diff options
Diffstat (limited to 'third_party/rust/regex-automata/src/dfa/remapper.rs')
| -rw-r--r-- | third_party/rust/regex-automata/src/dfa/remapper.rs | 242 |
1 files changed, 242 insertions, 0 deletions
diff --git a/third_party/rust/regex-automata/src/dfa/remapper.rs b/third_party/rust/regex-automata/src/dfa/remapper.rs new file mode 100644 index 0000000000..6e49646721 --- /dev/null +++ b/third_party/rust/regex-automata/src/dfa/remapper.rs @@ -0,0 +1,242 @@ +use alloc::vec::Vec; + +use crate::util::primitives::StateID; + +/// Remappable is a tightly coupled abstraction that facilitates remapping +/// state identifiers in DFAs. +/// +/// The main idea behind remapping state IDs is that DFAs often need to check +/// if a certain state is a "special" state of some kind (like a match state) +/// during a search. Since this is extremely perf critical code, we want this +/// check to be as fast as possible. Partitioning state IDs into, for example, +/// into "non-match" and "match" states means one can tell if a state is a +/// match state via a simple comparison of the state ID. +/// +/// The issue is that during the DFA construction process, it's not +/// particularly easy to partition the states. Instead, the simplest thing is +/// to often just do a pass over all of the states and shuffle them into their +/// desired partitionings. To do that, we need a mechanism for swapping states. +/// Hence, this abstraction. +/// +/// Normally, for such little code, I would just duplicate it. But this is a +/// key optimization and the implementation is a bit subtle. So the abstraction +/// is basically a ham-fisted attempt at DRY. The only place we use this is in +/// the dense and one-pass DFAs. +/// +/// See also src/dfa/special.rs for a more detailed explanation of how dense +/// DFAs are partitioned. +pub(super) trait Remappable: core::fmt::Debug { + /// Return the total number of states. + fn state_len(&self) -> usize; + /// Return the power-of-2 exponent that yields the stride. The pertinent + /// laws here are, where N=stride2: 2^N=stride and len(alphabet) <= stride. + fn stride2(&self) -> usize; + /// Swap the states pointed to by the given IDs. The underlying finite + /// state machine should be mutated such that all of the transitions in + /// `id1` are now in the memory region where the transitions for `id2` + /// were, and all of the transitions in `id2` are now in the memory region + /// where the transitions for `id1` were. + /// + /// Essentially, this "moves" `id1` to `id2` and `id2` to `id1`. + /// + /// It is expected that, after calling this, the underlying value will be + /// left in an inconsistent state, since any other transitions pointing to, + /// e.g., `id1` need to be updated to point to `id2`, since that's where + /// `id1` moved to. + /// + /// In order to "fix" the underlying inconsistent state, a `Remapper` + /// should be used to guarantee that `remap` is called at the appropriate + /// time. + fn swap_states(&mut self, id1: StateID, id2: StateID); + /// This must remap every single state ID in the underlying value according + /// to the function given. For example, in a DFA, this should remap every + /// transition and every starting state ID. + fn remap(&mut self, map: impl Fn(StateID) -> StateID); +} + +/// Remapper is an abstraction the manages the remapping of state IDs in a +/// finite state machine. This is useful when one wants to shuffle states into +/// different positions in the machine. +/// +/// One of the key complexities this manages is the ability to correctly move +/// one state multiple times. +/// +/// Once shuffling is complete, `remap` must be called, which will rewrite +/// all pertinent transitions to updated state IDs. Neglecting to call `remap` +/// will almost certainly result in a corrupt machine. +#[derive(Debug)] +pub(super) struct Remapper { + /// A map from the index of a state to its pre-multiplied identifier. + /// + /// When a state is swapped with another, then their corresponding + /// locations in this map are also swapped. Thus, its new position will + /// still point to its old pre-multiplied StateID. + /// + /// While there is a bit more to it, this then allows us to rewrite the + /// state IDs in a DFA's transition table in a single pass. This is done + /// by iterating over every ID in this map, then iterating over each + /// transition for the state at that ID and re-mapping the transition from + /// `old_id` to `map[dfa.to_index(old_id)]`. That is, we find the position + /// in this map where `old_id` *started*, and set it to where it ended up + /// after all swaps have been completed. + map: Vec<StateID>, + /// A mapper from state index to state ID (and back). + idxmap: IndexMapper, +} + +impl Remapper { + /// Create a new remapper from the given remappable implementation. The + /// remapper can then be used to swap states. The remappable value given + /// here must the same one given to `swap` and `remap`. + pub(super) fn new(r: &impl Remappable) -> Remapper { + let idxmap = IndexMapper { stride2: r.stride2() }; + let map = (0..r.state_len()).map(|i| idxmap.to_state_id(i)).collect(); + Remapper { map, idxmap } + } + + /// Swap two states. Once this is called, callers must follow through to + /// call `remap`, or else it's possible for the underlying remappable + /// value to be in a corrupt state. + pub(super) fn swap( + &mut self, + r: &mut impl Remappable, + id1: StateID, + id2: StateID, + ) { + if id1 == id2 { + return; + } + r.swap_states(id1, id2); + self.map.swap(self.idxmap.to_index(id1), self.idxmap.to_index(id2)); + } + + /// Complete the remapping process by rewriting all state IDs in the + /// remappable value according to the swaps performed. + pub(super) fn remap(mut self, r: &mut impl Remappable) { + // Update the map to account for states that have been swapped + // multiple times. For example, if (A, C) and (C, G) are swapped, then + // transitions previously pointing to A should now point to G. But if + // we don't update our map, they will erroneously be set to C. All we + // do is follow the swaps in our map until we see our original state + // ID. + // + // The intuition here is to think about how changes are made to the + // map: only through pairwise swaps. That means that starting at any + // given state, it is always possible to find the loop back to that + // state by following the swaps represented in the map (which might be + // 0 swaps). + // + // We are also careful to clone the map before starting in order to + // freeze it. We use the frozen map to find our loops, since we need to + // update our map as well. Without freezing it, our updates could break + // the loops referenced above and produce incorrect results. + let oldmap = self.map.clone(); + for i in 0..r.state_len() { + let cur_id = self.idxmap.to_state_id(i); + let mut new_id = oldmap[i]; + if cur_id == new_id { + continue; + } + loop { + let id = oldmap[self.idxmap.to_index(new_id)]; + if cur_id == id { + self.map[i] = new_id; + break; + } + new_id = id; + } + } + r.remap(|next| self.map[self.idxmap.to_index(next)]); + } +} + +/// A simple type for mapping between state indices and state IDs. +/// +/// The reason why this exists is because state IDs are "premultiplied." That +/// is, in order to get to the transitions for a particular state, one need +/// only use the state ID as-is, instead of having to multiple it by transition +/// table's stride. +/// +/// The downside of this is that it's inconvenient to map between state IDs +/// using a dense map, e.g., Vec<StateID>. That's because state IDs look like +/// `0`, `0+stride`, `0+2*stride`, `0+3*stride`, etc., instead of `0`, `1`, +/// `2`, `3`, etc. +/// +/// Since our state IDs are premultiplied, we can convert back-and-forth +/// between IDs and indices by simply unmultiplying the IDs and multiplying the +/// indices. +#[derive(Debug)] +struct IndexMapper { + /// The power of 2 corresponding to the stride of the corresponding + /// transition table. 'id >> stride2' de-multiplies an ID while 'index << + /// stride2' pre-multiplies an index to an ID. + stride2: usize, +} + +impl IndexMapper { + /// Convert a state ID to a state index. + fn to_index(&self, id: StateID) -> usize { + id.as_usize() >> self.stride2 + } + + /// Convert a state index to a state ID. + fn to_state_id(&self, index: usize) -> StateID { + // CORRECTNESS: If the given index is not valid, then it is not + // required for this to panic or return a valid state ID. We'll "just" + // wind up with panics or silent logic errors at some other point. + StateID::new_unchecked(index << self.stride2) + } +} + +#[cfg(feature = "dfa-build")] +mod dense { + use crate::{dfa::dense::OwnedDFA, util::primitives::StateID}; + + use super::Remappable; + + impl Remappable for OwnedDFA { + fn state_len(&self) -> usize { + OwnedDFA::state_len(self) + } + + fn stride2(&self) -> usize { + OwnedDFA::stride2(self) + } + + fn swap_states(&mut self, id1: StateID, id2: StateID) { + OwnedDFA::swap_states(self, id1, id2) + } + + fn remap(&mut self, map: impl Fn(StateID) -> StateID) { + OwnedDFA::remap(self, map) + } + } +} + +#[cfg(feature = "dfa-onepass")] +mod onepass { + use crate::{dfa::onepass::DFA, util::primitives::StateID}; + + use super::Remappable; + + impl Remappable for DFA { + fn state_len(&self) -> usize { + DFA::state_len(self) + } + + fn stride2(&self) -> usize { + // We don't do pre-multiplication for the one-pass DFA, so + // returning 0 has the effect of making state IDs and state indices + // equivalent. + 0 + } + + fn swap_states(&mut self, id1: StateID, id2: StateID) { + DFA::swap_states(self, id1, id2) + } + + fn remap(&mut self, map: impl Fn(StateID) -> StateID) { + DFA::remap(self, map) + } + } +} |