Vendor things

third-party/vendor/regex-automata/src/nfa/thompson/map.rs

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+// This module contains a couple simple and purpose built hash maps. The key
+// trade off they make is that they serve as caches rather than true maps. That
+// is, inserting a new entry may cause eviction of another entry. This gives
+// us two things. First, there's less overhead associated with inserts and
+// lookups. Secondly, it lets us control our memory usage.
+//
+// These maps are used in some fairly hot code when generating NFA states for
+// large Unicode character classes.
+//
+// Instead of exposing a rich hashmap entry API, we just permit the caller to
+// produce a hash of the key directly. The hash can then be reused for both
+// lookups and insertions at the cost of leaking abstraction a bit. But these
+// are for internal use only, so it's fine.
+//
+// The Utf8BoundedMap is used for Daciuk's algorithm for constructing a
+// (almost) minimal DFA for large Unicode character classes in linear time.
+// (Daciuk's algorithm is always used when compiling forward NFAs. For reverse
+// NFAs, it's only used when the compiler is configured to 'shrink' the NFA,
+// since there's a bit more expense in the reverse direction.)
+//
+// The Utf8SuffixMap is used when compiling large Unicode character classes for
+// reverse NFAs when 'shrink' is disabled. Specifically, it augments the naive
+// construction of UTF-8 automata by caching common suffixes. This doesn't
+// get the same space savings as Daciuk's algorithm, but it's basically as
+// fast as the naive approach and typically winds up using less memory (since
+// it generates smaller NFAs) despite the presence of the cache.
+//
+// These maps effectively represent caching mechanisms for sparse and
+// byte-range NFA states, respectively. The former represents a single NFA
+// state with many transitions of equivalent priority while the latter
+// represents a single NFA state with a single transition. (Neither state ever
+// has or is an epsilon transition.) Thus, they have different key types. It's
+// likely we could make one generic map, but the machinery didn't seem worth
+// it. They are simple enough.
+
+use alloc::{vec, vec::Vec};
+
+use crate::{
+    nfa::thompson::Transition,
+    util::{
+        int::{Usize, U64},
+        primitives::StateID,
+    },
+};
+
+// Basic FNV-1a hash constants as described in:
+// https://en.wikipedia.org/wiki/Fowler%E2%80%93Noll%E2%80%93Vo_hash_function
+const PRIME: u64 = 1099511628211;
+const INIT: u64 = 14695981039346656037;
+
+/// A bounded hash map where the key is a sequence of NFA transitions and the
+/// value is a pre-existing NFA state ID.
+///
+/// std's hashmap can be used for this, however, this map has two important
+/// advantages. Firstly, it has lower overhead. Secondly, it permits us to
+/// control our memory usage by limited the number of slots. In general, the
+/// cost here is that this map acts as a cache. That is, inserting a new entry
+/// may remove an old entry. We are okay with this, since it does not impact
+/// correctness in the cases where it is used. The only effect that dropping
+/// states from the cache has is that the resulting NFA generated may be bigger
+/// than it otherwise would be.
+///
+/// This improves benchmarks that compile large Unicode character classes,
+/// since it makes the generation of (almost) minimal UTF-8 automaton faster.
+/// Specifically, one could observe the difference with std's hashmap via
+/// something like the following benchmark:
+///
+///   hyperfine "regex-cli debug thompson -qr --captures none '\w{90} ecurB'"
+///
+/// But to observe that difference, you'd have to modify the code to use
+/// std's hashmap.
+///
+/// It is quite possible that there is a better way to approach this problem.
+/// For example, if there happens to be a very common state that collides with
+/// a lot of less frequent states, then we could wind up with very poor caching
+/// behavior. Alas, the effectiveness of this cache has not been measured.
+/// Instead, ad hoc experiments suggest that it is "good enough." Additional
+/// smarts (such as an LRU eviction policy) have to be weighed against the
+/// amount of extra time they cost.
+#[derive(Clone, Debug)]
+pub struct Utf8BoundedMap {
+    /// The current version of this map. Only entries with matching versions
+    /// are considered during lookups. If an entry is found with a mismatched
+    /// version, then the map behaves as if the entry does not exist.
+    ///
+    /// This makes it possible to clear the map by simply incrementing the
+    /// version number instead of actually deallocating any storage.
+    version: u16,
+    /// The total number of entries this map can store.
+    capacity: usize,
+    /// The actual entries, keyed by hash. Collisions between different states
+    /// result in the old state being dropped.
+    map: Vec<Utf8BoundedEntry>,
+}
+
+/// An entry in this map.
+#[derive(Clone, Debug, Default)]
+struct Utf8BoundedEntry {
+    /// The version of the map used to produce this entry. If this entry's
+    /// version does not match the current version of the map, then the map
+    /// should behave as if this entry does not exist.
+    version: u16,
+    /// The key, which is a sorted sequence of non-overlapping NFA transitions.
+    key: Vec<Transition>,
+    /// The state ID corresponding to the state containing the transitions in
+    /// this entry.
+    val: StateID,
+}
+
+impl Utf8BoundedMap {
+    /// Create a new bounded map with the given capacity. The map will never
+    /// grow beyond the given size.
+    ///
+    /// Note that this does not allocate. Instead, callers must call `clear`
+    /// before using this map. `clear` will allocate space if necessary.
+    ///
+    /// This avoids the need to pay for the allocation of this map when
+    /// compiling regexes that lack large Unicode character classes.
+    pub fn new(capacity: usize) -> Utf8BoundedMap {
+        assert!(capacity > 0);
+        Utf8BoundedMap { version: 0, capacity, map: vec![] }
+    }
+
+    /// Clear this map of all entries, but permit the reuse of allocation
+    /// if possible.
+    ///
+    /// This must be called before the map can be used.
+    pub fn clear(&mut self) {
+        if self.map.is_empty() {
+            self.map = vec![Utf8BoundedEntry::default(); self.capacity];
+        } else {
+            self.version = self.version.wrapping_add(1);
+            // If we loop back to version 0, then we forcefully clear the
+            // entire map. Otherwise, it might be possible to incorrectly
+            // match entries used to generate other NFAs.
+            if self.version == 0 {
+                self.map = vec![Utf8BoundedEntry::default(); self.capacity];
+            }
+        }
+    }
+
+    /// Return a hash of the given transitions.
+    pub fn hash(&self, key: &[Transition]) -> usize {
+        let mut h = INIT;
+        for t in key {
+            h = (h ^ u64::from(t.start)).wrapping_mul(PRIME);
+            h = (h ^ u64::from(t.end)).wrapping_mul(PRIME);
+            h = (h ^ t.next.as_u64()).wrapping_mul(PRIME);
+        }
+        (h % self.map.len().as_u64()).as_usize()
+    }
+
+    /// Retrieve the cached state ID corresponding to the given key. The hash
+    /// given must have been computed with `hash` using the same key value.
+    ///
+    /// If there is no cached state with the given transitions, then None is
+    /// returned.
+    pub fn get(&mut self, key: &[Transition], hash: usize) -> Option<StateID> {
+        let entry = &self.map[hash];
+        if entry.version != self.version {
+            return None;
+        }
+        // There may be a hash collision, so we need to confirm real equality.
+        if entry.key != key {
+            return None;
+        }
+        Some(entry.val)
+    }
+
+    /// Add a cached state to this map with the given key. Callers should
+    /// ensure that `state_id` points to a state that contains precisely the
+    /// NFA transitions given.
+    ///
+    /// `hash` must have been computed using the `hash` method with the same
+    /// key.
+    pub fn set(
+        &mut self,
+        key: Vec<Transition>,
+        hash: usize,
+        state_id: StateID,
+    ) {
+        self.map[hash] =
+            Utf8BoundedEntry { version: self.version, key, val: state_id };
+    }
+}
+
+/// A cache of suffixes used to modestly compress UTF-8 automata for large
+/// Unicode character classes.
+#[derive(Clone, Debug)]
+pub struct Utf8SuffixMap {
+    /// The current version of this map. Only entries with matching versions
+    /// are considered during lookups. If an entry is found with a mismatched
+    /// version, then the map behaves as if the entry does not exist.
+    version: u16,
+    /// The total number of entries this map can store.
+    capacity: usize,
+    /// The actual entries, keyed by hash. Collisions between different states
+    /// result in the old state being dropped.
+    map: Vec<Utf8SuffixEntry>,
+}
+
+/// A key that uniquely identifies an NFA state. It is a triple that represents
+/// a transition from one state for a particular byte range.
+#[derive(Clone, Debug, Default, Eq, PartialEq)]
+pub struct Utf8SuffixKey {
+    pub from: StateID,
+    pub start: u8,
+    pub end: u8,
+}
+
+/// An entry in this map.
+#[derive(Clone, Debug, Default)]
+struct Utf8SuffixEntry {
+    /// The version of the map used to produce this entry. If this entry's
+    /// version does not match the current version of the map, then the map
+    /// should behave as if this entry does not exist.
+    version: u16,
+    /// The key, which consists of a transition in a particular state.
+    key: Utf8SuffixKey,
+    /// The identifier that the transition in the key maps to.
+    val: StateID,
+}
+
+impl Utf8SuffixMap {
+    /// Create a new bounded map with the given capacity. The map will never
+    /// grow beyond the given size.
+    ///
+    /// Note that this does not allocate. Instead, callers must call `clear`
+    /// before using this map. `clear` will allocate space if necessary.
+    ///
+    /// This avoids the need to pay for the allocation of this map when
+    /// compiling regexes that lack large Unicode character classes.
+    pub fn new(capacity: usize) -> Utf8SuffixMap {
+        assert!(capacity > 0);
+        Utf8SuffixMap { version: 0, capacity, map: vec![] }
+    }
+
+    /// Clear this map of all entries, but permit the reuse of allocation
+    /// if possible.
+    ///
+    /// This must be called before the map can be used.
+    pub fn clear(&mut self) {
+        if self.map.is_empty() {
+            self.map = vec![Utf8SuffixEntry::default(); self.capacity];
+        } else {
+            self.version = self.version.wrapping_add(1);
+            if self.version == 0 {
+                self.map = vec![Utf8SuffixEntry::default(); self.capacity];
+            }
+        }
+    }
+
+    /// Return a hash of the given transition.
+    pub fn hash(&self, key: &Utf8SuffixKey) -> usize {
+        // Basic FNV-1a hash as described:
+        // https://en.wikipedia.org/wiki/Fowler%E2%80%93Noll%E2%80%93Vo_hash_function
+        const PRIME: u64 = 1099511628211;
+        const INIT: u64 = 14695981039346656037;
+
+        let mut h = INIT;
+        h = (h ^ key.from.as_u64()).wrapping_mul(PRIME);
+        h = (h ^ u64::from(key.start)).wrapping_mul(PRIME);
+        h = (h ^ u64::from(key.end)).wrapping_mul(PRIME);
+        (h % self.map.len().as_u64()).as_usize()
+    }
+
+    /// Retrieve the cached state ID corresponding to the given key. The hash
+    /// given must have been computed with `hash` using the same key value.
+    ///
+    /// If there is no cached state with the given key, then None is returned.
+    pub fn get(
+        &mut self,
+        key: &Utf8SuffixKey,
+        hash: usize,
+    ) -> Option<StateID> {
+        let entry = &self.map[hash];
+        if entry.version != self.version {
+            return None;
+        }
+        if key != &entry.key {
+            return None;
+        }
+        Some(entry.val)
+    }
+
+    /// Add a cached state to this map with the given key. Callers should
+    /// ensure that `state_id` points to a state that contains precisely the
+    /// NFA transition given.
+    ///
+    /// `hash` must have been computed using the `hash` method with the same
+    /// key.
+    pub fn set(&mut self, key: Utf8SuffixKey, hash: usize, state_id: StateID) {
+        self.map[hash] =
+            Utf8SuffixEntry { version: self.version, key, val: state_id };
+    }
+}