diff options
Diffstat (limited to 'third_party/rust/regex-automata/src/util/prefilter/teddy.rs')
| -rw-r--r-- | third_party/rust/regex-automata/src/util/prefilter/teddy.rs | 160 |
1 files changed, 160 insertions, 0 deletions
diff --git a/third_party/rust/regex-automata/src/util/prefilter/teddy.rs b/third_party/rust/regex-automata/src/util/prefilter/teddy.rs new file mode 100644 index 0000000000..fc79f2b2f3 --- /dev/null +++ b/third_party/rust/regex-automata/src/util/prefilter/teddy.rs @@ -0,0 +1,160 @@ +use crate::util::{ + prefilter::PrefilterI, + search::{MatchKind, Span}, +}; + +#[derive(Clone, Debug)] +pub(crate) struct Teddy { + #[cfg(not(feature = "perf-literal-multisubstring"))] + _unused: (), + /// The actual Teddy searcher. + /// + /// Technically, it's possible that Teddy doesn't actually get used, since + /// Teddy does require its haystack to at least be of a certain size + /// (usually around the size of whatever vector is being used, so ~16 + /// or ~32 bytes). For haystacks shorter than that, the implementation + /// currently uses Rabin-Karp. + #[cfg(feature = "perf-literal-multisubstring")] + searcher: aho_corasick::packed::Searcher, + /// When running an anchored search, the packed searcher can't handle it so + /// we defer to Aho-Corasick itself. Kind of sad, but changing the packed + /// searchers to support anchored search would be difficult at worst and + /// annoying at best. Since packed searchers only apply to small numbers of + /// literals, we content ourselves that this is not much of an added cost. + /// (That packed searchers only work with a small number of literals is + /// also why we use a DFA here. Otherwise, the memory usage of a DFA would + /// likely be unacceptable.) + #[cfg(feature = "perf-literal-multisubstring")] + anchored_ac: aho_corasick::dfa::DFA, + /// The length of the smallest literal we look for. + /// + /// We use this as a heuristic to figure out whether this will be "fast" or + /// not. Generally, the longer the better, because longer needles are more + /// discriminating and thus reduce false positive rate. + #[cfg(feature = "perf-literal-multisubstring")] + minimum_len: usize, +} + +impl Teddy { + pub(crate) fn new<B: AsRef<[u8]>>( + kind: MatchKind, + needles: &[B], + ) -> Option<Teddy> { + #[cfg(not(feature = "perf-literal-multisubstring"))] + { + None + } + #[cfg(feature = "perf-literal-multisubstring")] + { + // We only really support leftmost-first semantics. In + // theory we could at least support leftmost-longest, as the + // aho-corasick crate does, but regex-automata doesn't know about + // leftmost-longest currently. + // + // And like the aho-corasick prefilter, if we're using `All` + // semantics, then we can still use leftmost semantics for a + // prefilter. (This might be a suspicious choice for the literal + // engine, which uses a prefilter as a regex engine directly, but + // that only happens when using leftmost-first semantics.) + let (packed_match_kind, ac_match_kind) = match kind { + MatchKind::LeftmostFirst | MatchKind::All => ( + aho_corasick::packed::MatchKind::LeftmostFirst, + aho_corasick::MatchKind::LeftmostFirst, + ), + }; + let minimum_len = + needles.iter().map(|n| n.as_ref().len()).min().unwrap_or(0); + let packed = aho_corasick::packed::Config::new() + .match_kind(packed_match_kind) + .builder() + .extend(needles) + .build()?; + let anchored_ac = aho_corasick::dfa::DFA::builder() + .match_kind(ac_match_kind) + .start_kind(aho_corasick::StartKind::Anchored) + .prefilter(false) + .build(needles) + .ok()?; + Some(Teddy { searcher: packed, anchored_ac, minimum_len }) + } + } +} + +impl PrefilterI for Teddy { + fn find(&self, haystack: &[u8], span: Span) -> Option<Span> { + #[cfg(not(feature = "perf-literal-multisubstring"))] + { + unreachable!() + } + #[cfg(feature = "perf-literal-multisubstring")] + { + let ac_span = + aho_corasick::Span { start: span.start, end: span.end }; + self.searcher + .find_in(haystack, ac_span) + .map(|m| Span { start: m.start(), end: m.end() }) + } + } + + fn prefix(&self, haystack: &[u8], span: Span) -> Option<Span> { + #[cfg(not(feature = "perf-literal-multisubstring"))] + { + unreachable!() + } + #[cfg(feature = "perf-literal-multisubstring")] + { + use aho_corasick::automaton::Automaton; + let input = aho_corasick::Input::new(haystack) + .anchored(aho_corasick::Anchored::Yes) + .span(span.start..span.end); + self.anchored_ac + .try_find(&input) + // OK because we build the DFA with anchored support. + .expect("aho-corasick DFA should never fail") + .map(|m| Span { start: m.start(), end: m.end() }) + } + } + + fn memory_usage(&self) -> usize { + #[cfg(not(feature = "perf-literal-multisubstring"))] + { + unreachable!() + } + #[cfg(feature = "perf-literal-multisubstring")] + { + use aho_corasick::automaton::Automaton; + self.searcher.memory_usage() + self.anchored_ac.memory_usage() + } + } + + fn is_fast(&self) -> bool { + #[cfg(not(feature = "perf-literal-multisubstring"))] + { + unreachable!() + } + #[cfg(feature = "perf-literal-multisubstring")] + { + // Teddy is usually quite fast, but I have seen some cases where + // a large number of literals can overwhelm it and make it not so + // fast. We make an educated but conservative guess at a limit, at + // which point, we're not so comfortable thinking Teddy is "fast." + // + // Well... this used to incorporate a "limit" on the *number* + // of literals, but I have since changed it to a minimum on the + // *smallest* literal. Namely, when there is a very small literal + // (1 or 2 bytes), it is far more likely that it leads to a higher + // false positive rate. (Although, of course, not always. For + // example, 'zq' is likely to have a very low false positive rate.) + // But when we have 3 bytes, we have a really good chance of being + // quite discriminatory and thus fast. + // + // We may still want to add some kind of limit on the number of + // literals here, but keep in mind that Teddy already has its own + // somewhat small limit (64 at time of writing). The main issue + // here is that if 'is_fast' is false, it opens the door for the + // reverse inner optimization to kick in. We really only want to + // resort to the reverse inner optimization if we absolutely must. + self.minimum_len >= 3 + } + } +} |