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John Doty 2024-03-08 11:03:01 -08:00
parent 5deceec006
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third-party/vendor/regex-automata/src/dfa/determinize.rs vendored Normal file
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use alloc::{collections::BTreeMap, vec::Vec};
use crate::{
dfa::{
dense::{self, BuildError},
DEAD,
},
nfa::thompson,
util::{
self,
alphabet::{self, ByteSet},
determinize::{State, StateBuilderEmpty, StateBuilderNFA},
primitives::{PatternID, StateID},
search::{Anchored, MatchKind},
sparse_set::SparseSets,
start::Start,
},
};
/// A builder for configuring and running a DFA determinizer.
#[derive(Clone, Debug)]
pub(crate) struct Config {
match_kind: MatchKind,
quit: ByteSet,
dfa_size_limit: Option<usize>,
determinize_size_limit: Option<usize>,
}
impl Config {
/// Create a new default config for a determinizer. The determinizer may be
/// configured before calling `run`.
pub fn new() -> Config {
Config {
match_kind: MatchKind::LeftmostFirst,
quit: ByteSet::empty(),
dfa_size_limit: None,
determinize_size_limit: None,
}
}
/// Run determinization on the given NFA and write the resulting DFA into
/// the one given. The DFA given should be initialized but otherwise empty.
/// "Initialized" means that it is setup to handle the NFA's byte classes,
/// number of patterns and whether to build start states for each pattern.
pub fn run(
&self,
nfa: &thompson::NFA,
dfa: &mut dense::OwnedDFA,
) -> Result<(), BuildError> {
let dead = State::dead();
let quit = State::dead();
let mut cache = StateMap::default();
// We only insert the dead state here since its representation is
// identical to the quit state. And we never want anything pointing
// to the quit state other than specific transitions derived from the
// determinizer's configured "quit" bytes.
//
// We do put the quit state into 'builder_states' below. This ensures
// that a proper DFA state ID is allocated for it, and that no other
// DFA state uses the "location after the DEAD state." That is, it
// is assumed that the quit state is always the state immediately
// following the DEAD state.
cache.insert(dead.clone(), DEAD);
let runner = Runner {
config: self.clone(),
nfa,
dfa,
builder_states: alloc::vec![dead, quit],
cache,
memory_usage_state: 0,
sparses: SparseSets::new(nfa.states().len()),
stack: alloc::vec![],
scratch_state_builder: StateBuilderEmpty::new(),
};
runner.run()
}
/// The match semantics to use for determinization.
///
/// MatchKind::All corresponds to the standard textbook construction.
/// All possible match states are represented in the DFA.
/// MatchKind::LeftmostFirst permits greediness and otherwise tries to
/// simulate the match semantics of backtracking regex engines. Namely,
/// only a subset of match states are built, and dead states are used to
/// stop searches with an unanchored prefix.
///
/// The default is MatchKind::LeftmostFirst.
pub fn match_kind(&mut self, kind: MatchKind) -> &mut Config {
self.match_kind = kind;
self
}
/// The set of bytes to use that will cause the DFA to enter a quit state,
/// stop searching and return an error. By default, this is empty.
pub fn quit(&mut self, set: ByteSet) -> &mut Config {
self.quit = set;
self
}
/// The limit, in bytes of the heap, that the DFA is permitted to use. This
/// does not include the auxiliary heap storage used by determinization.
pub fn dfa_size_limit(&mut self, bytes: Option<usize>) -> &mut Config {
self.dfa_size_limit = bytes;
self
}
/// The limit, in bytes of the heap, that determinization itself is allowed
/// to use. This does not include the size of the DFA being built.
pub fn determinize_size_limit(
&mut self,
bytes: Option<usize>,
) -> &mut Config {
self.determinize_size_limit = bytes;
self
}
}
/// The actual implementation of determinization that converts an NFA to a DFA
/// through powerset construction.
///
/// This determinizer roughly follows the typical powerset construction, where
/// each DFA state is comprised of one or more NFA states. In the worst case,
/// there is one DFA state for every possible combination of NFA states. In
/// practice, this only happens in certain conditions, typically when there are
/// bounded repetitions.
///
/// The main differences between this implementation and typical deteminization
/// are that this implementation delays matches by one state and hackily makes
/// look-around work. Comments below attempt to explain this.
///
/// The lifetime variable `'a` refers to the lifetime of the NFA or DFA,
/// whichever is shorter.
#[derive(Debug)]
struct Runner<'a> {
/// The configuration used to initialize determinization.
config: Config,
/// The NFA we're converting into a DFA.
nfa: &'a thompson::NFA,
/// The DFA we're building.
dfa: &'a mut dense::OwnedDFA,
/// Each DFA state being built is defined as an *ordered* set of NFA
/// states, along with some meta facts about the ordered set of NFA states.
///
/// This is never empty. The first state is always a dummy state such that
/// a state id == 0 corresponds to a dead state. The second state is always
/// the quit state.
///
/// Why do we have states in both a `Vec` and in a cache map below?
/// Well, they serve two different roles based on access patterns.
/// `builder_states` is the canonical home of each state, and provides
/// constant random access by a DFA state's ID. The cache map below, on
/// the other hand, provides a quick way of searching for identical DFA
/// states by using the DFA state as a key in the map. Of course, we use
/// reference counting to avoid actually duplicating the state's data
/// itself. (Although this has never been benchmarked.) Note that the cache
/// map does not give us full minimization; it just lets us avoid some very
/// obvious redundant states.
///
/// Note that the index into this Vec isn't quite the DFA's state ID.
/// Rather, it's just an index. To get the state ID, you have to multiply
/// it by the DFA's stride. That's done by self.dfa.from_index. And the
/// inverse is self.dfa.to_index.
///
/// Moreover, DFA states don't usually retain the IDs assigned to them
/// by their position in this Vec. After determinization completes,
/// states are shuffled around to support other optimizations. See the
/// sibling 'special' module for more details on that. (The reason for
/// mentioning this is that if you print out the DFA for debugging during
/// determinization, and then print out the final DFA after it is fully
/// built, then the state IDs likely won't match up.)
builder_states: Vec<State>,
/// A cache of DFA states that already exist and can be easily looked up
/// via ordered sets of NFA states.
///
/// See `builder_states` docs for why we store states in two different
/// ways.
cache: StateMap,
/// The memory usage, in bytes, used by builder_states and cache. We track
/// this as new states are added since states use a variable amount of
/// heap. Tracking this as we add states makes it possible to compute the
/// total amount of memory used by the determinizer in constant time.
memory_usage_state: usize,
/// A pair of sparse sets for tracking ordered sets of NFA state IDs.
/// These are reused throughout determinization. A bounded sparse set
/// gives us constant time insertion, membership testing and clearing.
sparses: SparseSets,
/// Scratch space for a stack of NFA states to visit, for depth first
/// visiting without recursion.
stack: Vec<StateID>,
/// Scratch space for storing an ordered sequence of NFA states, for
/// amortizing allocation. This is principally useful for when we avoid
/// adding a new DFA state since it already exists. In order to detect this
/// case though, we still need an ordered set of NFA state IDs. So we use
/// this space to stage that ordered set before we know whether we need to
/// create a new DFA state or not.
scratch_state_builder: StateBuilderEmpty,
}
/// A map from states to state identifiers. When using std, we use a standard
/// hashmap, since it's a bit faster for this use case. (Other maps, like
/// one's based on FNV, have not yet been benchmarked.)
///
/// The main purpose of this map is to reuse states where possible. This won't
/// fully minimize the DFA, but it works well in a lot of cases.
#[cfg(feature = "std")]
type StateMap = std::collections::HashMap<State, StateID>;
#[cfg(not(feature = "std"))]
type StateMap = BTreeMap<State, StateID>;
impl<'a> Runner<'a> {
/// Build the DFA. If there was a problem constructing the DFA (e.g., if
/// the chosen state identifier representation is too small), then an error
/// is returned.
fn run(mut self) -> Result<(), BuildError> {
if self.nfa.look_set_any().contains_word_unicode()
&& !self.config.quit.contains_range(0x80, 0xFF)
{
return Err(BuildError::unsupported_dfa_word_boundary_unicode());
}
// A sequence of "representative" bytes drawn from each equivalence
// class. These representative bytes are fed to the NFA to compute
// state transitions. This allows us to avoid re-computing state
// transitions for bytes that are guaranteed to produce identical
// results. Since computing the representatives needs to do a little
// work, we do it once here because we'll be iterating over them a lot.
let representatives: Vec<alphabet::Unit> =
self.dfa.byte_classes().representatives(..).collect();
// The set of all DFA state IDs that still need to have their
// transitions set. We start by seeding this with all starting states.
let mut uncompiled = alloc::vec![];
self.add_all_starts(&mut uncompiled)?;
while let Some(dfa_id) = uncompiled.pop() {
for &unit in &representatives {
if unit.as_u8().map_or(false, |b| self.config.quit.contains(b))
{
continue;
}
// In many cases, the state we transition to has already been
// computed. 'cached_state' will do the minimal amount of work
// to check this, and if it exists, immediately return an
// already existing state ID.
let (next_dfa_id, is_new) = self.cached_state(dfa_id, unit)?;
self.dfa.set_transition(dfa_id, unit, next_dfa_id);
// If the state ID we got back is newly created, then we need
// to compile it, so add it to our uncompiled frontier.
if is_new {
uncompiled.push(next_dfa_id);
}
}
}
debug!(
"determinization complete, memory usage: {}, \
dense DFA size: {}, \
is reverse? {}",
self.memory_usage(),
self.dfa.memory_usage(),
self.nfa.is_reverse(),
);
// A map from DFA state ID to one or more NFA match IDs. Each NFA match
// ID corresponds to a distinct regex pattern that matches in the state
// corresponding to the key.
let mut matches: BTreeMap<StateID, Vec<PatternID>> = BTreeMap::new();
self.cache.clear();
#[cfg(feature = "logging")]
let mut total_pat_len = 0;
for (i, state) in self.builder_states.into_iter().enumerate() {
if let Some(pat_ids) = state.match_pattern_ids() {
let id = self.dfa.to_state_id(i);
log! {
total_pat_len += pat_ids.len();
}
matches.insert(id, pat_ids);
}
}
log! {
use core::mem::size_of;
let per_elem = size_of::<StateID>() + size_of::<Vec<PatternID>>();
let pats = total_pat_len * size_of::<PatternID>();
let mem = (matches.len() * per_elem) + pats;
log::debug!("matches map built, memory usage: {}", mem);
}
// At this point, we shuffle the "special" states in the final DFA.
// This permits a DFA's match loop to detect a match condition (among
// other things) by merely inspecting the current state's identifier,
// and avoids the need for any additional auxiliary storage.
self.dfa.shuffle(matches)?;
Ok(())
}
/// Return the identifier for the next DFA state given an existing DFA
/// state and an input byte. If the next DFA state already exists, then
/// return its identifier from the cache. Otherwise, build the state, cache
/// it and return its identifier.
///
/// This routine returns a boolean indicating whether a new state was
/// built. If a new state is built, then the caller needs to add it to its
/// frontier of uncompiled DFA states to compute transitions for.
fn cached_state(
&mut self,
dfa_id: StateID,
unit: alphabet::Unit,
) -> Result<(StateID, bool), BuildError> {
// Compute the set of all reachable NFA states, including epsilons.
let empty_builder = self.get_state_builder();
let builder = util::determinize::next(
self.nfa,
self.config.match_kind,
&mut self.sparses,
&mut self.stack,
&self.builder_states[self.dfa.to_index(dfa_id)],
unit,
empty_builder,
);
self.maybe_add_state(builder)
}
/// Compute the set of DFA start states and add their identifiers in
/// 'dfa_state_ids' (no duplicates are added).
fn add_all_starts(
&mut self,
dfa_state_ids: &mut Vec<StateID>,
) -> Result<(), BuildError> {
// These should be the first states added.
assert!(dfa_state_ids.is_empty());
// We only want to add (un)anchored starting states that is consistent
// with our DFA's configuration. Unconditionally adding both (although
// it is the default) can make DFAs quite a bit bigger.
if self.dfa.start_kind().has_unanchored() {
self.add_start_group(Anchored::No, dfa_state_ids)?;
}
if self.dfa.start_kind().has_anchored() {
self.add_start_group(Anchored::Yes, dfa_state_ids)?;
}
// I previously has an 'assert' here checking that either
// 'dfa_state_ids' was non-empty, or the NFA had zero patterns. But it
// turns out this isn't always true. For example, the NFA might have
// one or more patterns but where all such patterns are just 'fail'
// states. These will ultimately just compile down to DFA dead states,
// and since the dead state was added earlier, no new DFA states are
// added. And thus, it is valid and okay for 'dfa_state_ids' to be
// empty even if there are a non-zero number of patterns in the NFA.
// We only need to compute anchored start states for each pattern if it
// was requested to do so.
if self.dfa.starts_for_each_pattern() {
for pid in self.nfa.patterns() {
self.add_start_group(Anchored::Pattern(pid), dfa_state_ids)?;
}
}
Ok(())
}
/// Add a group of start states for the given match pattern ID. Any new
/// DFA states added are pushed on to 'dfa_state_ids'. (No duplicates are
/// pushed.)
///
/// When pattern_id is None, then this will compile a group of unanchored
/// start states (if the DFA is unanchored). When the pattern_id is
/// present, then this will compile a group of anchored start states that
/// only match the given pattern.
///
/// This panics if `anchored` corresponds to an invalid pattern ID.
fn add_start_group(
&mut self,
anchored: Anchored,
dfa_state_ids: &mut Vec<StateID>,
) -> Result<(), BuildError> {
let nfa_start = match anchored {
Anchored::No => self.nfa.start_unanchored(),
Anchored::Yes => self.nfa.start_anchored(),
Anchored::Pattern(pid) => {
self.nfa.start_pattern(pid).expect("valid pattern ID")
}
};
// When compiling start states, we're careful not to build additional
// states that aren't necessary. For example, if the NFA has no word
// boundary assertion, then there's no reason to have distinct start
// states for 'NonWordByte' and 'WordByte' starting configurations.
// Instead, the 'WordByte' starting configuration can just point
// directly to the start state for the 'NonWordByte' config.
//
// Note though that we only need to care about assertions in the prefix
// of an NFA since this only concerns the starting states. (Actually,
// the most precisely thing we could do it is look at the prefix
// assertions of each pattern when 'anchored == Anchored::Pattern',
// and then only compile extra states if the prefix is non-empty.) But
// we settle for simplicity here instead of absolute minimalism. It is
// somewhat rare, after all, for multiple patterns in the same regex to
// have different prefix look-arounds.
let (id, is_new) =
self.add_one_start(nfa_start, Start::NonWordByte)?;
self.dfa.set_start_state(anchored, Start::NonWordByte, id);
if is_new {
dfa_state_ids.push(id);
}
if !self.nfa.look_set_prefix_any().contains_word() {
self.dfa.set_start_state(anchored, Start::WordByte, id);
} else {
let (id, is_new) =
self.add_one_start(nfa_start, Start::WordByte)?;
self.dfa.set_start_state(anchored, Start::WordByte, id);
if is_new {
dfa_state_ids.push(id);
}
}
if !self.nfa.look_set_prefix_any().contains_anchor() {
self.dfa.set_start_state(anchored, Start::Text, id);
self.dfa.set_start_state(anchored, Start::LineLF, id);
self.dfa.set_start_state(anchored, Start::LineCR, id);
self.dfa.set_start_state(
anchored,
Start::CustomLineTerminator,
id,
);
} else {
let (id, is_new) = self.add_one_start(nfa_start, Start::Text)?;
self.dfa.set_start_state(anchored, Start::Text, id);
if is_new {
dfa_state_ids.push(id);
}
let (id, is_new) = self.add_one_start(nfa_start, Start::LineLF)?;
self.dfa.set_start_state(anchored, Start::LineLF, id);
if is_new {
dfa_state_ids.push(id);
}
let (id, is_new) = self.add_one_start(nfa_start, Start::LineCR)?;
self.dfa.set_start_state(anchored, Start::LineCR, id);
if is_new {
dfa_state_ids.push(id);
}
let (id, is_new) =
self.add_one_start(nfa_start, Start::CustomLineTerminator)?;
self.dfa.set_start_state(
anchored,
Start::CustomLineTerminator,
id,
);
if is_new {
dfa_state_ids.push(id);
}
}
Ok(())
}
/// Add a new DFA start state corresponding to the given starting NFA
/// state, and the starting search configuration. (The starting search
/// configuration essentially tells us which look-behind assertions are
/// true for this particular state.)
///
/// The boolean returned indicates whether the state ID returned is a newly
/// created state, or a previously cached state.
fn add_one_start(
&mut self,
nfa_start: StateID,
start: Start,
) -> Result<(StateID, bool), BuildError> {
// Compute the look-behind assertions that are true in this starting
// configuration, and the determine the epsilon closure. While
// computing the epsilon closure, we only follow condiional epsilon
// transitions that satisfy the look-behind assertions in 'look_have'.
let mut builder_matches = self.get_state_builder().into_matches();
util::determinize::set_lookbehind_from_start(
self.nfa,
&start,
&mut builder_matches,
);
self.sparses.set1.clear();
util::determinize::epsilon_closure(
self.nfa,
nfa_start,
builder_matches.look_have(),
&mut self.stack,
&mut self.sparses.set1,
);
let mut builder = builder_matches.into_nfa();
util::determinize::add_nfa_states(
&self.nfa,
&self.sparses.set1,
&mut builder,
);
self.maybe_add_state(builder)
}
/// Adds the given state to the DFA being built depending on whether it
/// already exists in this determinizer's cache.
///
/// If it does exist, then the memory used by 'state' is put back into the
/// determinizer and the previously created state's ID is returned. (Along
/// with 'false', indicating that no new state was added.)
///
/// If it does not exist, then the state is added to the DFA being built
/// and a fresh ID is allocated (if ID allocation fails, then an error is
/// returned) and returned. (Along with 'true', indicating that a new state
/// was added.)
fn maybe_add_state(
&mut self,
builder: StateBuilderNFA,
) -> Result<(StateID, bool), BuildError> {
if let Some(&cached_id) = self.cache.get(builder.as_bytes()) {
// Since we have a cached state, put the constructed state's
// memory back into our scratch space, so that it can be reused.
self.put_state_builder(builder);
return Ok((cached_id, false));
}
self.add_state(builder).map(|sid| (sid, true))
}
/// Add the given state to the DFA and make it available in the cache.
///
/// The state initially has no transitions. That is, it transitions to the
/// dead state for all possible inputs, and transitions to the quit state
/// for all quit bytes.
///
/// If adding the state would exceed the maximum value for StateID, then an
/// error is returned.
fn add_state(
&mut self,
builder: StateBuilderNFA,
) -> Result<StateID, BuildError> {
let id = self.dfa.add_empty_state()?;
if !self.config.quit.is_empty() {
for b in self.config.quit.iter() {
self.dfa.set_transition(
id,
alphabet::Unit::u8(b),
self.dfa.quit_id(),
);
}
}
let state = builder.to_state();
// States use reference counting internally, so we only need to count
// their memory usage once.
self.memory_usage_state += state.memory_usage();
self.builder_states.push(state.clone());
self.cache.insert(state, id);
self.put_state_builder(builder);
if let Some(limit) = self.config.dfa_size_limit {
if self.dfa.memory_usage() > limit {
return Err(BuildError::dfa_exceeded_size_limit(limit));
}
}
if let Some(limit) = self.config.determinize_size_limit {
if self.memory_usage() > limit {
return Err(BuildError::determinize_exceeded_size_limit(
limit,
));
}
}
Ok(id)
}
/// Returns a state builder from this determinizer that might have existing
/// capacity. This helps avoid allocs in cases where a state is built that
/// turns out to already be cached.
///
/// Callers must put the state builder back with 'put_state_builder',
/// otherwise the allocation reuse won't work.
fn get_state_builder(&mut self) -> StateBuilderEmpty {
core::mem::replace(
&mut self.scratch_state_builder,
StateBuilderEmpty::new(),
)
}
/// Puts the given state builder back into this determinizer for reuse.
///
/// Note that building a 'State' from a builder always creates a new
/// alloc, so callers should always put the builder back.
fn put_state_builder(&mut self, builder: StateBuilderNFA) {
let _ = core::mem::replace(
&mut self.scratch_state_builder,
builder.clear(),
);
}
/// Return the memory usage, in bytes, of this determinizer at the current
/// point in time. This does not include memory used by the NFA or the
/// dense DFA itself.
fn memory_usage(&self) -> usize {
use core::mem::size_of;
self.builder_states.len() * size_of::<State>()
// Maps likely use more memory than this, but it's probably close.
+ self.cache.len() * (size_of::<State>() + size_of::<StateID>())
+ self.memory_usage_state
+ self.stack.capacity() * size_of::<StateID>()
+ self.scratch_state_builder.capacity()
}
}