Issue 3673 - Merge closely matching rules

lib/abp2blocklist.js

Index: lib/abp2blocklist.js
===================================================================
--- a/lib/abp2blocklist.js
+++ b/lib/abp2blocklist.js
@@ -361,16 +361,375 @@
     newSelector.push('[id=', selector.substring(pos.start + 1, pos.end), ']');
     i = pos.end;
   }
   newSelector.push(selector.substring(i));
 
   return newSelector.join("");
 }
 
+function closeMatch(s, t)
+{
+  // This function returns an edit operation, one of "substitute", "delete",
+  // and "insert", along with an index in the source string where the edit must
+  // occur in order to arrive at the target string. If the strings are not a
+  // close match, it returns null.
+  //
+  // Two strings are considered to be a close match if they are one edit
+  // operation apart.
+  //
+  // Deletions or insertions of a contiguous range of characters from one
+  // string into the other, at the same index, are treated as a single edit.
+  // For example, "internal" and "international" are considered to be one edit
+  // apart and therefore a close match.
+
+  // A few things to note:
+  //
+  //   1) This function does not care about the format of the input strings.
+  //   For example, the caller may pass in regular expressions, where "[ab]"
+  //   and "[bc]" could be considered to be a close match, since the order
+  //   within the brackets doesn't matter. This function will still return null
+  //   for this set of inputs since they are two edits apart.
+  //
+  //   2) To be friendly to calling code that might be passing in regular
+  //   expressions, this function will simply return null if it encounters a
+  //   special character (e.g. "\", "?", "+", etc.) in the delta. For example,
+  //   given "Hello" and "Hello, how are you?", it will return null.
+  //
+  //   3) If the caller does indeed pass in regular expressions, it must make
+  //   the important assumption that the parts where two such regular
+  //   expressions may differ can always be treated as normal strings. For
+  //   example, "^https?://.*/ads" and "^https?://.*/adv" differ only in the
+  //   last character, therefore the regular expressions can safely be merged
+  //   into "^https?://.*/ad[sv]".
+
+  let diff = s.length - t.length;
+
+  // If target is longer than source, swap them for the purpose of our
+  // calculation.
+  if (diff < 0)
+  {
+    let tmp = s;
+    s = t;
+    t = tmp;
+  }
+
+  let edit = null;
+
+  let i = 0, j = 0;
+
+  for (; i < s.length; i++)
+  {
+    if (s[i] != t[i])
+      break;
+  }
+
+  for (; j < t.length; j++)
+  {
+    if (t.length - j == i || s[s.length - j - 1] != t[t.length - j - 1])
+      break;
+  }
+
+  if (diff == 0)
+  {
+    if (t.length - j - i != 1)
+      return null;
+  }
+  else if (i != t.length - j)
+  {
+    return null;
+  }
+
+  for (let k = i; k < s.length - j; k++)
+  {
+    // If there are any special characters in the delta, bail.
+    if (s[k] == "." || s[k] == "+" || s[k] == "$" || s[k] == "?" ||
+        s[k] == "{" || s[k] == "}" || s[k] == "(" || s[k] == ")" ||
+        s[k] == "[" || s[k] == "]" || s[k] == "\\")
+      return null;
+  }
+
+  if (diff == 0)
+  {
+    edit = {type: "substitute", index: i};
+  }
+  else if (diff > 0)
+  {
+    edit = {type: "delete", index: i};
+
+    if (diff > 1)
+      edit.endIndex = s.length - j;
+  }
+  else
+  {
+    edit = {type: "insert", index: i};
+
+    if (diff < -1)
+      edit.endIndex = s.length - j;
+  }
+
+  return edit;
+}
+
+function performRuleMerging(rulesInfo, exhaustive)
+{
+  // Closely matching rules are likely to be within a certain range. We only
+  // look for matches within this range. If we increase this value, it can give
+  // us more matches and a smaller resulting rule set, but possibly at a
+  // significant performance cost.
+  const heuristicRange = 100;
+
+  for (let i = 0; i < rulesInfo.length; i++)
+  {
+    let limit = exhaustive ? rulesInfo.length :
+                Math.min(i + heuristicRange, rulesInfo.length);
+
+    for (let j = i + 1; j < limit; j++)
+    {
+      let source = rulesInfo[i].rule.trigger["url-filter"];
+      let target = rulesInfo[j].rule.trigger["url-filter"];
+
+      let edit = closeMatch(source, target);
+
+      if (edit)
+      {
+        let urlFilter, ruleInfo, match = {edit};
+
+        if (edit.type == "insert")
+        {
+          // Convert the insertion into a deletion and stick it on the target
+          // rule instead. We can only group deletions and substitutions;
+          // therefore insertions must be treated as deletions on the target
+          // rule.
+          urlFilter = target;
+          ruleInfo = rulesInfo[j];
+          match.index = i;
+          edit.type = "delete";
+        }
+        else
+        {
+          urlFilter = source;
+          ruleInfo = rulesInfo[i];
+          match.index = j;
+        }
+
+        // If the edit has an end index, it represents a multiple character
+        // edit.
+        let multiEdit = !!edit.endIndex;
+
+        if (multiEdit)
+        {
+          // We only care about a single multiple character edit because the
+          // number of characters for such a match doesn't matter, we can
+          // only merge with one other rule.
+          if (!ruleInfo.multiEditMatch)
+            ruleInfo.multiEditMatch = match;
+        }
+        else
+        {
+          // For single character edits, multiple rules can be merged into
+          // one. e.g. "ad", "ads", and "adv" can be merged into "ad[sv]?".
+          if (!ruleInfo.matches)
+            ruleInfo.matches = new Array(urlFilter.length + 1);
+
+          // Matches at a particular index. For example, for a source string
+          // "ads", both target strings "ad" (deletion) and "adv"
+          // (substitution) match at index 2, hence they are grouped together
+          // to possibly be merged later into "ad[sv]?".
+          let matchesForIndex = ruleInfo.matches[edit.index];
+
+          if (matchesForIndex)
+          {
+            matchesForIndex.push(match);
+          }
+          else
+          {
+            matchesForIndex = [match];
+            ruleInfo.matches[edit.index] = matchesForIndex;
+          }
+
+          // Keep track of the best set of matches. We later sort by this to
+          // get best results.
+          if (!ruleInfo.bestMatches ||
+              matchesForIndex.length > ruleInfo.bestMatches.length)
+            ruleInfo.bestMatches = matchesForIndex;
+        }
+      }
+    }
+  }
+
+  // Filter out rules that have no matches at all.
+  let candidateRulesInfo = rulesInfo.filter(ruleInfo =>
+  {
+    return ruleInfo.bestMatches || ruleInfo.multiEditMatch
+  });
+
+  // For best results, we have to sort the candidates by the largest set of
+  // matches.
+  //
+  // For example, we want "ads", "bds", "adv", "bdv", "adx", and "bdx" to
+  // generate "ad[svx]" and "bd[svx]" (2 rules), not "[ab]ds", "[ab]dv", and
+  // "[ab]dx" (3 rules).
+  candidateRulesInfo.sort((ruleInfo1, ruleInfo2) =>
+  {
+    let weight1 = ruleInfo1.bestMatches ? ruleInfo1.bestMatches.length :
+                  ruleInfo1.multiEditMatch ? 1 : 0;
+    let weight2 = ruleInfo2.bestMatches ? ruleInfo2.bestMatches.length :
+                  ruleInfo2.multiEditMatch ? 1 : 0;
+
+    return weight2 - weight1;
+  });
+
+  for (let ruleInfo of candidateRulesInfo)
+  {
+    let rule = ruleInfo.rule;
+
+    // If this rule has already been merged into another rule, we skip it.
+    if (ruleInfo.merged)
+      continue;
+
+    // Find the best set of rules to group, which is simply the largest set.
+    let best = (ruleInfo.matches || []).reduce((best, matchesForIndex) =>
+    {
+      matchesForIndex = (matchesForIndex || []).filter(match =>
+      {
+        // Filter out rules that have either already been merged into other
+        // rules or have had other rules merged into them.
+        return !rulesInfo[match.index].merged &&
+               !rulesInfo[match.index].mergedInto;
+      });
+
+      return matchesForIndex.length > best.length ? matchesForIndex : best;
+    },
+    []);
+
+    let multiEdit = false;
+
+    // If we couldn't find a single rule to merge with, let's see if we have a
+    // multiple character edit. e.g. we could merge "ad" and "adserver" into
+    // "ad(server)?".
+    if (best.length == 0 && ruleInfo.multiEditMatch &&
+        !rulesInfo[ruleInfo.multiEditMatch.index].merged &&
+        !rulesInfo[ruleInfo.multiEditMatch.index].mergedInto)
+    {
+      best = [ruleInfo.multiEditMatch];
+      multiEdit = true;
+    }
+
+    if (best.length > 0)
+    {
+      let urlFilter = rule.trigger["url-filter"];
+
+      let editIndex = best[0].edit.index;
+
+      if (!multiEdit)
+      {
+        // Merge all the matching rules into this one.
+
+        let characters = [];
+        let quantifier = "";
+
+        for (let match of best)
+        {
+          if (match.edit.type == "delete")
+          {
+            quantifier = "?";
+          }
+          else
+          {
+            let character = rulesInfo[match.index].rule
+                            .trigger["url-filter"][editIndex];
+            characters.push(character);
+          }
+
+          // Mark the target rule as merged so other rules don't try to merge
+          // it again.
+          rulesInfo[match.index].merged = true;
+        }
+
+        urlFilter = urlFilter.substring(0, editIndex + 1) + quantifier +
+                    urlFilter.substring(editIndex + 1);
+        if (characters.length > 0)
+        {
+          urlFilter = urlFilter.substring(0, editIndex) + "[" +
+                      urlFilter[editIndex] + characters.join("") + "]" +
+                      urlFilter.substring(editIndex + 1);
+        }
+      }
+      else
+      {
+        let editEndIndex = best[0].edit.endIndex;
+
+        // Mark the target rule as merged so other rules don't try to merge it
+        // again.
+        rulesInfo[best[0].index].merged = true;
+
+        urlFilter = urlFilter.substring(0, editIndex) + "(" +
+                    urlFilter.substring(editIndex, editEndIndex) + ")?" +
+                    urlFilter.substring(editEndIndex);
+      }
+
+      rule.trigger["url-filter"] = urlFilter;
+
+      // Mark this rule as one that has had other rules merged into it.
+      ruleInfo.mergedInto = true;
+    }
+  }
+}
+
+function mergeCloselyMatchingRules(rules, options)
+{
+  const defaultOptions = {exhaustive: false};
+
+  options = Object.assign({}, defaultOptions, options);
+
+  let rulesInfo = new Array(rules.length);
+
+  let mergeableRulesInfoByGroup = new Map();
+
+  rules.forEach((rule, index) =>
+  {
+    rulesInfo[index] = {rule};
+
+    // Skip rules we do not want to merge, for whatever reason.
+    if (rule.action.type == "ignore-previous-rules")
+      return;
+
+    // Group rules based on all their properties except the URL filter.
+
+    let copy = {
+      trigger: Object.assign({}, rule.trigger),
+      action: Object.assign({}, rule.action)
+    };
+
+    delete copy.trigger["url-filter"];
+
+    let groupKey = JSON.stringify(copy);
+
+    let mergeableRulesInfo = mergeableRulesInfoByGroup.get(groupKey);
+
+    if (mergeableRulesInfo)
+      mergeableRulesInfo.push(rulesInfo[index]);
+    else
+      mergeableRulesInfoByGroup.set(groupKey, [rulesInfo[index]]);
+  });
+
+  mergeableRulesInfoByGroup.forEach(mergeableRulesInfo =>
+  {
+    if (mergeableRulesInfo.length > 1)
+    {
+      performRuleMerging(mergeableRulesInfo, options.exhaustive);
+    }
+  });
+
+  // Filter out rules that have been merged into other rules.
+  return rulesInfo.filter(ruleInfo => !ruleInfo.merged)
+         .map(ruleInfo => ruleInfo.rule);
+}
+
 let ContentBlockerList =
 /**
  * Create a new Adblock Plus filter to content blocker list converter
  *
  * @constructor
  */
 exports.ContentBlockerList = function ()
 {
@@ -419,18 +778,25 @@
   }
 };
 
 /**
  * Generate content blocker list for all filters that were added
  *
  * @returns   {Filter}   filter    Filter to convert
  */
-ContentBlockerList.prototype.generateRules = function(filter)
+ContentBlockerList.prototype.generateRules = function(options)
 {
+  const defaultOptions = {
+    merge: false,
+    exhaustiveMerge: false
+  };
+
+  options = Object.assign({}, defaultOptions, options);
+
   let rules = [];
 
   let groupedElemhideFilters = new Map();
   for (let filter of this.elemhideFilters)
   {
     let result = convertElemHideFilter(filter, this.elemhideSelectorExceptions);
     if (!result)
       continue;
@@ -467,10 +833,21 @@
 
   for (let filter of this.elemhideExceptions)
     convertFilterAddRules(rules, filter, "ignore-previous-rules", false);
   for (let filter of this.requestFilters)
     convertFilterAddRules(rules, filter, "block", true);
   for (let filter of this.requestExceptions)
     convertFilterAddRules(rules, filter, "ignore-previous-rules", true);
 
-  return rules.filter(rule => !hasNonASCI(rule));
+  rules = rules.filter(rule => !hasNonASCI(rule));
+
+  if (options.merge)
+  {
+    let mergeOptions = {
+      exhaustive: options.exhaustiveMerge
+    };
+
+    rules = mergeCloselyMatchingRules(rules, mergeOptions);
+  }
+
+  return rules;
 };