use core::mem;
use alloc::{sync::Arc, vec, vec::Vec};
use crate::{
nfa::thompson::{
error::BuildError,
nfa::{self, SparseTransitions, Transition, NFA},
},
util::{
look::{Look, LookMatcher},
primitives::{IteratorIndexExt, PatternID, SmallIndex, StateID},
},
};
/// An intermediate NFA state used during construction.
///
/// During construction of an NFA, it is often convenient to work with states
/// that are amenable to mutation and other carry more information than we
/// otherwise need once an NFA has been built. This type represents those
/// needs.
///
/// Once construction is finished, the builder will convert these states to a
/// [`nfa::thompson::State`](crate::nfa::thompson::State). This conversion not
/// only results in a simpler representation, but in some cases, entire classes
/// of states are completely removed (such as [`State::Empty`]).
#[derive(Clone, Debug, Eq, PartialEq)]
enum State {
/// An empty state whose only purpose is to forward the automaton to
/// another state via an unconditional epsilon transition.
///
/// Unconditional epsilon transitions are quite useful during the
/// construction of an NFA, as they permit the insertion of no-op
/// placeholders that make it easier to compose NFA sub-graphs. When
/// the Thompson NFA builder produces a final NFA, all unconditional
/// epsilon transitions are removed, and state identifiers are remapped
/// accordingly.
Empty {
/// The next state that this state should transition to.
next: StateID,
},
/// A state that only transitions to another state if the current input
/// byte is in a particular range of bytes.
ByteRange { trans: Transition },
/// A state with possibly many transitions, represented in a sparse
/// fashion. Transitions must be ordered lexicographically by input range
/// and be non-overlapping. As such, this may only be used when every
/// transition has equal priority. (In practice, this is only used for
/// encoding large UTF-8 automata.) In contrast, a `Union` state has each
/// alternate in order of priority. Priority is used to implement greedy
/// matching and also alternations themselves, e.g., `abc|a` where `abc`
/// has priority over `a`.
///
/// To clarify, it is possible to remove `Sparse` and represent all things
/// that `Sparse` is used for via `Union`. But this creates a more bloated
/// NFA with more epsilon transitions than is necessary in the special case
/// of character classes.
Sparse { transitions: Vec<Transition> },
/// A conditional epsilon transition satisfied via some sort of
/// look-around.
Look { look: Look, next: StateID },
/// An empty state that records the start of a capture location. This is an
/// unconditional epsilon transition like `Empty`, except it can be used to
/// record position information for a capture group when using the NFA for
/// search.
CaptureStart {
/// The ID of the pattern that this capture was defined.
pattern_id: PatternID,
/// The capture group index that this capture state corresponds to.
/// The capture group index is always relative to its corresponding
/// pattern. Therefore, in the presence of multiple patterns, both the
/// pattern ID and the capture group index are required to uniquely
/// identify a capturing group.
group_index: SmallIndex,
/// The next state that this state should transition to.
next: StateID,
},
/// An empty state that records the end of a capture location. This is an
/// unconditional epsilon transition like `Empty`, except it can be used to
/// record position information for a capture group when using the NFA for
/// search.
CaptureEnd {
/// The ID of the pattern that this capture was defined.
pattern_id: PatternID,
/// The capture group index that this capture state corresponds to.
/// The capture group index is always relative to its corresponding
/// pattern. Therefore, in the presence of multiple patterns, both the
/// pattern ID and the capture group index are required to uniquely
/// identify a capturing group.
group_index: SmallIndex,
/// The next state that this state should transition to.
next: StateID,
},
/// An alternation such that there exists an epsilon transition to all
/// states in `alternates`, where matches found via earlier transitions
/// are preferred over later transitions.
Union { alternates: Vec<StateID> },
/// An alternation such that there exists an epsilon transition to all
/// states in `alternates`, where matches found via later transitions are
/// preferred over earlier transitions.
///
/// This "reverse" state exists for convenience during compilation that
/// permits easy construction of non-greedy combinations of NFA states. At
/// the end of compilation, Union and UnionReverse states are merged into
/// one Union type of state, where the latter has its epsilon transitions
/// reversed to reflect the priority inversion.
///
/// The "convenience" here arises from the fact that as new states are
/// added to the list of `alternates`, we would like that add operation
/// to be amortized constant time. But if we used a `Union`, we'd need to
/// prepend the state, which takes O(n) time. There are other approaches we
/// could use to solve this, but this seems simple enough.
UnionReverse { alternates: Vec<StateID> },
/// A state that cannot be transitioned out of. This is useful for cases
/// where you want to prevent matching from occurring. For example, if your
/// regex parser permits empty character classes, then one could choose a
/// `Fail` state to represent it.
Fail,
/// A match state. There is at most one such occurrence of this state in
/// an NFA for each pattern compiled into the NFA. At time of writing, a
/// match state is always produced for every pattern given, but in theory,
/// if a pattern can never lead to a match, then the match state could be
/// omitted.
///
/// `pattern_id` refers to the ID of the pattern itself, which corresponds
/// to the pattern's index (starting at 0).
Match { pattern_id: PatternID },
}
impl State {
/// If this state is an unconditional epsilon transition, then this returns
/// the target of the transition.
fn goto(&self) -> Option<StateID> {
match *self {
State::Empty { next } => Some(next),
State::Union { ref alternates } if alternates.len() == 1 => {
Some(alternates[0])
}
State::UnionReverse { ref alternates }
if alternates.len() == 1 =>
{
Some(alternates[0])
}
_ => None,
}
}
/// Returns the heap memory usage, in bytes, of this state.
fn memory_usage(&self) -> usize {
match *self {
State::Empty { .. }
| State::ByteRange { .. }
| State::Look { .. }
| State::CaptureStart { .. }
| State::CaptureEnd { .. }
| State::Fail
| State::Match { .. } => 0,
State::Sparse { ref transitions } => {
transitions.len() * mem::size_of::<Transition>()
}
State::Union { ref alternates } => {
alternates.len() * mem::size_of::<StateID>()
}
State::UnionReverse { ref alternates } => {
alternates.len() * mem::size_of::<StateID>()
}
}
}
}
/// An abstraction for building Thompson NFAs by hand.
///
/// A builder is what a [`thompson::Compiler`](crate::nfa::thompson::Compiler)
/// uses internally to translate a regex's high-level intermediate
/// representation into an [`NFA`].
///
/// The primary function of this builder is to abstract away the internal
/// representation of an NFA and make it difficult to produce NFAs are that
/// internally invalid or inconsistent. This builder also provides a way to
/// add "empty" states (which can be thought of as unconditional epsilon
/// transitions), despite the fact that [`thompson::State`](nfa::State) does
/// not have any "empty" representation. The advantage of "empty" states is
/// that they make the code for constructing a Thompson NFA logically simpler.
///
/// Many of the routines on this builder may panic or return errors. Generally
/// speaking, panics occur when an invalid sequence of method calls were made,
/// where as an error occurs if things get too big. (Where "too big" might mean
/// exhausting identifier space or using up too much heap memory in accordance
/// with the configured [`size_limit`](Builder::set_size_limit).)
///
/// # Overview
///
/// ## Adding multiple patterns
///
/// Each pattern you add to an NFA should correspond to a pair of
/// [`Builder::start_pattern`] and [`Builder::finish_pattern`] calls, with
/// calls inbetween that add NFA states for that pattern. NFA states may be
/// added without first calling `start_pattern`, with the exception of adding
/// capturing states.
///
/// ## Adding NFA states
///
/// Here is a very brief overview of each of the methods that add NFA states.
/// Every method adds a single state.
///
/// * [`add_empty`](Builder::add_empty): Add a state with a single
/// unconditional epsilon transition to another state.
/// * [`add_union`](Builder::add_union): Adds a state with unconditional
/// epsilon transitions to two or more states, with earlier transitions
/// preferred over later ones.
/// * [`add_union_reverse`](Builder::add_union_reverse): Adds a state with
/// unconditional epsilon transitions to two or more states, with later
/// transitions preferred over earlier ones.
/// * [`add_range`](Builder::add_range): Adds a state with a single transition
/// to another state that can only be followed if the current input byte is
/// within the range given.
/// * [`add_sparse`](Builder::add_sparse): Adds a state with two or more
/// range transitions to other states, where a transition is only followed
/// if the current input byte is within one of the ranges. All transitions
/// in this state have equal priority, and the corresponding ranges must be
/// non-overlapping.
/// * [`add_look`](Builder::add_look): Adds a state with a single *conditional*
/// epsilon transition to another state, where the condition depends on a
/// limited look-around property.
/// * [`add_capture_start`](Builder::add_capture_start): Adds a state with
/// a single unconditional epsilon transition that also instructs an NFA
/// simulation to record the current input position to a specific location in
/// memory. This is intended to represent the starting location of a capturing
/// group.
/// * [`add_capture_end`](Builder::add_capture_end): Adds a state with
/// a single unconditional epsilon transition that also instructs an NFA
/// simulation to record the current input position to a specific location in
/// memory. This is intended to represent the ending location of a capturing
/// group.
/// * [`add_fail`](Builder::add_fail): Adds a state that never transitions to
/// another state.
/// * [`add_match`](Builder::add_match): Add a state that indicates a match has
/// been found for a particular pattern. A match state is a final state with
/// no outgoing transitions.
///
/// ## Setting transitions between NFA states
///
/// The [`Builder::patch`] method creates a transition from one state to the
/// next. If the `from` state corresponds to a state that supports multiple
/// outgoing transitions (such as "union"), then this adds the corresponding
/// transition. Otherwise, it sets the single transition. (This routine panics
/// if `from` corresponds to a state added by `add_sparse`, since sparse states
/// need more specialized handling.)
///
/// # Example
///
/// This annotated example shows how to hand construct the regex `[a-z]+`
/// (without an unanchored prefix).
///
/// ```
/// use regex_automata::{
/// nfa::thompson::{pikevm::PikeVM, Builder, Transition},
/// util::primitives::StateID,
/// Match,
/// };
///
/// let mut builder = Builder::new();
/// // Before adding NFA states for our pattern, we need to tell the builder
/// // that we are starting the pattern.
/// builder.start_pattern()?;
/// // Since we use the Pike VM below for searching, we need to add capturing
/// // states. If you're just going to build a DFA from the NFA, then capturing
/// // states do not need to be added.
/// let start = builder.add_capture_start(StateID::ZERO, 0, None)?;
/// let range = builder.add_range(Transition {
/// // We don't know the state ID of the 'next' state yet, so we just fill
/// // in a dummy 'ZERO' value.
/// start: b'a', end: b'z', next: StateID::ZERO,
/// })?;
/// // This state will point back to 'range', but also enable us to move ahead.
/// // That is, this implements the '+' repetition operator. We add 'range' and
/// // then 'end' below to this alternation.
/// let alt = builder.add_union(vec![])?;
/// // The final state before the match state, which serves to capture the
/// // end location of the match.
/// let end = builder.add_capture_end(StateID::ZERO, 0)?;
/// // The match state for our pattern.
/// let mat = builder.add_match()?;
/// // Now we fill in the transitions between states.
/// builder.patch(start, range)?;
/// builder.patch(range, alt)?;
/// // If we added 'end' before 'range', then we'd implement non-greedy
/// // matching, i.e., '+?'.
/// builder.patch(alt, range)?;
/// builder.patch(alt, end)?;
/// builder.patch(end, mat)?;
/// // We must explicitly finish pattern and provide the starting state ID for
/// // this particular pattern.
/// builder.finish_pattern(start)?;
/// // Finally, when we build the NFA, we provide the anchored and unanchored
/// // starting state IDs. Since we didn't bother with an unanchored prefix
/// // here, we only support anchored searching. Thus, both starting states are
/// // the same.
/// let nfa = builder.build(start, start)?;
///
/// // Now build a Pike VM from our NFA, and use it for searching. This shows
/// // how we can use a regex engine without ever worrying about syntax!
/// let re = PikeVM::new_from_nfa(nfa)?;
/// let mut cache = re.create_cache();
/// let mut caps = re.create_captures();
/// let expected = Some(Match::must(0, 0..3));
/// re.captures(&mut cache, "foo0", &mut caps);
/// assert_eq!(expected, caps.get_match());
///
/// # Ok::<(), Box<dyn std::error::Error>>(())
/// ```
#[derive(Clone, Debug, Default)]
pub struct Builder {
/// The ID of the pattern that we're currently building.
///
/// Callers are required to set (and unset) this by calling
/// {start,finish}_pattern. Otherwise, most methods will panic.
pattern_id: Option<PatternID>,
/// A sequence of intermediate NFA states. Once a state is added to this
/// sequence, it is assigned a state ID equivalent to its index. Once a
/// state is added, it is still expected to be mutated, e.g., to set its
/// transition to a state that didn't exist at the time it was added.
states: Vec<State>,
/// The starting states for each individual pattern. Starting at any
/// of these states will result in only an anchored search for the
/// corresponding pattern. The vec is indexed by pattern ID. When the NFA
/// contains a single regex, then `start_pattern[0]` and `start_anchored`
/// are always equivalent.
start_pattern: Vec<StateID>,
/// A map from pattern ID to capture group index to name. (If no name
/// exists, then a None entry is present. Thus, all capturing groups are
/// present in this mapping.)
///
/// The outer vec is indexed by pattern ID, while the inner vec is indexed
/// by capture index offset for the corresponding pattern.
///
/// The first capture group for each pattern is always unnamed and is thus
/// always None.
captures: Vec<Vec<Option<Arc<str>>>>,
/// The combined memory used by each of the 'State's in 'states'. This
/// only includes heap usage by each state, and not the size of the state
/// itself. In other words, this tracks heap memory used that isn't
/// captured via `size_of::<State>() * states.len()`.
memory_states: usize,
/// Whether this NFA only matches UTF-8 and whether regex engines using
/// this NFA for searching should report empty matches that split a
/// codepoint.
utf8: bool,
/// Whether this NFA should be matched in reverse or not.
reverse: bool,
/// The matcher to use for look-around assertions.
look_matcher: LookMatcher,
/// A size limit to respect when building an NFA. If the total heap memory
/// of the intermediate NFA states exceeds (or would exceed) this amount,
/// then an error is returned.
size_limit: Option<usize>,
}
impl Builder {
/// Create a new builder for hand-assembling NFAs.
pub fn new() -> Builder {
Builder::default()
}
/// Clear this builder.
///
/// Clearing removes all state associated with building an NFA, but does
/// not reset configuration (such as size limits and whether the NFA
/// should only match UTF-8). After clearing, the builder can be reused to
/// assemble an entirely new NFA.
pub fn clear(&mut self) {
self.pattern_id = None;
self.states.clear();
self.start_pattern.clear();
self.captures.clear();
self.memory_states = 0;
}
/// Assemble a [`NFA`] from the states added so far.
///
/// After building an NFA, more states may be added and `build` may be
/// called again. To reuse a builder to produce an entirely new NFA from
/// scratch, call the [`clear`](Builder::clear) method first.
///
/// `start_anchored` refers to the ID of the starting state that anchored
/// searches should use. That is, searches who matches are limited to the
/// starting position of the search.
///
/// `start_unanchored` refers to the ID of the starting state that
/// unanchored searches should use. This permits searches to report matches
/// that start after the beginning of the search. In cases where unanchored
/// searches are not supported, the unanchored starting state ID must be
/// the same as the anchored starting state ID.
///
/// # Errors
///
/// This returns an error if there was a problem producing the final NFA.
/// In particular, this might include an error if the capturing groups
/// added to this builder violate any of the invariants documented on
/// [`GroupInfo`](crate::util::captures::GroupInfo).
///
/// # Panics
///
/// If `start_pattern` was called, then `finish_pattern` must be called
/// before `build`, otherwise this panics.
///
/// This may panic for other invalid uses of a builder. For example, if
/// a "start capture" state was added without a corresponding "end capture"
/// state.
pub fn build(
&self,
start_anchored: StateID,
start_unanchored: StateID,
) -> Result<NFA, BuildError> {
assert!(self.pattern_id.is_none(), "must call 'finish_pattern' first");
debug!(
"intermediate NFA compilation via builder is complete, \
intermediate NFA size: {} states, {} bytes on heap",
self.states.len(),
self.memory_usage(),
);
let mut nfa = nfa::Inner::default();
nfa.set_utf8(self.utf8);
nfa.set_reverse(self.reverse);
nfa.set_look_matcher(self.look_matcher.clone());
// A set of compiler internal state IDs that correspond to states
// that are exclusively epsilon transitions, i.e., goto instructions,
// combined with the state that they point to. This is used to
// record said states while transforming the compiler's internal NFA
// representation to the external form.
let mut empties = vec![];
// A map used to re-map state IDs when translating this builder's
// internal NFA state representation to the final NFA representation.
let mut remap = vec![];
remap.resize(self.states.len(), StateID::ZERO);
nfa.set_starts(start_anchored, start_unanchored, &self.start_pattern);
nfa.set_captures(&self.captures).map_err(BuildError::captures)?;
// The idea here is to convert our intermediate states to their final
// form. The only real complexity here is the process of converting
// transitions, which are expressed in terms of state IDs. The new
// set of states will be smaller because of partial epsilon removal,
// so the state IDs will not be the same.
for (sid, state) in self.states.iter().with_state_ids() {
match *state {
State::Empty { next } => {
// Since we're removing empty states, we need to handle
// them later since we don't yet know which new state this
// empty state will be mapped to.
empties.push((sid, next));
}
State::ByteRange { trans } => {
remap[sid] = nfa.add(nfa::State::ByteRange { trans });
}
State::Sparse { ref transitions } => {
remap[sid] = match transitions.len() {
0 => nfa.add(nfa::State::Fail),
1 => nfa.add(nfa::State::ByteRange {
trans: transitions[0],
}),
_ => {
let transitions =
transitions.to_vec().into_boxed_slice();
let sparse = SparseTransitions { transitions };
nfa.add(nfa::State::Sparse(sparse))
}
}
}
State::Look { look, next } => {
remap[sid] = nfa.add(nfa::State::Look { look, next });
}
State::CaptureStart { pattern_id, group_index, next } => {
// We can't remove this empty state because of the side
// effect of capturing an offset for this capture slot.
let slot = nfa
.group_info()
.slot(pattern_id, group_index.as_usize())
.expect("invalid capture index");
let slot =
SmallIndex::new(slot).expect("a small enough slot");
remap[sid] = nfa.add(nfa::State::Capture {
next,
pattern_id,
group_index,
slot,
});
}
State::CaptureEnd { pattern_id, group_index, next } => {
// We can't remove this empty state because of the side
// effect of capturing an offset for this capture slot.
// Also, this always succeeds because we check that all
// slot indices are valid for all capture indices when they
// are initially added.
let slot = nfa
.group_info()
.slot(pattern_id, group_index.as_usize())
.expect("invalid capture index")
.checked_add(1)
.unwrap();
let slot =
SmallIndex::new(slot).expect("a small enough slot");
remap[sid] = nfa.add(nfa::State::Capture {
next,
pattern_id,
group_index,
slot,
});
}
State::Union { ref alternates } => {
if alternates.is_empty() {
remap[sid] = nfa.add(nfa::State::Fail);
} else if alternates.len() == 1 {
empties.push((sid, alternates[0]));
remap[sid] = alternates[0];
} else if alternates.len() == 2 {
remap[sid] = nfa.add(nfa::State::BinaryUnion {
alt1: alternates[0],
alt2: alternates[1],
});
} else {
let alternates =
alternates.to_vec().into_boxed_slice();
remap[sid] = nfa.add(nfa::State::Union { alternates });
}
}
State::UnionReverse { ref alternates } => {
if alternates.is_empty() {
remap[sid] = nfa.add(nfa::State::Fail);
} else if alternates.len() == 1 {
empties.push((sid, alternates[0]));
remap[sid] = alternates[0];
} else if alternates.len() == 2 {
remap[sid] = nfa.add(nfa::State::BinaryUnion {
alt1: alternates[1],
alt2: alternates[0],
});
} else {
let mut alternates =
alternates.to_vec().into_boxed_slice();
alternates.reverse();
remap[sid] = nfa.add(nfa::State::Union { alternates });
}
}
State::Fail => {
remap[sid] = nfa.add(nfa::State::Fail);
}
State::Match { pattern_id } => {
remap[sid] = nfa.add(nfa::State::Match { pattern_id });
}
}
}
// Some of the new states still point to empty state IDs, so we need to
// follow each of them and remap the empty state IDs to their non-empty
// state IDs.
//
// We also keep track of which states we've already mapped. This helps
// avoid quadratic behavior in a long chain of empty states. For
// example, in 'a{0}{50000}'.
let mut remapped = vec![false; self.states.len()];
for &(empty_id, empty_next) in empties.iter() {
if remapped[empty_id] {
continue;
}
// empty states can point to other empty states, forming a chain.
// So we must follow the chain until the end, which must end at
// a non-empty state, and therefore, a state that is correctly
// remapped. We are guaranteed to terminate because our compiler
// never builds a loop among only empty states.
let mut new_next = empty_next;
while let Some(next) = self.states[new_next].goto() {
new_next = next;
}
remap[empty_id] = remap[new_next];
remapped[empty_id] = true;
// Now that we've remapped the main 'empty_id' above, we re-follow
// the chain from above and remap every empty state we found along
// the way to our ultimate non-empty target. We are careful to set
// 'remapped' to true for each such state. We thus will not need
// to re-compute this chain for any subsequent empty states in
// 'empties' that are part of this chain.
let mut next2 = empty_next;
while let Some(next) = self.states[next2].goto() {
remap[next2] = remap[new_next];
remapped[next2] = true;
next2 = next;
}
}
// Finally remap all of the state IDs.
nfa.remap(&remap);
let final_nfa = nfa.into_nfa();
debug!(
"NFA compilation via builder complete, \
final NFA size: {} states, {} bytes on heap, \
has empty? {:?}, utf8? {:?}",
final_nfa.states().len(),
final_nfa.memory_usage(),
final_nfa.has_empty(),
final_nfa.is_utf8(),
);
Ok(final_nfa)
}
/// Start the assembly of a pattern in this NFA.
///
/// Upon success, this returns the identifier for the new pattern.
/// Identifiers start at `0` and are incremented by 1 for each new pattern.
///
/// It is necessary to call this routine before adding capturing states.
/// Otherwise, any other NFA state may be added before starting a pattern.
///
/// # Errors
///
/// If the pattern identifier space is exhausted, then this returns an
/// error.
///
/// # Panics
///
/// If this is called while assembling another pattern (i.e., before
/// `finish_pattern` is called), then this panics.
pub fn start_pattern(&mut self) -> Result<PatternID, BuildError> {
assert!(self.pattern_id.is_none(), "must call 'finish_pattern' first");
let proposed = self.start_pattern.len();
let pid = PatternID::new(proposed)
.map_err(|_| BuildError::too_many_patterns(proposed))?;
self.pattern_id = Some(pid);
// This gets filled in when 'finish_pattern' is called.
self.start_pattern.push(StateID::ZERO);
Ok(pid)
}
/// Finish the assembly of a pattern in this NFA.
///
/// Upon success, this returns the identifier for the new pattern.
/// Identifiers start at `0` and are incremented by 1 for each new
/// pattern. This is the same identifier returned by the corresponding
/// `start_pattern` call.
///
/// Note that `start_pattern` and `finish_pattern` pairs cannot be
/// interleaved or nested. A correct `finish_pattern` call _always_
/// corresponds to the most recently called `start_pattern` routine.
///
/// # Errors
///
/// This currently never returns an error, but this is subject to change.
///
/// # Panics
///
/// If this is called without a corresponding `start_pattern` call, then
/// this panics.
pub fn finish_pattern(
&mut self,
start_id: StateID,
) -> Result<PatternID, BuildError> {
let pid = self.current_pattern_id();
self.start_pattern[pid] = start_id;
self.pattern_id = None;
Ok(pid)
}
/// Returns the pattern identifier of the current pattern.
///
/// # Panics
///
/// If this doesn't occur after a `start_pattern` call and before the
/// corresponding `finish_pattern` call, then this panics.
pub fn current_pattern_id(&self) -> PatternID {
self.pattern_id.expect("must call 'start_pattern' first")
}
/// Returns the number of patterns added to this builder so far.
///
/// This only includes patterns that have had `finish_pattern` called
/// for them.
pub fn pattern_len(&self) -> usize {
self.start_pattern.len()
}
/// Add an "empty" NFA state.
///
/// An "empty" NFA state is a state with a single unconditional epsilon
/// transition to another NFA state. Such empty states are removed before
/// building the final [`NFA`] (which has no such "empty" states), but they
/// can be quite useful in the construction process of an NFA.
///
/// # Errors
///
/// This returns an error if the state identifier space is exhausted, or if
/// the configured heap size limit has been exceeded.
pub fn add_empty(&mut self) -> Result<StateID, BuildError> {
self.add(State::Empty { next: StateID::ZERO })
}
/// Add a "union" NFA state.
///
/// A "union" NFA state that contains zero or more unconditional epsilon
/// transitions to other NFA states. The order of these transitions
/// reflects a priority order where earlier transitions are preferred over
/// later transitions.
///
/// Callers may provide an empty set of alternates to this method call, and
/// then later add transitions via `patch`. At final build time, a "union"
/// state with no alternates is converted to a "fail" state, and a "union"
/// state with exactly one alternate is treated as if it were an "empty"
/// state.
///
/// # Errors
///
/// This returns an error if the state identifier space is exhausted, or if
/// the configured heap size limit has been exceeded.
pub fn add_union(
&mut self,
alternates: Vec<StateID>,
) -> Result<StateID, BuildError> {
self.add(State::Union { alternates })
}
/// Add a "reverse union" NFA state.
///
/// A "reverse union" NFA state contains zero or more unconditional epsilon
/// transitions to other NFA states. The order of these transitions
/// reflects a priority order where later transitions are preferred
/// over earlier transitions. This is an inverted priority order when
/// compared to `add_union`. This is useful, for example, for implementing
/// non-greedy repetition operators.
///
/// Callers may provide an empty set of alternates to this method call, and
/// then later add transitions via `patch`. At final build time, a "reverse
/// union" state with no alternates is converted to a "fail" state, and a
/// "reverse union" state with exactly one alternate is treated as if it
/// were an "empty" state.
///
/// # Errors
///
/// This returns an error if the state identifier space is exhausted, or if
/// the configured heap size limit has been exceeded.
pub fn add_union_reverse(
&mut self,
alternates: Vec<StateID>,
) -> Result<StateID, BuildError> {
self.add(State::UnionReverse { alternates })
}
/// Add a "range" NFA state.
///
/// A "range" NFA state is a state with one outgoing transition to another
/// state, where that transition may only be followed if the current input
/// byte falls between a range of bytes given.
///
/// # Errors
///
/// This returns an error if the state identifier space is exhausted, or if
/// the configured heap size limit has been exceeded.
pub fn add_range(
&mut self,
trans: Transition,
) -> Result<StateID, BuildError> {
self.add(State::ByteRange { trans })
}
/// Add a "sparse" NFA state.
///
/// A "sparse" NFA state contains zero or more outgoing transitions, where
/// the transition to be followed (if any) is chosen based on whether the
/// current input byte falls in the range of one such transition. The
/// transitions given *must* be non-overlapping and in ascending order. (A
/// "sparse" state with no transitions is equivalent to a "fail" state.)
///
/// A "sparse" state is like adding a "union" state and pointing it at a
/// bunch of "range" states, except that the different alternates have
/// equal priority.
///
/// Note that a "sparse" state is the only state that cannot be patched.
/// This is because a "sparse" state has many transitions, each of which
/// may point to a different NFA state. Moreover, adding more such
/// transitions requires more than just an NFA state ID to point to. It
/// also requires a byte range. The `patch` routine does not support the
/// additional information required. Therefore, callers must ensure that
/// all outgoing transitions for this state are included when `add_sparse`
/// is called. There is no way to add more later.
///
/// # Errors
///
/// This returns an error if the state identifier space is exhausted, or if
/// the configured heap size limit has been exceeded.
///
/// # Panics
///
/// This routine _may_ panic if the transitions given overlap or are not
/// in ascending order.
pub fn add_sparse(
&mut self,
transitions: Vec<Transition>,
) -> Result<StateID, BuildError> {
self.add(State::Sparse { transitions })
}
/// Add a "look" NFA state.
///
/// A "look" NFA state corresponds to a state with exactly one
/// *conditional* epsilon transition to another NFA state. Namely, it
/// represents one of a small set of simplistic look-around operators.
///
/// Callers may provide a "dummy" state ID (typically [`StateID::ZERO`]),
/// and then change it later with [`patch`](Builder::patch).
///
/// # Errors
///
/// This returns an error if the state identifier space is exhausted, or if
/// the configured heap size limit has been exceeded.
pub fn add_look(
&mut self,
next: StateID,
look: Look,
) -> Result<StateID, BuildError> {
self.add(State::Look { look, next })
}
/// Add a "start capture" NFA state.
///
/// A "start capture" NFA state corresponds to a state with exactly one
/// outgoing unconditional epsilon transition to another state. Unlike
/// "empty" states, a "start capture" state also carries with it an
/// instruction for saving the current position of input to a particular
/// location in memory. NFA simulations, like the Pike VM, may use this
/// information to report the match locations of capturing groups in a
/// regex pattern.
///
/// If the corresponding capturing group has a name, then callers should
/// include it here.
///
/// Callers may provide a "dummy" state ID (typically [`StateID::ZERO`]),
/// and then change it later with [`patch`](Builder::patch).
///
/// Note that unlike `start_pattern`/`finish_pattern`, capturing start and
/// end states may be interleaved. Indeed, it is typical for many "start
/// capture" NFA states to appear before the first "end capture" state.
///
/// # Errors
///
/// This returns an error if the state identifier space is exhausted, or if
/// the configured heap size limit has been exceeded or if the given
/// capture index overflows `usize`.
///
/// While the above are the only conditions in which this routine can
/// currently return an error, it is possible to call this method with an
/// inputs that results in the final `build()` step failing to produce an
/// NFA. For example, if one adds two distinct capturing groups with the
/// same name, then that will result in `build()` failing with an error.
///
/// See the [`GroupInfo`](crate::util::captures::GroupInfo) type for
/// more information on what qualifies as valid capturing groups.
///
/// # Example
///
/// This example shows that an error occurs when one tries to add multiple
/// capturing groups with the same name to the same pattern.
///
/// ```
/// use regex_automata::{
/// nfa::thompson::Builder,
/// util::primitives::StateID,
/// };
///
/// let name = Some(std::sync::Arc::from("foo"));
/// let mut builder = Builder::new();
/// builder.start_pattern()?;
/// // 0th capture group should always be unnamed.
/// let start = builder.add_capture_start(StateID::ZERO, 0, None)?;
/// // OK
/// builder.add_capture_start(StateID::ZERO, 1, name.clone())?;
/// // This is not OK, but 'add_capture_start' still succeeds. We don't
/// // get an error until we call 'build' below. Without this call, the
/// // call to 'build' below would succeed.
/// builder.add_capture_start(StateID::ZERO, 2, name.clone())?;
/// // Finish our pattern so we can try to build the NFA.
/// builder.finish_pattern(start)?;
/// let result = builder.build(start, start);
/// assert!(result.is_err());
///
/// # Ok::<(), Box<dyn std::error::Error>>(())
/// ```
///
/// However, adding multiple capturing groups with the same name to
/// distinct patterns is okay:
///
/// ```
/// use std::sync::Arc;
///
/// use regex_automata::{
/// nfa::thompson::{pikevm::PikeVM, Builder, Transition},
/// util::{
/// captures::Captures,
/// primitives::{PatternID, StateID},
/// },
/// Span,
/// };
///
/// // Hand-compile the patterns '(?P<foo>[a-z])' and '(?P<foo>[A-Z])'.
/// let mut builder = Builder::new();
/// // We compile them to support an unanchored search, which requires
/// // adding an implicit '(?s-u:.)*?' prefix before adding either pattern.
/// let unanchored_prefix = builder.add_union_reverse(vec![])?;
/// let any = builder.add_range(Transition {
/// start: b'\x00', end: b'\xFF', next: StateID::ZERO,
/// })?;
/// builder.patch(unanchored_prefix, any)?;
/// builder.patch(any, unanchored_prefix)?;
///
/// // Compile an alternation that permits matching multiple patterns.
/// let alt = builder.add_union(vec![])?;
/// builder.patch(unanchored_prefix, alt)?;
///
/// // Compile '(?P<foo>[a-z]+)'.
/// builder.start_pattern()?;
/// let start0 = builder.add_capture_start(StateID::ZERO, 0, None)?;
/// // N.B. 0th capture group must always be unnamed.
/// let foo_start0 = builder.add_capture_start(
/// StateID::ZERO, 1, Some(Arc::from("foo")),
/// )?;
/// let lowercase = builder.add_range(Transition {
/// start: b'a', end: b'z', next: StateID::ZERO,
/// })?;
/// let foo_end0 = builder.add_capture_end(StateID::ZERO, 1)?;
/// let end0 = builder.add_capture_end(StateID::ZERO, 0)?;
/// let match0 = builder.add_match()?;
/// builder.patch(start0, foo_start0)?;
/// builder.patch(foo_start0, lowercase)?;
/// builder.patch(lowercase, foo_end0)?;
/// builder.patch(foo_end0, end0)?;
/// builder.patch(end0, match0)?;
/// builder.finish_pattern(start0)?;
///
/// // Compile '(?P<foo>[A-Z]+)'.
/// builder.start_pattern()?;
/// let start1 = builder.add_capture_start(StateID::ZERO, 0, None)?;
/// // N.B. 0th capture group must always be unnamed.
/// let foo_start1 = builder.add_capture_start(
/// StateID::ZERO, 1, Some(Arc::from("foo")),
/// )?;
/// let uppercase = builder.add_range(Transition {
/// start: b'A', end: b'Z', next: StateID::ZERO,
/// })?;
/// let foo_end1 = builder.add_capture_end(StateID::ZERO, 1)?;
/// let end1 = builder.add_capture_end(StateID::ZERO, 0)?;
/// let match1 = builder.add_match()?;
/// builder.patch(start1, foo_start1)?;
/// builder.patch(foo_start1, uppercase)?;
/// builder.patch(uppercase, foo_end1)?;
/// builder.patch(foo_end1, end1)?;
/// builder.patch(end1, match1)?;
/// builder.finish_pattern(start1)?;
///
/// // Now add the patterns to our alternation that we started above.
/// builder.patch(alt, start0)?;
/// builder.patch(alt, start1)?;
///
/// // Finally build the NFA. The first argument is the anchored starting
/// // state (the pattern alternation) where as the second is the
/// // unanchored starting state (the unanchored prefix).
/// let nfa = builder.build(alt, unanchored_prefix)?;
///
/// // Now build a Pike VM from our NFA and access the 'foo' capture
/// // group regardless of which pattern matched, since it is defined
/// // for both patterns.
/// let vm = PikeVM::new_from_nfa(nfa)?;
/// let mut cache = vm.create_cache();
/// let caps: Vec<Captures> =
/// vm.captures_iter(&mut cache, "0123aAaAA").collect();
/// assert_eq!(5, caps.len());
///
/// assert_eq!(Some(PatternID::must(0)), caps[0].pattern());
/// assert_eq!(Some(Span::from(4..5)), caps[0].get_group_by_name("foo"));
///
/// assert_eq!(Some(PatternID::must(1)), caps[1].pattern());
/// assert_eq!(Some(Span::from(5..6)), caps[1].get_group_by_name("foo"));
///
/// assert_eq!(Some(PatternID::must(0)), caps[2].pattern());
/// assert_eq!(Some(Span::from(6..7)), caps[2].get_group_by_name("foo"));
///
/// assert_eq!(Some(PatternID::must(1)), caps[3].pattern());
/// assert_eq!(Some(Span::from(7..8)), caps[3].get_group_by_name("foo"));
///
/// assert_eq!(Some(PatternID::must(1)), caps[4].pattern());
/// assert_eq!(Some(Span::from(8..9)), caps[4].get_group_by_name("foo"));
///
/// # Ok::<(), Box<dyn std::error::Error>>(())
/// ```
pub fn add_capture_start(
&mut self,
next: StateID,
group_index: u32,
name: Option<Arc<str>>,
) -> Result<StateID, BuildError> {
let pid = self.current_pattern_id();
let group_index = match SmallIndex::try_from(group_index) {
Err(_) => {
return Err(BuildError::invalid_capture_index(group_index))
}
Ok(group_index) => group_index,
};
// Make sure we have space to insert our (pid,index)|-->name mapping.
if pid.as_usize() >= self.captures.len() {
for _ in 0..=(pid.as_usize() - self.captures.len()) {
self.captures.push(vec![]);
}
}
// In the case where 'group_index < self.captures[pid].len()', it means
// that we are adding a duplicate capture group. This is somewhat
// weird, but permissible because the capture group itself can be
// repeated in the syntax. For example, '([a-z]){4}' will produce 4
// capture groups. In practice, only the last will be set at search
// time when a match occurs. For duplicates, we don't need to push
// anything other than a CaptureStart NFA state.
if group_index.as_usize() >= self.captures[pid].len() {
// For discontiguous indices, push placeholders for earlier capture
// groups that weren't explicitly added.
for _ in 0..(group_index.as_usize() - self.captures[pid].len()) {
self.captures[pid].push(None);
}
self.captures[pid].push(name);
}
self.add(State::CaptureStart { pattern_id: pid, group_index, next })
}
/// Add a "end capture" NFA state.
///
/// A "end capture" NFA state corresponds to a state with exactly one
/// outgoing unconditional epsilon transition to another state. Unlike
/// "empty" states, a "end capture" state also carries with it an
/// instruction for saving the current position of input to a particular
/// location in memory. NFA simulations, like the Pike VM, may use this
/// information to report the match locations of capturing groups in a
///
/// Callers may provide a "dummy" state ID (typically [`StateID::ZERO`]),
/// and then change it later with [`patch`](Builder::patch).
///
/// Note that unlike `start_pattern`/`finish_pattern`, capturing start and
/// end states may be interleaved. Indeed, it is typical for many "start
/// capture" NFA states to appear before the first "end capture" state.
///
/// # Errors
///
/// This returns an error if the state identifier space is exhausted, or if
/// the configured heap size limit has been exceeded or if the given
/// capture index overflows `usize`.
///
/// While the above are the only conditions in which this routine can
/// currently return an error, it is possible to call this method with an
/// inputs that results in the final `build()` step failing to produce an
/// NFA. For example, if one adds two distinct capturing groups with the
/// same name, then that will result in `build()` failing with an error.
///
/// See the [`GroupInfo`](crate::util::captures::GroupInfo) type for
/// more information on what qualifies as valid capturing groups.
pub fn add_capture_end(
&mut self,
next: StateID,
group_index: u32,
) -> Result<StateID, BuildError> {
let pid = self.current_pattern_id();
let group_index = match SmallIndex::try_from(group_index) {
Err(_) => {
return Err(BuildError::invalid_capture_index(group_index))
}
Ok(group_index) => group_index,
};
self.add(State::CaptureEnd { pattern_id: pid, group_index, next })
}
/// Adds a "fail" NFA state.
///
/// A "fail" state is simply a state that has no outgoing transitions. It
/// acts as a way to cause a search to stop without reporting a match.
/// For example, one way to represent an NFA with zero patterns is with a
/// single "fail" state.
///
/// # Errors
///
/// This returns an error if the state identifier space is exhausted, or if
/// the configured heap size limit has been exceeded.
pub fn add_fail(&mut self) -> Result<StateID, BuildError> {
self.add(State::Fail)
}
/// Adds a "match" NFA state.
///
/// A "match" state has no outgoing transitions (just like a "fail"
/// state), but it has special significance in that if a search enters
/// this state, then a match has been found. The match state that is added
/// automatically has the current pattern ID associated with it. This is
/// used to report the matching pattern ID at search time.
///
/// # Errors
///
/// This returns an error if the state identifier space is exhausted, or if
/// the configured heap size limit has been exceeded.
///
/// # Panics
///
/// This must be called after a `start_pattern` call but before the
/// corresponding `finish_pattern` call. Otherwise, it panics.
pub fn add_match(&mut self) -> Result<StateID, BuildError> {
let pattern_id = self.current_pattern_id();
let sid = self.add(State::Match { pattern_id })?;
Ok(sid)
}
/// The common implementation of "add a state." It handles the common
/// error cases of state ID exhausting (by owning state ID allocation) and
/// whether the size limit has been exceeded.
fn add(&mut self, state: State) -> Result<StateID, BuildError> {
let id = StateID::new(self.states.len())
.map_err(|_| BuildError::too_many_states(self.states.len()))?;
self.memory_states += state.memory_usage();
self.states.push(state);
self.check_size_limit()?;
Ok(id)
}
/// Add a transition from one state to another.
///
/// This routine is called "patch" since it is very common to add the
/// states you want, typically with "dummy" state ID transitions, and then
/// "patch" in the real state IDs later. This is because you don't always
/// know all of the necessary state IDs to add because they might not
/// exist yet.
///
/// # Errors
///
/// This may error if patching leads to an increase in heap usage beyond
/// the configured size limit. Heap usage only grows when patching adds a
/// new transition (as in the case of a "union" state).
///
/// # Panics
///
/// This panics if `from` corresponds to a "sparse" state. When "sparse"
/// states are added, there is no way to patch them after-the-fact. (If you
/// have a use case where this would be helpful, please file an issue. It
/// will likely require a new API.)
pub fn patch(
&mut self,
from: StateID,
to: StateID,
) -> Result<(), BuildError> {
let old_memory_states = self.memory_states;
match self.states[from] {
State::Empty { ref mut next } => {
*next = to;
}
State::ByteRange { ref mut trans } => {
trans.next = to;
}
State::Sparse { .. } => {
panic!("cannot patch from a sparse NFA state")
}
State::Look { ref mut next, .. } => {
*next = to;
}
State::Union { ref mut alternates } => {
alternates.push(to);
self.memory_states += mem::size_of::<StateID>();
}
State::UnionReverse { ref mut alternates } => {
alternates.push(to);
self.memory_states += mem::size_of::<StateID>();
}
State::CaptureStart { ref mut next, .. } => {
*next = to;
}
State::CaptureEnd { ref mut next, .. } => {
*next = to;
}
State::Fail => {}
State::Match { .. } => {}
}
if old_memory_states != self.memory_states {
self.check_size_limit()?;
}
Ok(())
}
/// Set whether the NFA produced by this builder should only match UTF-8.
///
/// This should be set when both of the following are true:
///
/// 1. The caller guarantees that the NFA created by this build will only
/// report non-empty matches with spans that are valid UTF-8.
/// 2. The caller desires regex engines using this NFA to avoid reporting
/// empty matches with a span that splits a valid UTF-8 encoded codepoint.
///
/// Property (1) is not checked. Instead, this requires the caller to
/// promise that it is true. Property (2) corresponds to the behavior of
/// regex engines using the NFA created by this builder. Namely, there
/// is no way in the NFA's graph itself to say that empty matches found
/// by, for example, the regex `a*` will fall on valid UTF-8 boundaries.
/// Instead, this option is used to communicate the UTF-8 semantic to regex
/// engines that will typically implement it as a post-processing step by
/// filtering out empty matches that don't fall on UTF-8 boundaries.
///
/// If you're building an NFA from an HIR (and not using a
/// [`thompson::Compiler`](crate::nfa::thompson::Compiler)), then you can
/// use the [`syntax::Config::utf8`](crate::util::syntax::Config::utf8)
/// option to guarantee that if the HIR detects a non-empty match, then it
/// is guaranteed to be valid UTF-8.
///
/// Note that property (2) does *not* specify the behavior of executing
/// a search on a haystack that is not valid UTF-8. Therefore, if you're
/// *not* running this NFA on strings that are guaranteed to be valid
/// UTF-8, you almost certainly do not want to enable this option.
/// Similarly, if you are running the NFA on strings that *are* guaranteed
/// to be valid UTF-8, then you almost certainly want to enable this option
/// unless you can guarantee that your NFA will never produce a zero-width
/// match.
///
/// It is disabled by default.
pub fn set_utf8(&mut self, yes: bool) {
self.utf8 = yes;
}
/// Returns whether UTF-8 mode is enabled for this builder.
///
/// See [`Builder::set_utf8`] for more details about what "UTF-8 mode" is.
pub fn get_utf8(&self) -> bool {
self.utf8
}
/// Sets whether the NFA produced by this builder should be matched in
/// reverse or not. Generally speaking, when enabled, the NFA produced
/// should be matched by moving backwards through a haystack, from a higher
/// memory address to a lower memory address.
///
/// See also [`NFA::is_reverse`] for more details.
///
/// This is disabled by default, which means NFAs are by default matched
/// in the forward direction.
pub fn set_reverse(&mut self, yes: bool) {
self.reverse = yes;
}
/// Returns whether reverse mode is enabled for this builder.
///
/// See [`Builder::set_reverse`] for more details about what "reverse mode"
/// is.
pub fn get_reverse(&self) -> bool {
self.reverse
}
/// Sets the look-around matcher that should be used for the resulting NFA.
///
/// A look-around matcher can be used to configure how look-around
/// assertions are matched. For example, a matcher might carry
/// configuration that changes the line terminator used for `(?m:^)` and
/// `(?m:$)` assertions.
pub fn set_look_matcher(&mut self, m: LookMatcher) {
self.look_matcher = m;
}
/// Returns the look-around matcher used for this builder.
///
/// If a matcher was not explicitly set, then `LookMatcher::default()` is
/// returned.
pub fn get_look_matcher(&self) -> &LookMatcher {
&self.look_matcher
}
/// Set the size limit on this builder.
///
/// Setting the size limit will also check whether the NFA built so far
/// fits within the given size limit. If it doesn't, then an error is
/// returned.
///
/// By default, there is no configured size limit.
pub fn set_size_limit(
&mut self,
limit: Option<usize>,
) -> Result<(), BuildError> {
self.size_limit = limit;
self.check_size_limit()
}
/// Return the currently configured size limit.
///
/// By default, this returns `None`, which corresponds to no configured
/// size limit.
pub fn get_size_limit(&self) -> Option<usize> {
self.size_limit
}
/// Returns the heap memory usage, in bytes, used by the NFA states added
/// so far.
///
/// Note that this is an approximation of how big the final NFA will be.
/// In practice, the final NFA will likely be a bit smaller because of
/// its simpler state representation. (For example, using things like
/// `Box<[StateID]>` instead of `Vec<StateID>`.)
pub fn memory_usage(&self) -> usize {
self.states.len() * mem::size_of::<State>() + self.memory_states
}
fn check_size_limit(&self) -> Result<(), BuildError> {
if let Some(limit) = self.size_limit {
if self.memory_usage() > limit {
return Err(BuildError::exceeded_size_limit(limit));
}
}
Ok(())
}
}
#[cfg(test)]
mod tests {
use super::*;
// This asserts that a builder state doesn't have its size changed. It is
// *really* easy to accidentally increase the size, and thus potentially
// dramatically increase the memory usage of NFA builder.
//
// This assert doesn't mean we absolutely cannot increase the size of a
// builder state. We can. It's just here to make sure we do it knowingly
// and intentionally.
//
// A builder state is unfortunately a little bigger than an NFA state,
// since we really want to support adding things to a pre-existing state.
// i.e., We use Vec<thing> instead of Box<[thing]>. So we end up using an
// extra 8 bytes per state. Sad, but at least it gets freed once the NFA
// is built.
#[test]
fn state_has_small_size() {
#[cfg(target_pointer_width = "64")]
assert_eq!(32, core::mem::size_of::<State>());
#[cfg(target_pointer_width = "32")]
assert_eq!(16, core::mem::size_of::<State>());
}
}
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