// This is the backtracking matching engine. It has the same exact capability
// as the full NFA simulation, except it is artificially restricted to small
// regexes on small inputs because of its memory requirements.
//
// In particular, this is a *bounded* backtracking engine. It retains worst
// case linear time by keeping track of the states that it has visited (using a
// bitmap). Namely, once a state is visited, it is never visited again. Since a
// state is keyed by `(instruction index, input index)`, we have that its time
// complexity is `O(mn)` (i.e., linear in the size of the search text).
//
// The backtracking engine can beat out the NFA simulation on small
// regexes/inputs because it doesn't have to keep track of multiple copies of
// the capture groups. In benchmarks, the backtracking engine is roughly twice
// as fast as the full NFA simulation. Note though that its performance doesn't
// scale, even if you're willing to live with the memory requirements. Namely,
// the bitset has to be zeroed on each execution, which becomes quite expensive
// on large bitsets.
use exec::ProgramCache;
use input::{Input, InputAt};
use prog::{InstPtr, Program};
use re_trait::Slot;
type Bits = u32;
const BIT_SIZE: usize = 32;
const MAX_SIZE_BYTES: usize = 256 * (1 << 10); // 256 KB
/// Returns true iff the given regex and input should be executed by this
/// engine with reasonable memory usage.
pub fn should_exec(num_insts: usize, text_len: usize) -> bool {
// Total memory usage in bytes is determined by:
//
// ((len(insts) * (len(input) + 1) + bits - 1) / bits) * (size_of(u32))
//
// The actual limit picked is pretty much a heuristic.
// See: https://github.com/rust-lang/regex/issues/215
let size = ((num_insts * (text_len + 1) + BIT_SIZE - 1) / BIT_SIZE) * 4;
size <= MAX_SIZE_BYTES
}
/// A backtracking matching engine.
#[derive(Debug)]
pub struct Bounded<'a, 'm, 'r, 's, I> {
prog: &'r Program,
input: I,
matches: &'m mut [bool],
slots: &'s mut [Slot],
m: &'a mut Cache,
}
/// Shared cached state between multiple invocations of a backtracking engine
/// in the same thread.
#[derive(Clone, Debug)]
pub struct Cache {
jobs: Vec<Job>,
visited: Vec<Bits>,
}
impl Cache {
/// Create new empty cache for the backtracking engine.
pub fn new(_prog: &Program) -> Self {
Cache { jobs: vec![], visited: vec![] }
}
}
/// A job is an explicit unit of stack space in the backtracking engine.
///
/// The "normal" representation is a single state transition, which corresponds
/// to an NFA state and a character in the input. However, the backtracking
/// engine must keep track of old capture group values. We use the explicit
/// stack to do it.
#[derive(Clone, Copy, Debug)]
enum Job {
Inst { ip: InstPtr, at: InputAt },
SaveRestore { slot: usize, old_pos: Option<usize> },
}
impl<'a, 'm, 'r, 's, I: Input> Bounded<'a, 'm, 'r, 's, I> {
/// Execute the backtracking matching engine.
///
/// If there's a match, `exec` returns `true` and populates the given
/// captures accordingly.
pub fn exec(
prog: &'r Program,
cache: &ProgramCache,
matches: &'m mut [bool],
slots: &'s mut [Slot],
input: I,
start: usize,
end: usize,
) -> bool {
let mut cache = cache.borrow_mut();
let cache = &mut cache.backtrack;
let start = input.at(start);
let mut b = Bounded {
prog: prog,
input: input,
matches: matches,
slots: slots,
m: cache,
};
b.exec_(start, end)
}
/// Clears the cache such that the backtracking engine can be executed
/// on some input of fixed length.
fn clear(&mut self) {
// Reset the job memory so that we start fresh.
self.m.jobs.clear();
// Now we need to clear the bit state set.
// We do this by figuring out how much space we need to keep track
// of the states we've visited.
// Then we reset all existing allocated space to 0.
// Finally, we request more space if we need it.
//
// This is all a little circuitous, but doing this unsafely
// doesn't seem to have a measurable impact on performance.
// (Probably because backtracking is limited to such small
// inputs/regexes in the first place.)
let visited_len =
(self.prog.len() * (self.input.len() + 1) + BIT_SIZE - 1)
/ BIT_SIZE;
self.m.visited.truncate(visited_len);
for v in &mut self.m.visited {
*v = 0;
}
if visited_len > self.m.visited.len() {
let len = self.m.visited.len();
self.m.visited.reserve_exact(visited_len - len);
for _ in 0..(visited_len - len) {
self.m.visited.push(0);
}
}
}
/// Start backtracking at the given position in the input, but also look
/// for literal prefixes.
fn exec_(&mut self, mut at: InputAt, end: usize) -> bool {
self.clear();
// If this is an anchored regex at the beginning of the input, then
// we're either already done or we only need to try backtracking once.
if self.prog.is_anchored_start {
return if !at.is_start() { false } else { self.backtrack(at) };
}
let mut matched = false;
loop {
if !self.prog.prefixes.is_empty() {
at = match self.input.prefix_at(&self.prog.prefixes, at) {
None => break,
Some(at) => at,
};
}
matched = self.backtrack(at) || matched;
if matched && self.prog.matches.len() == 1 {
return true;
}
if at.pos() >= end {
break;
}
at = self.input.at(at.next_pos());
}
matched
}
/// The main backtracking loop starting at the given input position.
fn backtrack(&mut self, start: InputAt) -> bool {
// N.B. We use an explicit stack to avoid recursion.
// To avoid excessive pushing and popping, most transitions are handled
// in the `step` helper function, which only pushes to the stack when
// there's a capture or a branch.
let mut matched = false;
self.m.jobs.push(Job::Inst { ip: 0, at: start });
while let Some(job) = self.m.jobs.pop() {
match job {
Job::Inst { ip, at } => {
if self.step(ip, at) {
// Only quit if we're matching one regex.
// If we're matching a regex set, then mush on and
// try to find other matches (if we want them).
if self.prog.matches.len() == 1 {
return true;
}
matched = true;
}
}
Job::SaveRestore { slot, old_pos } => {
if slot < self.slots.len() {
self.slots[slot] = old_pos;
}
}
}
}
matched
}
fn step(&mut self, mut ip: InstPtr, mut at: InputAt) -> bool {
use prog::Inst::*;
loop {
// This loop is an optimization to avoid constantly pushing/popping
// from the stack. Namely, if we're pushing a job only to run it
// next, avoid the push and just mutate `ip` (and possibly `at`)
// in place.
if self.has_visited(ip, at) {
return false;
}
match self.prog[ip] {
Match(slot) => {
if slot < self.matches.len() {
self.matches[slot] = true;
}
return true;
}
Save(ref inst) => {
if let Some(&old_pos) = self.slots.get(inst.slot) {
// If this path doesn't work out, then we save the old
// capture index (if one exists) in an alternate
// job. If the next path fails, then the alternate
// job is popped and the old capture index is restored.
self.m.jobs.push(Job::SaveRestore {
slot: inst.slot,
old_pos: old_pos,
});
self.slots[inst.slot] = Some(at.pos());
}
ip = inst.goto;
}
Split(ref inst) => {
self.m.jobs.push(Job::Inst { ip: inst.goto2, at: at });
ip = inst.goto1;
}
EmptyLook(ref inst) => {
if self.input.is_empty_match(at, inst) {
ip = inst.goto;
} else {
return false;
}
}
Char(ref inst) => {
if inst.c == at.char() {
ip = inst.goto;
at = self.input.at(at.next_pos());
} else {
return false;
}
}
Ranges(ref inst) => {
if inst.matches(at.char()) {
ip = inst.goto;
at = self.input.at(at.next_pos());
} else {
return false;
}
}
Bytes(ref inst) => {
if let Some(b) = at.byte() {
if inst.matches(b) {
ip = inst.goto;
at = self.input.at(at.next_pos());
continue;
}
}
return false;
}
}
}
}
fn has_visited(&mut self, ip: InstPtr, at: InputAt) -> bool {
let k = ip * (self.input.len() + 1) + at.pos();
let k1 = k / BIT_SIZE;
let k2 = usize_to_u32(1 << (k & (BIT_SIZE - 1)));
if self.m.visited[k1] & k2 == 0 {
self.m.visited[k1] |= k2;
false
} else {
true
}
}
}
fn usize_to_u32(n: usize) -> u32 {
if (n as u64) > (::std::u32::MAX as u64) {
panic!("BUG: {} is too big to fit into u32", n)
}
n as u32
}