Issue 3673 - Merge closely matching rules

lib/abp2blocklist.js

Index: lib/abp2blocklist.js
===================================================================
--- a/lib/abp2blocklist.js
+++ b/lib/abp2blocklist.js
@@ -361,16 +361,239 @@
     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
+  // should occur in order to arrive at the target string.
+
+  let diff = s.length - t.length;
+
+  // If the string lenghts differ by more than one character, we cannot arrive
+  // at target from source in a single edit operation.
+  if (diff < -1 || diff > 1)
+    return null;
+
+  // If target is longer than source, swap them for the purpose of our
+  // calculation.
+  if (diff == -1)
+  {
+    let tmp = s;
+    s = t;
+    t = tmp;
+  }
+
+  let edit = null;
+
+  for (let i = 0, j = 0; i < s.length; i++)
+  {
+    if (s[i] == t[j])
+    {
+      j++;
+    }
+    else if (edit)
+    {
+      // Since we want one and only one edit operation, we must bail here.
+      return null;
+    }
+    else if ((s[i] == "." || s[i] == "+" || s[i] == "$" || s[i] == "?" ||
+              s[i] == "{" || s[i] == "}" || s[i] == "(" || s[i] == ")" ||
+              s[i] == "[" || s[i] == "]" || s[i] == "\\") ||
+             (t[j] == "." || t[j] == "+" || t[j] == "$" || t[j] == "?" ||
+              t[j] == "{" || t[j] == "}" || t[j] == "(" || t[j] == ")" ||
+              t[j] == "[" || t[j] == "]" || t[j] == "\\"))
+    {
+      // We don't deal with special characters for now.

[Manish Jethani, 2017/05/02 15:39:53]

It should be possible to merge "\[" and "\(" into "[\[\(]" but it's probably not
worth the added complexity.

+      return null;
+    }
+    else
+    {
+      switch (diff)
+      {
+        case 0:
+          // If both strings are equal in length, this is a substitution.
+          edit = {type: "substitute", index: i};
+          j++;
+          break;
+        case 1:
+          // If the source string is longer, this is a deletion.
+          edit = {type: "delete", index: i};
+          break;
+        default:
+          edit = {type: "insert", index: i};
+      }
+    }
+  }
+
+  return edit;
+}
+
+function ruleWithoutURLFilter(rule)
+{
+  let copy = {
+    trigger: Object.assign({}, rule.trigger),
+    action: Object.assign({}, rule.action)
+  };
+
+  delete copy.trigger["url-filter"];
+
+  return copy;
+}
+
+function mergeCloselyMatchingRules(rules)
+{
+  let rulesInfo = new Array(rules.length);
+
+  rules.forEach((rule, index) =>
+  {
+    let skip = false;
+
+    // For these rules we're getting dimishing returns. i.e. there aren't too
+    // many of these that qualify anyway, they just slow us down.
+    if (rule.action.type == "css-display-none" ||
+        rule.action.type == "ignore-previous-rules" ||
+        rule.trigger["url-filter-is-case-sensitive"] ||
+        rule.trigger["load-type"] ||
+        rule.trigger["if-domain"] ||
+        rule.trigger["unless-domain"])
+      skip = true;
+
+    if (skip)
+    {
+      rulesInfo[index] = {skip: true};
+    }
+    else
+    {
+      // Save a stringified version of the rule, but without the URL filter. We
+      // use this for comparison later.
+      rulesInfo[index] = {
+        stringifiedWithoutURLFilter: JSON.stringify(ruleWithoutURLFilter(rule))
+      };
+    }
+  });
+
+  for (let i = 0; i < rules.length; i++)
+  {
+    if (rulesInfo[i].skip)
+      continue;
+
+    for (let j = i + 1; j < rules.length; j++)
+    {
+      if (rulesInfo[j].skip)
+        continue;
+
+      // Check if the rules are identical except for the URL filter.
+      if (rulesInfo[i].stringifiedWithoutURLFilter ==
+          rulesInfo[j].stringifiedWithoutURLFilter)
+      {
+        let source = rules[i].trigger["url-filter"];
+        let target = rules[j].trigger["url-filter"];
+
+        // Find out if the Levenshtein distance between the rules is 1.
+        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, to be dealt with later.
+            urlFilter = target;
+            ruleInfo = rulesInfo[j];
+            match.index = i;
+            edit.type = "delete";
+          }
+          else
+          {
+            urlFilter = source;
+            ruleInfo = rulesInfo[i];
+            match.index = j;
+          }
+
+          if (!ruleInfo.matches)
+            ruleInfo.matches = new Array(urlFilter.length + 1);
+
+          let matchesForIndex = ruleInfo.matches[edit.index];
+
+          if (matchesForIndex)
+            matchesForIndex.push(match);
+          else
+            ruleInfo.matches[edit.index] = [match];
+        }
+      }
+    }
+  }
+
+  let mergedRules = [];
+
+  rules.forEach((rule, index) =>
+  {
+    let ruleInfo = rulesInfo[index];
+
+    if (ruleInfo.merged)
+      return;
+
+    // Find the best set of rules to group, which is simply the largest set.
+    let best = null;
+    for (let matchesForIndex of ruleInfo.matches || [])
+    {
+      if (matchesForIndex && (!best || matchesForIndex.length > best.length))
+        best = matchesForIndex;
+    }
+
+    if (best)
+    {
+      // Merge all the matching rules into this one.
+
+      let editIndex = best[0].edit.index;
+
+      let characters = [];
+      let quantifier = "";
+
+      for (let match of best)
+      {
+        if (match.edit.type == "delete")
+          quantifier = "?";
+        else
+          characters.push(rules[match.index].trigger["url-filter"][editIndex]);
+
+        rulesInfo[match.index].merged = true;
+      }
+
+      let urlFilter = rule.trigger["url-filter"];
+
+      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);
+      }
+
+      rule.trigger["url-filter"] = urlFilter;
+    }
+
+    mergedRules.push(rule);
+  });
+
+  return mergedRules;
+}
+
 let ContentBlockerList =
 /**
  * Create a new Adblock Plus filter to content blocker list converter
  *
  * @constructor
  */
 exports.ContentBlockerList = function ()
 {
@@ -467,10 +690,14 @@
 
   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));
+
+  rules = mergeCloselyMatchingRules(rules);
+
+  return rules;
 };