                        dfa, cache, &input, state.at,
+                                    )?;
+                                }
+                                continue;
+                            }
+                        }
+                    }
+                }
+            } else if sid.is_match() {
+                state.next_match_index = Some(1);
+                let pattern = dfa.match_pattern(cache, sid, 0);
+                state.mat = Some(HalfMatch::new(pattern, state.at));
+                cache.search_finish(state.at);
+                return Ok(());
+            } else if sid.is_dead() {
+                cache.search_finish(state.at);
+                return Ok(());
+            } else if sid.is_quit() {
+                cache.search_finish(state.at);
+                return Err(MatchError::quit(
+                    input.haystack()[state.at],
+                    state.at,
+                ));
+            } else {
+                debug_assert!(sid.is_unknown());
+                unreachable!("sid being unknown is a bug");
+            }
+        }
+        state.at += 1;
+        cache.search_update(state.at);
+    }
+
+    let result = eoi_fwd(dfa, cache, input, &mut sid, &mut state.mat);
+    state.id = Some(sid);
+    if state.mat.is_some() {
+        // '1' is always correct here since if we get to this point, this
+        // always corresponds to the first (index '0') match discovered at
+        // this position. So the next match to report at this position (if
+        // it exists) is at index '1'.
+        state.next_match_index = Some(1);
+    }
+    cache.search_finish(input.end());
+    result
+}
+
+#[inline(never)]
+pub(crate) fn find_overlapping_rev(
+    dfa: &DFA,
+    cache: &mut Cache,
+    input: &Input<'_>,
+    state: &mut OverlappingState,
+) -> Result<(), MatchError> {
+    state.mat = None;
+    if input.is_done() {
+        return Ok(());
+    }
+    let mut sid = match state.id {
+        None => {
+            let sid = init_rev(dfa, cache, input)?;
+            state.id = Some(sid);
+            if input.start() == input.end() {
+                state.rev_eoi = true;
+            } else {
+                state.at = input.end() - 1;
+            }
+            sid
+        }
+        Some(sid) => {
+            if let Some(match_index) = state.next_match_index {
+                let match_len = dfa.match_len(cache, sid);
+                if match_index < match_len {
+                    state.next_match_index = Some(match_index + 1);
+                    let pattern = dfa.match_pattern(cache, sid, match_index);
+                    state.mat = Some(HalfMatch::new(pattern, state.at));
+                    return Ok(());
+                }
+            }
+            // Once we've reported all matches at a given position, we need
+            // to advance the search to the next position. However, if we've
+            // already followed the EOI transition, then we know we're done
+            // with the search and there cannot be any more matches to report.
+            if state.rev_eoi {
+                return Ok(());
+            } else if state.at == input.start() {
+                // At this point, we should follow the EOI transition. This
+                // will cause us the skip the main loop below and fall through
+                // to the final 'eoi_rev' transition.
+                state.rev_eoi = true;
+            } else {
+                // We haven't hit the end of the search yet, so move on.
+                state.at -= 1;
+            }
+            sid
+        }
+    };
+    cache.search_start(state.at);
+    while !state.rev_eoi {
+        sid = dfa
+            .next_state(cache, sid, input.haystack()[state.at])
+            .map_err(|_| gave_up(state.at))?;
+        if sid.is_tagged() {
+            state.id = Some(sid);
+            if sid.is_start() {
+                // do nothing
+            } else if sid.is_match() {
+                state.next_match_index = Some(1);
+                let pattern = dfa.match_pattern(cache, sid, 0);
+                state.mat = Some(HalfMatch::new(pattern, state.at + 1));
+                cache.search_finish(state.at);
+                return Ok(());
+            } else if sid.is_dead() {
+                cache.search_finish(state.at);
+                return Ok(());
+            } else if sid.is_quit() {
+                cache.search_finish(state.at);
+                return Err(MatchError::quit(
+                    input.haystack()[state.at],
+                    state.at,
+                ));
+            } else {
+                debug_assert!(sid.is_unknown());
+                unreachable!("sid being unknown is a bug");
+            }
+        }
+        if state.at == input.start() {
+            break;
+        }
+        state.at -= 1;
+        cache.search_update(state.at);
+    }
+
+    let result = eoi_rev(dfa, cache, input, &mut sid, &mut state.mat);
+    state.rev_eoi = true;
+    state.id = Some(sid);
+    if state.mat.is_some() {
+        // '1' is always correct here since if we get to this point, this
+        // always corresponds to the first (index '0') match discovered at
+        // this position. So the next match to report at this position (if
+        // it exists) is at index '1'.
+        state.next_match_index = Some(1);
+    }
+    cache.search_finish(input.start());
+    result
+}
+
+#[cfg_attr(feature = "perf-inline", inline(always))]
+fn init_fwd(
+    dfa: &DFA,
+    cache: &mut Cache,
+    input: &Input<'_>,
+) -> Result<LazyStateID, MatchError> {
+    let sid = dfa.start_state_forward(cache, input)?;
+    // Start states can never be match states, since all matches are delayed
+    // by 1 byte.
+    debug_assert!(!sid.is_match());
+    Ok(sid)
+}
+
+#[cfg_attr(feature = "perf-inline", inline(always))]
+fn init_rev(
+    dfa: &DFA,
+    cache: &mut Cache,
+    input: &Input<'_>,
+) -> Result<LazyStateID, MatchError> {
+    let sid = dfa.start_state_reverse(cache, input)?;
+    // Start states can never be match states, since all matches are delayed
+    // by 1 byte.
+    debug_assert!(!sid.is_match());
+    Ok(sid)
+}
+
+#[cfg_attr(feature = "perf-inline", inline(always))]
+fn eoi_fwd(
+    dfa: &DFA,
+    cache: &mut Cache,
+    input: &Input<'_>,
+    sid: &mut LazyStateID,
+    mat: &mut Option<HalfMatch>,
+) -> Result<(), MatchError> {
+    let sp = input.get_span();
+    match input.haystack().get(sp.end) {
+        Some(&b) => {
+            *sid =
+                dfa.next_state(cache, *sid, b).map_err(|_| gave_up(sp.end))?;
+            if sid.is_match() {
+                let pattern = dfa.match_pattern(cache, *sid, 0);
+                *mat = Some(HalfMatch::new(pattern, sp.end));
+            } else if sid.is_quit() {
+                return Err(MatchError::quit(b, sp.end));
+            }
+        }
+        None => {
+            *sid = dfa
+                .next_eoi_state(cache, *sid)
+                .map_err(|_| gave_up(input.haystack().len()))?;
+            if sid.is_match() {
+                let pattern = dfa.match_pattern(cache, *sid, 0);
+                *mat = Some(HalfMatch::new(pattern, input.haystack().len()));
+            }
+            // N.B. We don't have to check 'is_quit' here because the EOI
+            // transition can never lead to a quit state.
+            debug_assert!(!sid.is_quit());
+        }
+    }
+    Ok(())
+}
+
+#[cfg_attr(feature = "perf-inline", inline(always))]
+fn eoi_rev(
+    dfa: &DFA,
+    cache: &mut Cache,
+    input: &Input<'_>,
+    sid: &mut LazyStateID,
+    mat: &mut Option<HalfMatch>,
+) -> Result<(), MatchError> {
+    let sp = input.get_span();
+    if sp.start > 0 {
+        let byte = input.haystack()[sp.start - 1];
+        *sid = dfa
+            .next_state(cache, *sid, byte)
+            .map_err(|_| gave_up(sp.start))?;
+        if sid.is_match() {
+            let pattern = dfa.match_pattern(cache, *sid, 0);
+            *mat = Some(HalfMatch::new(pattern, sp.start));
+        } else if sid.is_quit() {
+            return Err(MatchError::quit(byte, sp.start - 1));
+        }
+    } else {
+        *sid =
+            dfa.next_eoi_state(cache, *sid).map_err(|_| gave_up(sp.start))?;
+        if sid.is_match() {
+            let pattern = dfa.match_pattern(cache, *sid, 0);
+            *mat = Some(HalfMatch::new(pattern, 0));
+        }
+        // N.B. We don't have to check 'is_quit' here because the EOI
+        // transition can never lead to a quit state.
+        debug_assert!(!sid.is_quit());
+    }
+    Ok(())
+}
+
+/// Re-compute the starting state that a DFA should be in after finding a
+/// prefilter candidate match at the position `at`.
+///
+/// It is always correct to call this, but not always necessary. Namely,
+/// whenever the DFA has a universal start state, the DFA can remain in the
+/// start state that it was in when it ran the prefilter. Why? Because in that
+/// case, there is only one start state.
+///
+/// When does a DFA have a universal start state? In precisely cases where
+/// it has no look-around assertions in its prefix. So for example, `\bfoo`
+/// does not have a universal start state because the start state depends on
+/// whether the byte immediately before the start position is a word byte or
+/// not. However, `foo\b` does have a universal start state because the word
+/// boundary does not appear in the pattern's prefix.
+///
+/// So... most cases don't need this, but when a pattern doesn't have a
+/// universal start state, then after a prefilter candidate has been found, the
+/// current state *must* be re-litigated as if computing the start state at the
+/// beginning of the search because it might change. That is, not all start
+/// states are created equal.
+///
+/// Why avoid it? Because while it's not super expensive, it isn't a trivial
+/// operation to compute the start state. It is much better to avoid it and
+/// just state in the current state if you know it to be correct.
+#[cfg_attr(feature = "perf-inline", inline(always))]
+fn prefilter_restart(
+    dfa: &DFA,
+    cache: &mut Cache,
+    input: &Input<'_>,
+    at: usize,
+) -> Result<LazyStateID, MatchError> {
+    let mut input = input.clone();
+    input.set_start(at);
+    init_fwd(dfa, cache, &input)
+}
+
+/// A convenience routine for constructing a "gave up" match error.
+#[cfg_attr(feature = "perf-inline", inline(always))]
+fn gave_up(offset: usize) -> MatchError {
+    MatchError::gave_up(offset)
+}