Fix quadratic time on backticks

This commit adds a cache `StateInline->backticks` which remembers
positions for every possible backtick closer (`, ``, ```, etc.).

Algorithm is similar to one described here:
mity/md4c@685b714

close #733

CHANGELOG.md

@@ -10,6 +10,7 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 ### Fixed
 - `[](<foo<bar>)` is no longer a valid link.
 - Fix performance issues when parsing links, #732.
+- Fix performance issues when parsing backticks, #733.
 
 
 ## [12.0.2] - 2020-10-23

lib/rules_inline/backticks.js

@@ -2,8 +2,18 @@
 
 'use strict';
 
+
+function addCodeToken(state, marker, pos, matchStart) {
+  var token     = state.push('code_inline', 'code', 0);
+  token.markup  = marker;
+  token.content = state.src.slice(pos, matchStart)
+    .replace(/\n/g, ' ')
+    .replace(/^ (.+) $/, '$1');
+}
+
+
 module.exports = function backtick(state, silent) {
-  var start, max, marker, matchStart, matchEnd, token,
+  var start, max, marker, matchStart, matchEnd, startLength, endLength,
       pos = state.pos,
       ch = state.src.charCodeAt(pos);
 
@@ -13,31 +23,54 @@ module.exports = function backtick(state, silent) {
   pos++;
   max = state.posMax;
 
+  // scan marker length
   while (pos < max && state.src.charCodeAt(pos) === 0x60/* ` */) { pos++; }
 
   marker = state.src.slice(start, pos);
+  startLength = marker.length;
+
+  // Look for required marker length in the cache first
+  if (state.backticks[startLength] && state.backticks[startLength] > start) {
+    if (state.backticks[startLength] === Infinity) {
+      if (!silent) state.pending += marker;
+      state.pos += startLength;
+    } else {
+      if (!silent) addCodeToken(state, marker, pos, state.backticks[startLength]);
+      state.pos = matchEnd;
+    }
+    return true;
+  }
 
   matchStart = matchEnd = pos;
 
+  // Nothing found in the cache, scan until the end of the line (or until marker is found)
   while ((matchStart = state.src.indexOf('`', matchEnd)) !== -1) {
     matchEnd = matchStart + 1;
 
+    // scan marker length
     while (matchEnd < max && state.src.charCodeAt(matchEnd) === 0x60/* ` */) { matchEnd++; }
 
-    if (matchEnd - matchStart === marker.length) {
-      if (!silent) {
-        token         = state.push('code_inline', 'code', 0);
-        token.markup  = marker;
-        token.content = state.src.slice(pos, matchStart)
-          .replace(/\n/g, ' ')
-          .replace(/^ (.+) $/, '$1');
-      }
+    endLength = matchEnd - matchStart;
+
+    if (endLength === marker.length) {
+      // Found matching closer length.
+      if (!silent) addCodeToken(state, marker, pos, matchStart);
       state.pos = matchEnd;
       return true;
     }
+
+    // Some different length found, put it in cache just in case
+    if (!state.backticks[endLength] || state.backticks[endLength] <= start) {
+      state.backticks[endLength] = matchStart;
+    }
+
+    // Scanned through the end, didn't find anything. Mark "no matches" for this length;
+    if (matchEnd >= max) {
+      state.backticks[startLength] = Infinity;
+    }
   }
 
-  if (!silent) { state.pending += marker; }
-  state.pos += marker.length;
+  if (!silent) state.pending += marker;
+  state.pos += startLength;
   return true;
 };

lib/rules_inline/state_inline.js

@@ -31,6 +31,9 @@ function StateInline(src, md, env, outTokens) {
 
   // Stack of delimiter lists for upper level tags
   this._prev_delimiters = [];
+
+  // backtick length => last seen position
+  this.backticks = {};
 }