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<!DOCTYPE html><html lang="en"><head><meta charset="utf-8"><meta name="viewport" content="width=device-width, initial-scale=1.0"><meta name="generator" content="rustdoc"><meta name="description" content="Source of the Rust file `/root/.cargo/registry/src/github.com-1ecc6299db9ec823/regex-1.8.3/src/backtrack.rs`."><meta name="keywords" content="rust, rustlang, rust-lang"><title>backtrack.rs - source</title><link rel="preload" as="font" type="font/woff2" crossorigin href="../../SourceSerif4-Regular.ttf.woff2"><link rel="preload" as="font" type="font/woff2" crossorigin href="../../FiraSans-Regular.woff2"><link rel="preload" as="font" type="font/woff2" crossorigin href="../../FiraSans-Medium.woff2"><link rel="preload" as="font" type="font/woff2" crossorigin href="../../SourceCodePro-Regular.ttf.woff2"><link rel="preload" as="font" type="font/woff2" crossorigin href="../../SourceSerif4-Bold.ttf.woff2"><link rel="preload" as="font" type="font/woff2" crossorigin href="../../SourceCodePro-Semibold.ttf.woff2"><link rel="stylesheet" href="../../normalize.css"><link rel="stylesheet" href="../../rustdoc.css" id="mainThemeStyle"><link rel="stylesheet" href="../../ayu.css" disabled><link rel="stylesheet" href="../../dark.css" disabled><link rel="stylesheet" href="../../light.css" id="themeStyle"><script id="default-settings" ></script><script src="../../storage.js"></script><script defer src="../../source-script.js"></script><script defer src="../../source-files.js"></script><script defer src="../../main.js"></script><noscript><link rel="stylesheet" href="../../noscript.css"></noscript><link rel="alternate icon" type="image/png" href="../../favicon-16x16.png"><link rel="alternate icon" type="image/png" href="../../favicon-32x32.png"><link rel="icon" type="image/svg+xml" href="../../favicon.svg"></head><body class="rustdoc source"><!--[if lte IE 11]><div class="warning">This old browser is unsupported and will most likely display funky things.</div><![endif]--><nav class="sidebar"><a class="sidebar-logo" href="../../regex/index.html"><div class="logo-container"><img class="rust-logo" src="../../rust-logo.svg" alt="logo"></div></a></nav><main><div class="width-limiter"><nav class="sub"><a class="sub-logo-container" href="../../regex/index.html"><img class="rust-logo" src="../../rust-logo.svg" alt="logo"></a><form class="search-form"><div class="search-container"><span></span><input class="search-input" name="search" autocomplete="off" spellcheck="false" placeholder="Click or press ‘S’ to search, ‘?’ for more options…" type="search"><div id="help-button" title="help" tabindex="-1"><a href="../../help.html">?</a></div><div id="settings-menu" tabindex="-1"><a href="../../settings.html" title="settings"><img width="22" height="22" alt="Change settings" src="../../wheel.svg"></a></div></div></form></nav><section id="main-content" class="content"><div class="example-wrap"><pre class="src-line-numbers"><span id="1">1</span>
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</pre><pre class="rust"><code><span class="comment">// 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&#39;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&#39;t
// scale, even if you&#39;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.
</span><span class="kw">use </span><span class="kw">crate</span>::exec::ProgramCache;
<span class="kw">use </span><span class="kw">crate</span>::input::{Input, InputAt};
<span class="kw">use </span><span class="kw">crate</span>::prog::{InstPtr, Program};
<span class="kw">use </span><span class="kw">crate</span>::re_trait::Slot;
<span class="kw">type </span>Bits = u32;
<span class="kw">const </span>BIT_SIZE: usize = <span class="number">32</span>;
<span class="kw">const </span>MAX_SIZE_BYTES: usize = <span class="number">256 </span>* (<span class="number">1 </span>&lt;&lt; <span class="number">10</span>); <span class="comment">// 256 KB
</span><span class="doccomment">/// Returns true iff the given regex and input should be executed by this
/// engine with reasonable memory usage.
</span><span class="kw">pub fn </span>should_exec(num_insts: usize, text_len: usize) -&gt; bool {
<span class="comment">// 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
</span><span class="kw">let </span>size = ((num_insts * (text_len + <span class="number">1</span>) + BIT_SIZE - <span class="number">1</span>) / BIT_SIZE) * <span class="number">4</span>;
size &lt;= MAX_SIZE_BYTES
}
<span class="doccomment">/// A backtracking matching engine.
</span><span class="attribute">#[derive(Debug)]
</span><span class="kw">pub struct </span>Bounded&lt;<span class="lifetime">&#39;a</span>, <span class="lifetime">&#39;m</span>, <span class="lifetime">&#39;r</span>, <span class="lifetime">&#39;s</span>, I&gt; {
prog: <span class="kw-2">&amp;</span><span class="lifetime">&#39;r </span>Program,
input: I,
matches: <span class="kw-2">&amp;</span><span class="lifetime">&#39;m </span><span class="kw-2">mut </span>[bool],
slots: <span class="kw-2">&amp;</span><span class="lifetime">&#39;s </span><span class="kw-2">mut </span>[Slot],
m: <span class="kw-2">&amp;</span><span class="lifetime">&#39;a </span><span class="kw-2">mut </span>Cache,
}
<span class="doccomment">/// Shared cached state between multiple invocations of a backtracking engine
/// in the same thread.
</span><span class="attribute">#[derive(Clone, Debug)]
</span><span class="kw">pub struct </span>Cache {
jobs: Vec&lt;Job&gt;,
visited: Vec&lt;Bits&gt;,
}
<span class="kw">impl </span>Cache {
<span class="doccomment">/// Create new empty cache for the backtracking engine.
</span><span class="kw">pub fn </span>new(_prog: <span class="kw-2">&amp;</span>Program) -&gt; <span class="self">Self </span>{
Cache { jobs: <span class="macro">vec!</span>[], visited: <span class="macro">vec!</span>[] }
}
}
<span class="doccomment">/// A job is an explicit unit of stack space in the backtracking engine.
///
/// The &quot;normal&quot; 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.
</span><span class="attribute">#[derive(Clone, Copy, Debug)]
</span><span class="kw">enum </span>Job {
Inst { ip: InstPtr, at: InputAt },
SaveRestore { slot: usize, old_pos: <span class="prelude-ty">Option</span>&lt;usize&gt; },
}
<span class="kw">impl</span>&lt;<span class="lifetime">&#39;a</span>, <span class="lifetime">&#39;m</span>, <span class="lifetime">&#39;r</span>, <span class="lifetime">&#39;s</span>, I: Input&gt; Bounded&lt;<span class="lifetime">&#39;a</span>, <span class="lifetime">&#39;m</span>, <span class="lifetime">&#39;r</span>, <span class="lifetime">&#39;s</span>, I&gt; {
<span class="doccomment">/// Execute the backtracking matching engine.
///
/// If there&#39;s a match, `exec` returns `true` and populates the given
/// captures accordingly.
</span><span class="kw">pub fn </span>exec(
prog: <span class="kw-2">&amp;</span><span class="lifetime">&#39;r </span>Program,
cache: <span class="kw-2">&amp;</span>ProgramCache,
matches: <span class="kw-2">&amp;</span><span class="lifetime">&#39;m </span><span class="kw-2">mut </span>[bool],
slots: <span class="kw-2">&amp;</span><span class="lifetime">&#39;s </span><span class="kw-2">mut </span>[Slot],
input: I,
start: usize,
end: usize,
) -&gt; bool {
<span class="kw">let </span><span class="kw-2">mut </span>cache = cache.borrow_mut();
<span class="kw">let </span>cache = <span class="kw-2">&amp;mut </span>cache.backtrack;
<span class="kw">let </span>start = input.at(start);
<span class="kw">let </span><span class="kw-2">mut </span>b = Bounded { prog, input, matches, slots, m: cache };
b.exec_(start, end)
}
<span class="doccomment">/// Clears the cache such that the backtracking engine can be executed
/// on some input of fixed length.
</span><span class="kw">fn </span>clear(<span class="kw-2">&amp;mut </span><span class="self">self</span>) {
<span class="comment">// Reset the job memory so that we start fresh.
</span><span class="self">self</span>.m.jobs.clear();
<span class="comment">// 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&#39;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 using unchecked
// operations doesn&#39;t seem to have a measurable impact on performance.
// (Probably because backtracking is limited to such small
// inputs/regexes in the first place.)
</span><span class="kw">let </span>visited_len =
(<span class="self">self</span>.prog.len() * (<span class="self">self</span>.input.len() + <span class="number">1</span>) + BIT_SIZE - <span class="number">1</span>)
/ BIT_SIZE;
<span class="self">self</span>.m.visited.truncate(visited_len);
<span class="kw">for </span>v <span class="kw">in </span><span class="kw-2">&amp;mut </span><span class="self">self</span>.m.visited {
<span class="kw-2">*</span>v = <span class="number">0</span>;
}
<span class="kw">if </span>visited_len &gt; <span class="self">self</span>.m.visited.len() {
<span class="kw">let </span>len = <span class="self">self</span>.m.visited.len();
<span class="self">self</span>.m.visited.reserve_exact(visited_len - len);
<span class="kw">for _ in </span><span class="number">0</span>..(visited_len - len) {
<span class="self">self</span>.m.visited.push(<span class="number">0</span>);
}
}
}
<span class="doccomment">/// Start backtracking at the given position in the input, but also look
/// for literal prefixes.
</span><span class="kw">fn </span>exec_(<span class="kw-2">&amp;mut </span><span class="self">self</span>, <span class="kw-2">mut </span>at: InputAt, end: usize) -&gt; bool {
<span class="self">self</span>.clear();
<span class="comment">// If this is an anchored regex at the beginning of the input, then
// we&#39;re either already done or we only need to try backtracking once.
</span><span class="kw">if </span><span class="self">self</span>.prog.is_anchored_start {
<span class="kw">return if </span>!at.is_start() { <span class="bool-val">false </span>} <span class="kw">else </span>{ <span class="self">self</span>.backtrack(at) };
}
<span class="kw">let </span><span class="kw-2">mut </span>matched = <span class="bool-val">false</span>;
<span class="kw">loop </span>{
<span class="kw">if </span>!<span class="self">self</span>.prog.prefixes.is_empty() {
at = <span class="kw">match </span><span class="self">self</span>.input.prefix_at(<span class="kw-2">&amp;</span><span class="self">self</span>.prog.prefixes, at) {
<span class="prelude-val">None </span>=&gt; <span class="kw">break</span>,
<span class="prelude-val">Some</span>(at) =&gt; at,
};
}
matched = <span class="self">self</span>.backtrack(at) || matched;
<span class="kw">if </span>matched &amp;&amp; <span class="self">self</span>.prog.matches.len() == <span class="number">1 </span>{
<span class="kw">return </span><span class="bool-val">true</span>;
}
<span class="kw">if </span>at.pos() &gt;= end {
<span class="kw">break</span>;
}
at = <span class="self">self</span>.input.at(at.next_pos());
}
matched
}
<span class="doccomment">/// The main backtracking loop starting at the given input position.
</span><span class="kw">fn </span>backtrack(<span class="kw-2">&amp;mut </span><span class="self">self</span>, start: InputAt) -&gt; bool {
<span class="comment">// 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&#39;s a capture or a branch.
</span><span class="kw">let </span><span class="kw-2">mut </span>matched = <span class="bool-val">false</span>;
<span class="self">self</span>.m.jobs.push(Job::Inst { ip: <span class="number">0</span>, at: start });
<span class="kw">while let </span><span class="prelude-val">Some</span>(job) = <span class="self">self</span>.m.jobs.pop() {
<span class="kw">match </span>job {
Job::Inst { ip, at } =&gt; {
<span class="kw">if </span><span class="self">self</span>.step(ip, at) {
<span class="comment">// Only quit if we&#39;re matching one regex.
// If we&#39;re matching a regex set, then mush on and
// try to find other matches (if we want them).
</span><span class="kw">if </span><span class="self">self</span>.prog.matches.len() == <span class="number">1 </span>{
<span class="kw">return </span><span class="bool-val">true</span>;
}
matched = <span class="bool-val">true</span>;
}
}
Job::SaveRestore { slot, old_pos } =&gt; {
<span class="kw">if </span>slot &lt; <span class="self">self</span>.slots.len() {
<span class="self">self</span>.slots[slot] = old_pos;
}
}
}
}
matched
}
<span class="kw">fn </span>step(<span class="kw-2">&amp;mut </span><span class="self">self</span>, <span class="kw-2">mut </span>ip: InstPtr, <span class="kw-2">mut </span>at: InputAt) -&gt; bool {
<span class="kw">use </span><span class="kw">crate</span>::prog::Inst::<span class="kw-2">*</span>;
<span class="kw">loop </span>{
<span class="comment">// This loop is an optimization to avoid constantly pushing/popping
// from the stack. Namely, if we&#39;re pushing a job only to run it
// next, avoid the push and just mutate `ip` (and possibly `at`)
// in place.
</span><span class="kw">if </span><span class="self">self</span>.has_visited(ip, at) {
<span class="kw">return </span><span class="bool-val">false</span>;
}
<span class="kw">match </span><span class="self">self</span>.prog[ip] {
Match(slot) =&gt; {
<span class="kw">if </span>slot &lt; <span class="self">self</span>.matches.len() {
<span class="self">self</span>.matches[slot] = <span class="bool-val">true</span>;
}
<span class="kw">return </span><span class="bool-val">true</span>;
}
Save(<span class="kw-2">ref </span>inst) =&gt; {
<span class="kw">if let </span><span class="prelude-val">Some</span>(<span class="kw-2">&amp;</span>old_pos) = <span class="self">self</span>.slots.get(inst.slot) {
<span class="comment">// If this path doesn&#39;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.
</span><span class="self">self</span>.m.jobs.push(Job::SaveRestore {
slot: inst.slot,
old_pos,
});
<span class="self">self</span>.slots[inst.slot] = <span class="prelude-val">Some</span>(at.pos());
}
ip = inst.goto;
}
Split(<span class="kw-2">ref </span>inst) =&gt; {
<span class="self">self</span>.m.jobs.push(Job::Inst { ip: inst.goto2, at });
ip = inst.goto1;
}
EmptyLook(<span class="kw-2">ref </span>inst) =&gt; {
<span class="kw">if </span><span class="self">self</span>.input.is_empty_match(at, inst) {
ip = inst.goto;
} <span class="kw">else </span>{
<span class="kw">return </span><span class="bool-val">false</span>;
}
}
Char(<span class="kw-2">ref </span>inst) =&gt; {
<span class="kw">if </span>inst.c == at.char() {
ip = inst.goto;
at = <span class="self">self</span>.input.at(at.next_pos());
} <span class="kw">else </span>{
<span class="kw">return </span><span class="bool-val">false</span>;
}
}
Ranges(<span class="kw-2">ref </span>inst) =&gt; {
<span class="kw">if </span>inst.matches(at.char()) {
ip = inst.goto;
at = <span class="self">self</span>.input.at(at.next_pos());
} <span class="kw">else </span>{
<span class="kw">return </span><span class="bool-val">false</span>;
}
}
Bytes(<span class="kw-2">ref </span>inst) =&gt; {
<span class="kw">if let </span><span class="prelude-val">Some</span>(b) = at.byte() {
<span class="kw">if </span>inst.matches(b) {
ip = inst.goto;
at = <span class="self">self</span>.input.at(at.next_pos());
<span class="kw">continue</span>;
}
}
<span class="kw">return </span><span class="bool-val">false</span>;
}
}
}
}
<span class="kw">fn </span>has_visited(<span class="kw-2">&amp;mut </span><span class="self">self</span>, ip: InstPtr, at: InputAt) -&gt; bool {
<span class="kw">let </span>k = ip * (<span class="self">self</span>.input.len() + <span class="number">1</span>) + at.pos();
<span class="kw">let </span>k1 = k / BIT_SIZE;
<span class="kw">let </span>k2 = usize_to_u32(<span class="number">1 </span>&lt;&lt; (k &amp; (BIT_SIZE - <span class="number">1</span>)));
<span class="kw">if </span><span class="self">self</span>.m.visited[k1] &amp; k2 == <span class="number">0 </span>{
<span class="self">self</span>.m.visited[k1] |= k2;
<span class="bool-val">false
</span>} <span class="kw">else </span>{
<span class="bool-val">true
</span>}
}
}
<span class="kw">fn </span>usize_to_u32(n: usize) -&gt; u32 {
<span class="kw">if </span>(n <span class="kw">as </span>u64) &gt; (::std::u32::MAX <span class="kw">as </span>u64) {
<span class="macro">panic!</span>(<span class="string">&quot;BUG: {} is too big to fit into u32&quot;</span>, n)
}
n <span class="kw">as </span>u32
}
</code></pre></div>
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