limited.rs

/*!
This module defines two bespoke reverse DFA searching routines. (One for the
lazy DFA and one for the fully compiled DFA.) These routines differ from the
usual ones by permitting the caller to specify a minimum starting position.
That is, the search will begin at `input.end()` and will usually stop at
`input.start()`, unless `min_start > input.start()`, in which case, the search
will stop at `min_start`.

In other words, this lets you say, "no, the search must not extend past this
point, even if it's within the bounds of the given `Input`." And if the search
*does* want to go past that point, it stops and returns a "may be quadratic"
error, which indicates that the caller should retry using some other technique.

These routines specifically exist to protect against quadratic behavior when
employing the "reverse suffix" and "reverse inner" optimizations. Without the
backstop these routines provide, it is possible for parts of the haystack to
get re-scanned over and over again. The backstop not only prevents this, but
*tells you when it is happening* so that you can change the strategy.

Why can't we just use the normal search routines? We could use the normal
search routines and just set the start bound on the provided `Input` to our
`min_start` position. The problem here is that it's impossible to distinguish
between "no match because we reached the end of input" and "determined there
was no match well before the end of input." The former case is what we care
about with respect to quadratic behavior. The latter case is totally fine.

Why don't we modify the normal search routines to report the position at which
the search stops? I considered this, and I still wonder if it is indeed the
right thing to do. However, I think the straight-forward thing to do there
would be to complicate the return type signature of almost every search routine
in this crate, which I really do not want to do. It therefore might make more
sense to provide a richer way for search routines to report meta data, but that
was beyond my bandwidth to work on at the time of writing.

See the 'opt/reverse-inner' and 'opt/reverse-suffix' benchmarks in rebar for a
real demonstration of how quadratic behavior is mitigated.
*/

use crate::{
    meta::error::{RetryError, RetryQuadraticError},
    HalfMatch, Input, MatchError,
};

#[cfg(feature = "dfa-build")]
pub(crate) fn dfa_try_search_half_rev(
    dfa: &crate::dfa::dense::DFA<alloc::vec::Vec<u32>>,
    input: &Input<'_>,
    min_start: usize,
) -> Result<Option<HalfMatch>, RetryError> {
    use crate::dfa::Automaton;

    let mut mat = None;
    let mut sid = dfa.start_state_reverse(input)?;
    if input.start() == input.end() {
        dfa_eoi_rev(dfa, input, &mut sid, &mut mat)?;
        return Ok(mat);
    }
    let mut at = input.end() - 1;
    loop {
        sid = dfa.next_state(sid, input.haystack()[at]);
        if dfa.is_special_state(sid) {
            if dfa.is_match_state(sid) {
                let pattern = dfa.match_pattern(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));
            } else if dfa.is_dead_state(sid) {
                return Ok(mat);
            } else if dfa.is_quit_state(sid) {
                if mat.is_some() {
                    return Ok(mat);
                }
                return Err(MatchError::quit(input.haystack()[at], at).into());
            }
        }
        if at == input.start() {
            break;
        }
        at -= 1;
        if at < min_start {
            trace!(
                "reached position {} which is before the previous literal \
				 match, quitting to avoid quadratic behavior",
                at,
            );
            return Err(RetryError::Quadratic(RetryQuadraticError::new()));
        }
    }
    let was_dead = dfa.is_dead_state(sid);
    dfa_eoi_rev(dfa, input, &mut sid, &mut mat)?;
    // If we reach the beginning of the search and we could otherwise still
    // potentially keep matching if there was more to match, then we actually
    // return an error to indicate giving up on this optimization. Why? Because
    // we can't prove that the real match begins at where we would report it.
    //
    // This only happens when all of the following are true:
    //
    // 1) We reach the starting point of our search span.
    // 2) The match we found is before the starting point.
    // 3) The FSM reports we could possibly find a longer match.
    //
    // We need (1) because otherwise the search stopped before the starting
    // point and there is no possible way to find a more leftmost position.
    //
    // We need (2) because if the match found has an offset equal to the minimum
    // possible offset, then there is no possible more leftmost match.
    //
    // We need (3) because if the FSM couldn't continue anyway (i.e., it's in
    // a dead state), then we know we couldn't find anything more leftmost
    // than what we have. (We have to check the state we were in prior to the
    // EOI transition since the EOI transition will usually bring us to a dead
    // state by virtue of it represents the end-of-input.)
    if at == input.start()
        && mat.map_or(false, |m| m.offset() > input.start())
        && !was_dead
    {
        trace!(
            "reached beginning of search at offset {} without hitting \
             a dead state, quitting to avoid potential false positive match",
            at,
        );
        return Err(RetryError::Quadratic(RetryQuadraticError::new()));
    }
    Ok(mat)
}

#[cfg(feature = "hybrid")]
pub(crate) fn hybrid_try_search_half_rev(
    dfa: &crate::hybrid::dfa::DFA,
    cache: &mut crate::hybrid::dfa::Cache,
    input: &Input<'_>,
    min_start: usize,
) -> Result<Option<HalfMatch>, RetryError> {
    let mut mat = None;
    let mut sid = dfa.start_state_reverse(cache, input)?;
    if input.start() == input.end() {
        hybrid_eoi_rev(dfa, cache, input, &mut sid, &mut mat)?;
        return Ok(mat);
    }
    let mut at = input.end() - 1;
    loop {
        sid = dfa
            .next_state(cache, sid, input.haystack()[at])
            .map_err(|_| MatchError::gave_up(at))?;
        if sid.is_tagged() {
            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));
            } else if sid.is_dead() {
                return Ok(mat);
            } else if sid.is_quit() {
                if mat.is_some() {
                    return Ok(mat);
                }
                return Err(MatchError::quit(input.haystack()[at], at).into());
            }
        }
        if at == input.start() {
            break;
        }
        at -= 1;
        if at < min_start {
            trace!(
                "reached position {} which is before the previous literal \
				 match, quitting to avoid quadratic behavior",
                at,
            );
            return Err(RetryError::Quadratic(RetryQuadraticError::new()));
        }
    }
    let was_dead = sid.is_dead();
    hybrid_eoi_rev(dfa, cache, input, &mut sid, &mut mat)?;
    // See the comments in the full DFA routine above for why we need this.
    if at == input.start()
        && mat.map_or(false, |m| m.offset() > input.start())
        && !was_dead
    {
        trace!(
            "reached beginning of search at offset {} without hitting \
             a dead state, quitting to avoid potential false positive match",
            at,
        );
        return Err(RetryError::Quadratic(RetryQuadraticError::new()));
    }
    Ok(mat)
}

#[cfg(feature = "dfa-build")]
#[cfg_attr(feature = "perf-inline", inline(always))]
fn dfa_eoi_rev(
    dfa: &crate::dfa::dense::DFA<alloc::vec::Vec<u32>>,
    input: &Input<'_>,
    sid: &mut crate::util::primitives::StateID,
    mat: &mut Option<HalfMatch>,
) -> Result<(), MatchError> {
    use crate::dfa::Automaton;

    let sp = input.get_span();
    if sp.start > 0 {
        let byte = input.haystack()[sp.start - 1];
        *sid = dfa.next_state(*sid, byte);
        if dfa.is_match_state(*sid) {
            let pattern = dfa.match_pattern(*sid, 0);
            *mat = Some(HalfMatch::new(pattern, sp.start));
        } else if dfa.is_quit_state(*sid) {
            if mat.is_some() {
                return Ok(());
            }
            return Err(MatchError::quit(byte, sp.start - 1));
        }
    } else {
        *sid = dfa.next_eoi_state(*sid);
        if dfa.is_match_state(*sid) {
            let pattern = dfa.match_pattern(*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!(!dfa.is_quit_state(*sid));
    }
    Ok(())
}

#[cfg(feature = "hybrid")]
#[cfg_attr(feature = "perf-inline", inline(always))]
fn hybrid_eoi_rev(
    dfa: &crate::hybrid::dfa::DFA,
    cache: &mut crate::hybrid::dfa::Cache,
    input: &Input<'_>,
    sid: &mut crate::hybrid::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(|_| MatchError::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() {
            if mat.is_some() {
                return Ok(());
            }
            return Err(MatchError::quit(byte, sp.start - 1));
        }
    } else {
        *sid = dfa
            .next_eoi_state(cache, *sid)
            .map_err(|_| MatchError::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(())
}