Issue 3673 - Merge closely matching rules

lib/abp2blocklist.js

 /*
  * This file is part of Adblock Plus <https://adblockplus.org/>,
  * Copyright (C) 2006-2017 eyeo GmbH
  *
  * Adblock Plus is free software: you can redistribute it and/or modify
  * it under the terms of the GNU General Public License version 3 as
  * published by the Free Software Foundation.
  *
  * Adblock Plus is distributed in the hope that it will be useful,
  * but WITHOUT ANY WARRANTY; without even the implied warranty of
  * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
  * GNU General Public License for more details.
  *
  * You should have received a copy of the GNU General Public License
  * along with Adblock Plus.  If not, see <http://www.gnu.org/licenses/>.
  */
 
 /** @module abp2blocklist */
 
 "use strict";
 
 let filterClasses = require("filterClasses");
-let tldjs = require("tldjs");
 let punycode = require("punycode");
 
 const selectorLimit = 5000;
 const typeMap = filterClasses.RegExpFilter.typeMap;
-const whitelistableRequestTypes = (typeMap.IMAGE
-                                   | typeMap.STYLESHEET
-                                   | typeMap.SCRIPT
-                                   | typeMap.FONT
-                                   | typeMap.MEDIA
-                                   | typeMap.POPUP
-                                   | typeMap.OBJECT
-                                   | typeMap.OBJECT_SUBREQUEST
-                                   | typeMap.XMLHTTPREQUEST
-                                   | typeMap.PING
-                                   | typeMap.SUBDOCUMENT
-                                   | typeMap.OTHER);
+
+const httpRequestTypes = typeMap.IMAGE |
+                         typeMap.STYLESHEET |
+                         typeMap.SCRIPT |
+                         typeMap.FONT |
+                         typeMap.MEDIA |
+                         typeMap.POPUP |
+                         typeMap.OBJECT |
+                         typeMap.OBJECT_SUBREQUEST |
+                         typeMap.XMLHTTPREQUEST |
+                         typeMap.PING |
+                         typeMap.SUBDOCUMENT |
+                         typeMap.OTHER;
+const rawRequestTypes = typeMap.XMLHTTPREQUEST |
+                        typeMap.WEBSOCKET |
+                        typeMap.WEBRTC |
+                        typeMap.OBJECT_SUBREQUEST |
+                        typeMap.PING |
+                        typeMap.OTHER;
+const whitelistableRequestTypes = httpRequestTypes |
+                                  typeMap.WEBSOCKET |
+                                  typeMap.WEBRTC;
+
+function callLater(func)
+{
+  return new Promise(resolve =>
+  {
+    let call = () => resolve(func());
+
+    // If this looks like Node.js, call process.nextTick, otherwise call
+    // setTimeout.
+    if (typeof process != "undefined")
+      process.nextTick(call);
+    else
+      setTimeout(call, 0);
+  });
+}
+
+function async(callees, mapFunction)
+{
+  if (!(Symbol.iterator in callees))
+    callees = [callees];
+
+  let lastPause = Date.now();
+  let index = 0;
+
+  let promise = Promise.resolve();
+
+  for (let next of callees)
+  {
+    let currentIndex = index;
+
+    promise = promise.then(() =>
+    {
+      if (mapFunction)
+        next = mapFunction(next, currentIndex);
+
+      // If it has been 100ms or longer since the last call, take a pause. This
+      // keeps the browser from freezing up.
+      let now = Date.now();
+      if (now - lastPause >= 100)
+      {
+        lastPause = now;
+        return callLater(next);
+      }
+
+      return next();
+    });
+
+    index++;
+  }
+
+  return promise;
+}
 
 function parseDomains(domains, included, excluded)
 {
   for (let domain in domains)
   {
     if (domain != "")
     {
       let enabled = domains[domain];
       domain = punycode.toASCII(domain.toLowerCase());
 
       if (!enabled)
         excluded.push(domain);
       else if (!domains[""])
         included.push(domain);
     }
   }
 }
 
 function escapeRegExp(s)
 {
   return s.replace(/[.*+?^${}()|[\]\\]/g, "\\$&");
 }
 
 function matchDomain(domain)
 {
+  if (!domain)
+    return "^https?://";
+
   return "^https?://([^/:]*\\.)?" + escapeRegExp(domain).toLowerCase() + "[/:]";
+}
+
+function getURLSchemes(contentType)
+{
+  // If the given content type includes all supported URL schemes, simply
+  // return a single generic URL scheme pattern. This minimizes the size of the
+  // generated rule set. The downside to this is that it will also match
+  // schemes that we do not want to match (e.g. "ftp://"), but this can be
+  // mitigated by adding exceptions for those schemes.
+  if (contentType & typeMap.WEBSOCKET && contentType & typeMap.WEBRTC &&
+      contentType & httpRequestTypes)
+    return ["[^:]+:(//)?"];
+
+  let urlSchemes = [];
+
+  if (contentType & typeMap.WEBSOCKET)
+    urlSchemes.push("wss?://");
+
+  if (contentType & typeMap.WEBRTC)
+    urlSchemes.push("stuns?:", "turns?:");
+
+  if (contentType & httpRequestTypes)
+    urlSchemes.push("https?://");
+
+  return urlSchemes;
+}
+
+function findSubdomainsInList(domain, list)
+{
+  let subdomains = [];
+  let suffixLength = domain.length + 1;
+
+  for (let name of list)
+  {
+    if (name.length > suffixLength && name.slice(-suffixLength) == "." + domain)
+      subdomains.push(name.slice(0, -suffixLength));
+  }
+
+  return subdomains;
+}
+
+function extractFilterDomains(filters)
+{
+  let domains = new Set();
+  for (let filter of filters)
+  {
+    let parsed = parseFilterRegexpSource(filter.regexpSource);
+    if (parsed.justHostname)
+      domains.add(parsed.hostname);
+  }
+  return domains;
 }
 
 function convertElemHideFilter(filter, elemhideSelectorExceptions)
 {
   let included = [];
   let excluded = [];
-  let rules = [];
 
   parseDomains(filter.domains, included, excluded);
 
   if (excluded.length == 0 && !(filter.selector in elemhideSelectorExceptions))
-    return {matchDomains: included.map(matchDomain), selector: filter.selector};
+    return {matchDomains: included, selector: filter.selector};
 }
 
 /**
  * Parse the given filter "regexpSource" string. Producing a regular expression,
  * extracting the hostname (if any), deciding if the regular expression is safe
  * to be converted + matched as lower case and noting if the source contains
  * anything after the hostname.)
  *
  * @param   {string} text regexpSource property of a filter
+ * @param   {string} urlScheme The URL scheme to use in the regular expression
  * @returns {object} An object containing a regular expression string, a bool
  *                   indicating if the filter can be safely matched as lower
  *                   case, a hostname string (or undefined) and a bool
  *                   indicating if the source only contains a hostname or not:
  *                     {regexp: "...",
  *                      canSafelyMatchAsLowercase: true/false,
  *                      hostname: "...",
  *                      justHostname: true/false}
  */
-function parseFilterRegexpSource(text)
+function parseFilterRegexpSource(text, urlScheme)
 {
   let regexp = [];
-  let lastIndex = text.length - 1;
+
+  // Convert the text into an array of Unicode characters.
+  //
+  // In the case of surrogate pairs (the smiley emoji, for example), one
+  // Unicode code point is represented by two JavaScript characters together.
+  // We want to iterate over Unicode code points rather than JavaScript
+  // characters.
+  let characters = Array.from(text);
+
+  let lastIndex = characters.length - 1;
   let hostname;
   let hostnameStart = null;
   let hostnameFinished = false;
   let justHostname = false;
   let canSafelyMatchAsLowercase = false;
 
-  for (let i = 0; i < text.length; i++)
-  {
-    let c = text[i];
+  if (!urlScheme)
+    urlScheme = getURLSchemes()[0];
+
+  for (let i = 0; i < characters.length; i++)
+  {
+    let c = characters[i];
 
     if (hostnameFinished)
       justHostname = false;
 
     // If we're currently inside the hostname we have to be careful not to
     // escape any characters until after we have converted it to punycode.
     if (hostnameStart != null && !hostnameFinished)
     {
       let endingChar = (c == "*" || c == "^" ||
                         c == "?" || c == "/" || c == "|");
       if (!endingChar && i != lastIndex)
         continue;
 
       hostname = punycode.toASCII(
-        text.substring(hostnameStart, endingChar ? i : i + 1)
+        characters.slice(hostnameStart, endingChar ? i : i + 1).join("")
+                  .toLowerCase()
       );
       hostnameFinished = justHostname = true;
       regexp.push(escapeRegExp(hostname));
       if (!endingChar)
         break;
     }
 
     switch (c)
     {
       case "*":
-        if (regexp.length > 0 && i < lastIndex && text[i + 1] != "*")
+        if (regexp.length > 0 && i < lastIndex && characters[i + 1] != "*")
           regexp.push(".*");
         break;
       case "^":
-        if (i < lastIndex)
-          regexp.push(".");
+        let alphabet = "a-z";
+        // If justHostname is true and we've encountered a "^", it means we're
+        // still in the hostname part of the URL. Since hostnames are always
+        // lower case (Punycode), there's no need to include "A-Z" in the
+        // pattern. Further, subsequent code may lower-case the entire regular
+        // expression (if the URL contains only the hostname part), leaving us
+        // with "a-za-z", which would be redundant.
+        if (!justHostname)
+          alphabet = "A-Z" + alphabet;
+        let digits = "0-9";
+        // Note that the "-" must appear first here in order to retain its
+        // literal meaning within the brackets.
+        let specialCharacters = "-_.%";
+        let separator = "[^" + specialCharacters + alphabet + digits + "]";
+        if (i == 0)
+          regexp.push("^" + urlScheme + "(.*" + separator + ")?");
+        else if (i == lastIndex)
+          regexp.push("(" + separator + ".*)?$");
+        else
+          regexp.push(separator);
         break;
       case "|":
         if (i == 0)
         {
           regexp.push("^");
           break;
         }
         if (i == lastIndex)
         {
           regexp.push("$");
           break;
         }
-        if (i == 1 && text[0] == "|")
+        if (i == 1 && characters[0] == "|")
         {
           hostnameStart = i + 1;
           canSafelyMatchAsLowercase = true;
-          regexp.push("https?://([^/]+\\.)?");
+          regexp.push(urlScheme + "([^/]+\\.)?");
           break;
         }
         regexp.push("\\|");
         break;
       case "/":
         if (!hostnameFinished &&
-            text.charAt(i-2) == ":" && text.charAt(i-1) == "/")
+            characters[i - 2] == ":" && characters[i - 1] == "/")
         {
           hostnameStart = i + 1;
           canSafelyMatchAsLowercase = true;
         }
         regexp.push("/");
         break;
       case ".": case "+": case "$": case "?":
       case "{": case "}": case "(": case ")":
       case "[": case "]": case "\\":
         regexp.push("\\", c);
         break;
       default:
         if (hostnameFinished && (c >= "a" && c <= "z" ||
                                  c >= "A" && c <= "Z"))
           canSafelyMatchAsLowercase = false;
-        regexp.push(c);
+        regexp.push(c == "%" ? c : encodeURI(c));
     }
   }
 
   return {
     regexp: regexp.join(""),
     canSafelyMatchAsLowercase: canSafelyMatchAsLowercase,
     hostname: hostname,
     justHostname: justHostname
   };
 }
 
-function getResourceTypes(filter)
+function getResourceTypes(contentType)
 {
   let types = [];
 
-  if (filter.contentType & typeMap.IMAGE)
+  if (contentType & typeMap.IMAGE)
     types.push("image");
-  if (filter.contentType & typeMap.STYLESHEET)
+  if (contentType & typeMap.STYLESHEET)
     types.push("style-sheet");
-  if (filter.contentType & typeMap.SCRIPT)
+  if (contentType & typeMap.SCRIPT)
     types.push("script");
-  if (filter.contentType & typeMap.FONT)
+  if (contentType & typeMap.FONT)
     types.push("font");
-  if (filter.contentType & (typeMap.MEDIA | typeMap.OBJECT))
+  if (contentType & (typeMap.MEDIA | typeMap.OBJECT))
     types.push("media");
-  if (filter.contentType & typeMap.POPUP)
+  if (contentType & typeMap.POPUP)
     types.push("popup");
-  if (filter.contentType & (typeMap.XMLHTTPREQUEST |
-                            typeMap.OBJECT_SUBREQUEST |
-                            typeMap.PING |
-                            typeMap.OTHER))
+  if (contentType & rawRequestTypes)
     types.push("raw");
-  if (filter.contentType & typeMap.SUBDOCUMENT)
+  if (contentType & typeMap.SUBDOCUMENT)
     types.push("document");
 
   return types;
 }
 
-function addDomainPrefix(domains)
-{
-  let result = [];
-
-  for (let domain of domains)
-  {
-    result.push(domain);
-
-    if (tldjs.getDomain(domain) == domain)
-      result.push("www." + domain);
-  }
-
-  return result;
-}
-
-function convertFilterAddRules(rules, filter, action, withResourceTypes)
-{
-  let parsed = parseFilterRegexpSource(filter.regexpSource);
+function makeRuleCopies(trigger, action, urlSchemes)
+{
+  let copies = [];
+
+  // Always make a deep copy of the rule, since rules may have to be
+  // manipulated individually at a later stage.
+  let stringifiedTrigger = JSON.stringify(trigger);
+
+  let filterPattern = trigger["url-filter"].substring(1);
+  let startIndex = 0;
+
+  // If the URL filter already begins with the first URL scheme pattern, skip
+  // it.
+  if (trigger["url-filter"].startsWith("^" + urlSchemes[0]))
+  {
+    filterPattern = filterPattern.substring(urlSchemes[0].length);
+    startIndex = 1;
+  }
+  else
+  {
+    filterPattern = ".*" + filterPattern;
+  }
+
+  for (let i = startIndex; i < urlSchemes.length; i++)
+  {
+    let copyTrigger = Object.assign(JSON.parse(stringifiedTrigger), {
+      "url-filter": "^" + urlSchemes[i] + filterPattern
+    });
+    copies.push({trigger: copyTrigger, action});
+  }
+
+  return copies;
+}
+
+function excludeTopURLFromTrigger(trigger)
+{
+  trigger["unless-top-url"] = [trigger["url-filter"]];
+  if (trigger["url-filter-is-case-sensitive"])
+    trigger["top-url-filter-is-case-sensitive"] = true;
+}
+
+function convertFilterAddRules(rules, filter, action, withResourceTypes,
+                               exceptionDomains, contentType)
+{
+  if (!contentType)
+    contentType = filter.contentType;
+
+  // If WebSocket or WebRTC are given along with other options but not
+  // including all three of WebSocket, WebRTC, and at least one HTTP raw type,
+  // we must generate multiple rules. For example, for the filter
+  // "foo$websocket,image", we must generate one rule with "^wss?://" and "raw"
+  // and another rule with "^https?://" and "image". If we merge the two, we
+  // end up blocking requests of all HTTP raw types (e.g. XMLHttpRequest)
+  // inadvertently.
+  if ((contentType & typeMap.WEBSOCKET && contentType != typeMap.WEBSOCKET &&
+       !(contentType & typeMap.WEBRTC &&
+         contentType & rawRequestTypes & httpRequestTypes)) ||
+      (contentType & typeMap.WEBRTC && contentType != typeMap.WEBRTC &&
+       !(contentType & typeMap.WEBSOCKET &&
+         contentType & rawRequestTypes & httpRequestTypes)))
+  {
+    if (contentType & typeMap.WEBSOCKET)
+    {
+      convertFilterAddRules(rules, filter, action, withResourceTypes,
+                            exceptionDomains, typeMap.WEBSOCKET);
+    }
+
+    if (contentType & typeMap.WEBRTC)
+    {
+      convertFilterAddRules(rules, filter, action, withResourceTypes,
+                            exceptionDomains, typeMap.WEBRTC);
+    }
+
+    contentType &= ~(typeMap.WEBSOCKET | typeMap.WEBRTC);
+
+    if (!contentType)
+      return;
+  }
+
+  let urlSchemes = getURLSchemes(contentType);
+  let parsed = parseFilterRegexpSource(filter.regexpSource, urlSchemes[0]);
 
   // For the special case of $document whitelisting filters with just a domain
   // we can generate an equivalent blocking rule exception using if-domain.
   if (filter instanceof filterClasses.WhitelistFilter &&
-      filter.contentType & typeMap.DOCUMENT &&
+      contentType & typeMap.DOCUMENT &&
       parsed.justHostname)
   {
     rules.push({
       trigger: {
         "url-filter": ".*",
-        "if-domain": addDomainPrefix([parsed.hostname])
+        "if-domain": ["*" + parsed.hostname]
       },
       action: {type: "ignore-previous-rules"}
     });
     // If the filter contains other supported options we'll need to generate
     // further rules for it, but if not we can simply return now.
-    if (!(filter.contentType & whitelistableRequestTypes))
+    if (!(contentType & whitelistableRequestTypes))
       return;
   }
 
   let trigger = {"url-filter": parsed.regexp};
 
-  // Limit rules to HTTP(S) URLs
-  if (!/^(\^|http)/i.test(trigger["url-filter"]))
-    trigger["url-filter"] = "^https?://.*" + trigger["url-filter"];
+  // If the URL filter begins with one of the URL schemes for this content
+  // type, we generate additional rules for all the URL scheme patterns;
+  // otherwise, if the start of the URL filter literally matches the first URL
+  // scheme pattern, we just generate additional rules for the remaining URL
+  // scheme patterns.
+  //
+  // For example, "stun:foo$webrtc" will give us "stun:foo", then we add a "^"
+  // in front of this and generate two additional rules for
+  // "^stuns?:.*stun:foo" and "^turns?:.*stun:foo". On the other hand,
+  // "||foo$webrtc" will give us "^stuns?:([^/]+\\.)?foo", so we just generate
+  // "^turns?:([^/]+\\.)?foo" in addition.
+  //
+  // Note that the filter can be already anchored to the beginning
+  // (e.g. "|stun:foo$webrtc"), in which case we do not generate any additional
+  // rules.
+  let needAltRules = trigger["url-filter"][0] != "^" ||
+                     trigger["url-filter"].startsWith("^" + urlSchemes[0]);
+
+  if (trigger["url-filter"][0] != "^")
+  {
+    if (!urlSchemes.some(scheme => new RegExp("^" + scheme)
+                                   .test(trigger["url-filter"])))
+    {
+      trigger["url-filter"] = urlSchemes[0] + ".*" + trigger["url-filter"];
+    }
+
+    trigger["url-filter"] = "^" + trigger["url-filter"];
+  }
 
   // For rules containing only a hostname we know that we're matching against
   // a lowercase string unless the matchCase option was passed.
   if (parsed.canSafelyMatchAsLowercase && !filter.matchCase)
     trigger["url-filter"] = trigger["url-filter"].toLowerCase();
 
   if (parsed.canSafelyMatchAsLowercase || filter.matchCase)
     trigger["url-filter-is-case-sensitive"] = true;
 
   let included = [];
   let excluded = [];
 
   parseDomains(filter.domains, included, excluded);
 
+  if (exceptionDomains)
+    excluded = excluded.concat(exceptionDomains);
+
   if (withResourceTypes)
   {
-    trigger["resource-type"] = getResourceTypes(filter);
-
-    if (trigger["resource-type"].length == 0)
+    let resourceTypes = getResourceTypes(contentType);
+
+    // Content blocker rules can't differentiate between sub-document requests
+    // (iframes) and top-level document requests. To avoid too many false
+    // positives, we prevent rules with no hostname part from blocking document
+    // requests.
+    //
+    // Once Safari 11 becomes our minimum supported version, we could change
+    // our approach here to use the new "unless-top-url" property instead.
+    if (filter instanceof filterClasses.BlockingFilter && !parsed.hostname)
+      resourceTypes = resourceTypes.filter(type => type != "document");
+
+    if (resourceTypes.length == 0)
       return;
+
+    trigger["resource-type"] = resourceTypes;
   }
 
   if (filter.thirdParty != null)
     trigger["load-type"] = [filter.thirdParty ? "third-party" : "first-party"];
 
+  let addTopLevelException = false;
+
   if (included.length > 0)
-    trigger["if-domain"] = addDomainPrefix(included);
+  {
+    trigger["if-domain"] = [];
+
+    for (let name of included)
+    {
+      // If this is a blocking filter or an element hiding filter, add the
+      // subdomain wildcard only if no subdomains have been excluded.
+      let notSubdomains = null;
+      if ((filter instanceof filterClasses.BlockingFilter ||
+           filter instanceof filterClasses.ElemHideFilter) &&
+          (notSubdomains = findSubdomainsInList(name, excluded)).length > 0)
+      {
+        trigger["if-domain"].push(name);
+
+        // Add the "www" prefix but only if it hasn't been excluded.
+        if (!notSubdomains.includes("www"))
+          trigger["if-domain"].push("www." + name);
+      }
+      else
+      {
+        trigger["if-domain"].push("*" + name);
+      }
+    }
+  }
   else if (excluded.length > 0)
-    trigger["unless-domain"] = addDomainPrefix(excluded);
+  {
+    trigger["unless-domain"] = excluded.map(name => "*" + name);
+  }
+  else if (filter instanceof filterClasses.BlockingFilter &&
+           filter.contentType & typeMap.SUBDOCUMENT && parsed.hostname)
+  {
+    // Rules with a hostname part are still allowed to block document requests,
+    // but we add an exception for top-level documents.
+    //
+    // Note that we can only do this if there's no "unless-domain" property for
+    // now. This also only works in Safari 11 onwards, while older versions
+    // simply ignore this property. Once Safari 11 becomes our minimum
+    // supported version, we can merge "unless-domain" into "unless-top-url".
+    addTopLevelException = true;
+    excludeTopURLFromTrigger(trigger);
+  }
 
   rules.push({trigger: trigger, action: {type: action}});
-}
-
-function hasNonASCI(obj)
-{
-  if (typeof obj == "string")
-  {
-    if (/[^\x00-\x7F]/.test(obj))
-      return true;
-  }
-
-  if (typeof obj == "object")
-  {
-    if (obj instanceof Array)
-      for (let item of obj)
-        if (hasNonASCI(item))
-          return true;
-
-    let names = Object.getOwnPropertyNames(obj);
-    for (let name of names)
-      if (hasNonASCI(obj[name]))
-        return true;
-  }
-
-  return false;
+
+  if (needAltRules)
+  {
+    // Generate additional rules for any alternative URL schemes.
+    for (let altRule of makeRuleCopies(trigger, {type: action}, urlSchemes))
+    {
+      if (addTopLevelException)
+        excludeTopURLFromTrigger(altRule.trigger);
+
+      rules.push(altRule);
+    }
+  }
 }
 
 function convertIDSelectorsToAttributeSelectors(selector)
 {
   // First we figure out where all the IDs are
   let sep = "";
   let start = null;
   let positions = [];
   for (let i = 0; i < selector.length; i++)
   {
@@ skipping 33 matching lines @@
   {
     newSelector.push(selector.substring(i, pos.start));
     newSelector.push('[id=', selector.substring(pos.start + 1, pos.end), ']');
     i = pos.end;
   }
   newSelector.push(selector.substring(i));
 
   return newSelector.join("");
 }
 
+function addCSSRules(rules, selectors, domain, exceptionDomains)
+{
+  let unlessDomain = exceptionDomains.size > 0 ? [] : null;
+
+  exceptionDomains.forEach(name =>
+  {
+    // For domain-specific filters, include the exception domains only if
+    // they're subdomains of the given domain.
+    if (!domain || name.substr(-domain.length - 1) == "." + domain)
+      unlessDomain.push("*" + name);
+  });
+
+  while (selectors.length)
+  {
+    let selector = selectors.splice(0, selectorLimit).join(", ");
+
+    // As of Safari 9.0 element IDs are matched as lowercase. We work around
+    // this by converting to the attribute format [id="elementID"]
+    selector = convertIDSelectorsToAttributeSelectors(selector);
+
+    let rule = {
+      trigger: {"url-filter": matchDomain(domain),
+                "url-filter-is-case-sensitive": true},
+      action: {type: "css-display-none",
+               selector: selector}
+    };
+
+    if (unlessDomain)
+      rule.trigger["unless-domain"] = unlessDomain;
+
+    rules.push(rule);
+  }
+}
+
+/**
+ * Check if two strings are a close match
+ *
+ * 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?://example.com/ads" and "^https?://example.com/adv" differ only in
+ *   the last character, therefore the regular expressions can safely be merged
+ *   into "^https?://example.com/ad[sv]".
+ *
+ * @param {string} s The source string
+ * @param {string} t The target string
+ *
+ * @returns {object} An object describing the single edit operation that must
+ *                   occur in the source string in order to arrive at the
+ *                   target string
+ */
 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)
+  if (diff < 0)
   {
     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.
+  let i = 0;
+  let j = 0;
+
+  // Start from the beginning and keep going until we hit a character that
+  // doesn't match.
+  for (; i < s.length; i++)
+  {
+    if (s[i] != t[i])
+      break;
+  }
+
+  // Now do exactly the same from the end, but also stop if we reach the
+  // position where we terminated the previous loop.
+  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 the strings are equal in length and the delta isn't exactly one
+    // character, it's not a close match.
+    if (t.length - j - i != 1)
       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.
+  }
+  else if (i != t.length - j)
+  {
+    // For strings of unequal length, if we haven't found a match for every
+    // single character in the shorter string counting from both the beginning
+    // and the end, it's not a close match.
+    return null;
+  }
+
+  for (let k = i; k < s.length - j; k++)
+  {
+    // If the delta contains any special characters, it's not a close match.
+    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;
-    }
-    else
-    {
-      switch (diff)
-      {
-        case 0:
-          // If both strings are equal in length, this is a substitution.
-          edit = {type: "substitute", index: i};
-          j++;
+  }
+
+  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 eliminateRedundantRulesByURLFilter(rulesInfo, exhaustive)
+{
+  const heuristicRange = 1000;
+
+  let ol = rulesInfo.length;
+
+  // Throw out obviously redundant rules.
+  return async(rulesInfo, (ruleInfo, index) => () =>
+  {
+    // If this rule is already marked as redundant, don't bother comparing it
+    // with other rules.
+    if (rulesInfo[index].redundant)
+      return;
+
+    let limit = exhaustive ? rulesInfo.length :
+                Math.min(index + heuristicRange, rulesInfo.length);
+
+    for (let i = index, j = i + 1; j < limit; j++)
+    {
+      if (rulesInfo[j].redundant)
+        continue;
+
+      let source = rulesInfo[i].rule.trigger["url-filter"];
+      let target = rulesInfo[j].rule.trigger["url-filter"];
+
+      if (source.length >= target.length)
+      {
+        // If one URL filter is a substring of the other starting at the
+        // beginning, the other one is clearly redundant.
+        if (source.substring(0, target.length) == target)
+        {
+          rulesInfo[i].redundant = true;
           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)
+      else if (target.substring(0, source.length) == source)
+      {
+        rulesInfo[j].redundant = true;
+      }
+    }
+  })
+  .then(() => rulesInfo.filter(ruleInfo => !ruleInfo.redundant));
+}
+
+function findMatchesForRuleByURLFilter(rulesInfo, index, 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;
-
-  let rulesInfo = new Array(rules.length);
-
-  rules.forEach((rule, index) =>
-  {
-    rulesInfo[index] = {rule};
-
-    if (rule.action.type == "ignore-previous-rules")
-    {
-      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));
-    }
+  // look for matches within this range by default. If we increase this value,
+  // it can give us more matches and a smaller resulting rule set, but possibly
+  // at a significant performance cost.
+  //
+  // If the exhaustive option is true, we simply ignore this value and look for
+  // matches throughout the rule set.
+  const heuristicRange = 1000;
+
+  let limit = exhaustive ? rulesInfo.length :
+              Math.min(index + heuristicRange, rulesInfo.length);
+
+  for (let i = index, 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);
+
+        // 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;
+      }
+    }
+  }
+}
+
+function mergeCandidateRulesByURLFilter(rulesInfo)
+{
+  // Filter out rules that have no matches at all.
+  let candidateRulesInfo = rulesInfo.filter(ruleInfo =>
+  {
+    return ruleInfo.bestMatches || ruleInfo.multiEditMatch
   });
 
-  for (let i = 0; i < rules.length; i++)
-  {
-    if (rulesInfo[i].skip)
+  // 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;
 
-    for (let j = i + 1; j < i + heuristicRange && 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)
+    // 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 = [urlFilter[editIndex]];
+        let quantifier = "";
+
+        for (let match of best)
         {
-          let urlFilter, ruleInfo, match = {edit};
-
-          if (edit.type == "insert")
+          if (match.edit.type == "delete")
           {
-            // 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";
+            quantifier = "?";
           }
           else
           {
-            urlFilter = source;
-            ruleInfo = rulesInfo[i];
-            match.index = j;
+            let character = rulesInfo[match.index].rule
+                            .trigger["url-filter"][editIndex];
+
+            // Insert any hyphen at the beginning so it gets interpreted as a
+            // literal hyphen.
+            if (character == "-")
+              characters.unshift(character);
+            else
+              characters.push(character);
           }
 
-          if (!ruleInfo.matches)
-            ruleInfo.matches = new Array(urlFilter.length + 1);
-
-          let matchesForIndex = ruleInfo.matches[edit.index];
-
-          if (matchesForIndex)
-          {
-            matchesForIndex.push(match);
-          }
-          else
-          {
-            matchesForIndex = [match];
-            ruleInfo.matches[edit.index] = matchesForIndex;
-          }
-
-          if (!ruleInfo.bestMatches ||
-              matchesForIndex.length > ruleInfo.bestMatches.length)
-            ruleInfo.bestMatches = matchesForIndex;
+          // 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 > 1)
+        {
+          urlFilter = urlFilter.substring(0, editIndex) + "[" +
+                      characters.join("") + "]" +
+                      urlFilter.substring(editIndex + 1);
         }
       }
-    }
-  }
-
-  let candidateRulesInfo = rulesInfo.filter(ruleInfo => ruleInfo.bestMatches);
-
-  // For best results, we have to sort the candidates by the number of matches.
-  // For example, we want "ads", "bds", "adv", "bdv", and "bdx" to generate
-  // "ad[sv]" and "bd[svx]" (2 rules), not "[ab]ds", "[ab]dv", and "bdx" (3
-  // rules).
-  candidateRulesInfo.sort((ruleInfo1, ruleInfo2) =>
-  {
-    return ruleInfo2.bestMatches.length - ruleInfo1.bestMatches.length;
-  });
-
-  for (let ruleInfo of candidateRulesInfo)
-  {
-    let rule = ruleInfo.rule;
-
-    if (rule._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 !rules[match.index]._merged &&
-               !rulesInfo[match.index].mergedInto;
-      });
-
-      return matchesForIndex.length > best.length ? matchesForIndex : best;
-    },
-    []);
-
-    if (best.length > 0)
-    {
-      // 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]);
-
-        rules[match.index]._merged = true;
+      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);
       }
 
-      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;
 
+      // Mark this rule as one that has had other rules merged into it.
       ruleInfo.mergedInto = true;
     }
   }
-
-  return rules.filter(rule => !rule._merged);
+}
+
+function mergeRulesByURLFilter(rulesInfo, exhaustive)
+{
+  return async(rulesInfo, (ruleInfo, index) => () =>
+    findMatchesForRuleByURLFilter(rulesInfo, index, exhaustive)
+  )
+  .then(() => mergeCandidateRulesByURLFilter(rulesInfo));
+}
+
+function mergeRulesByArrayProperty(rulesInfo, propertyType, property)
+{
+  if (rulesInfo.length <= 1)
+    return;
+
+  let valueSet = new Set(rulesInfo[0].rule[propertyType][property]);
+
+  for (let i = 1; i < rulesInfo.length; i++)
+  {
+    for (let value of rulesInfo[i].rule[propertyType][property] || [])
+      valueSet.add(value);
+
+    rulesInfo[i].merged = true;
+  }
+
+  if (valueSet.size > 0)
+    rulesInfo[0].rule[propertyType][property] = Array.from(valueSet);
+
+  rulesInfo[0].mergedInto = true;
+}
+
+function groupRulesByMergeableProperty(rulesInfo, propertyType, property)
+{
+  let mergeableRulesInfoByGroup = new Map();
+
+  for (let ruleInfo of rulesInfo)
+  {
+    let copy = {
+      trigger: Object.assign({}, ruleInfo.rule.trigger),
+      action: Object.assign({}, ruleInfo.rule.action)
+    };
+
+    delete copy[propertyType][property];
+
+    let groupKey = JSON.stringify(copy);
+
+    let mergeableRulesInfo = mergeableRulesInfoByGroup.get(groupKey);
+
+    if (mergeableRulesInfo)
+      mergeableRulesInfo.push(ruleInfo);
+    else
+      mergeableRulesInfoByGroup.set(groupKey, [ruleInfo]);
+  }
+
+  return mergeableRulesInfoByGroup;
+}
+
+function mergeRules(rules, exhaustive)
+{
+  let rulesInfo = rules.map(rule => ({rule}));
+
+  let arrayPropertiesToMergeBy = ["resource-type", "if-domain"];
+
+  return async(() =>
+  {
+    let map = groupRulesByMergeableProperty(rulesInfo, "trigger", "url-filter");
+    return async(map.values(), mergeableRulesInfo => () =>
+      eliminateRedundantRulesByURLFilter(mergeableRulesInfo, exhaustive)
+      .then(rulesInfo => mergeRulesByURLFilter(rulesInfo, exhaustive))
+    )
+    .then(() =>
+    {
+      // Filter out rules that are redundant or have been merged into other
+      // rules.
+      rulesInfo = rulesInfo.filter(ruleInfo => !ruleInfo.redundant &&
+                                               !ruleInfo.merged);
+    });
+  })
+  .then(() => async(arrayPropertiesToMergeBy, arrayProperty => () =>
+  {
+    let map = groupRulesByMergeableProperty(rulesInfo, "trigger",
+                                            arrayProperty);
+    return async(map.values(), mergeableRulesInfo => () =>
+      mergeRulesByArrayProperty(mergeableRulesInfo, "trigger", arrayProperty)
+    )
+    .then(() =>
+    {
+      rulesInfo = rulesInfo.filter(ruleInfo => !ruleInfo.merged);
+    });
+  }))
+  .then(() => rulesInfo.map(ruleInfo => ruleInfo.rule));
 }
 
 let ContentBlockerList =
 /**
  * Create a new Adblock Plus filter to content blocker list converter
  *
+ * @param {object} options Options for content blocker list generation
+ *
  * @constructor
  */
-exports.ContentBlockerList = function ()
-{
+exports.ContentBlockerList = function (options)
+{
+  const defaultOptions = {
+    merge: "auto"
+  };
+
+  this.options = Object.assign({}, defaultOptions, options);
+
   this.requestFilters = [];
   this.requestExceptions = [];
   this.elemhideFilters = [];
   this.elemhideExceptions =  [];
+  this.genericblockExceptions = [];
+  this.generichideExceptions = [];
   this.elemhideSelectorExceptions = new Map();
 };
 
 /**
  * Add Adblock Plus filter to be converted
  *
  * @param {Filter} filter Filter to convert
  */
 ContentBlockerList.prototype.addFilter = function(filter)
 {
   if (filter.sitekeys)
     return;
   if (filter instanceof filterClasses.RegExpFilter &&
       filter.regexpSource == null)
     return;
 
   if (filter instanceof filterClasses.BlockingFilter)
     this.requestFilters.push(filter);
 
   if (filter instanceof filterClasses.WhitelistFilter)
   {
     if (filter.contentType & (typeMap.DOCUMENT | whitelistableRequestTypes))
       this.requestExceptions.push(filter);
 
-      if (filter.contentType & typeMap.ELEMHIDE)
-        this.elemhideExceptions.push(filter);
+    if (filter.contentType & typeMap.GENERICBLOCK)
+      this.genericblockExceptions.push(filter);
+
+    if (filter.contentType & typeMap.ELEMHIDE)
+      this.elemhideExceptions.push(filter);
+    else if (filter.contentType & typeMap.GENERICHIDE)
+      this.generichideExceptions.push(filter);
   }
 
   if (filter instanceof filterClasses.ElemHideFilter)
     this.elemhideFilters.push(filter);
 
   if (filter instanceof filterClasses.ElemHideException)
   {
     let domains = this.elemhideSelectorExceptions[filter.selector];
     if (!domains)
       domains = this.elemhideSelectorExceptions[filter.selector] = [];
 
     parseDomains(filter.domains, domains, []);
   }
 };
 
 /**
  * Generate content blocker list for all filters that were added
- *
- * @returns   {Filter}   filter    Filter to convert
  */
-ContentBlockerList.prototype.generateRules = function({merge = false} = {})
-{
-  let rules = [];
-
+ContentBlockerList.prototype.generateRules = function()
+{
+  let cssRules = [];
+  let cssExceptionRules = [];
+  let blockingRules = [];
+  let blockingExceptionRules = [];
+
+  let ruleGroups = [cssRules, cssExceptionRules,
+                    blockingRules, blockingExceptionRules];
+
+  let genericSelectors = [];
   let groupedElemhideFilters = new Map();
+
   for (let filter of this.elemhideFilters)
   {
     let result = convertElemHideFilter(filter, this.elemhideSelectorExceptions);
     if (!result)
       continue;
 
     if (result.matchDomains.length == 0)
-      result.matchDomains = ["^https?://"];
-
-    for (let matchDomain of result.matchDomains)
-    {
-      let group = groupedElemhideFilters.get(matchDomain) || [];
-      group.push(result.selector);
-      groupedElemhideFilters.set(matchDomain, group);
-    }
-  }
+    {
+      genericSelectors.push(result.selector);
+    }
+    else
+    {
+      for (let matchDomain of result.matchDomains)
+      {
+        let group = groupedElemhideFilters.get(matchDomain) || [];
+        group.push(result.selector);
+        groupedElemhideFilters.set(matchDomain, group);
+      }
+    }
+  }
+
+  // Separate out the element hiding exceptions that have only a hostname part
+  // from the rest. This allows us to implement a workaround for issue #5345
+  // (WebKit bug #167423), but as a bonus it also reduces the number of
+  // generated rules. The downside is that the exception will only apply to the
+  // top-level document, not to iframes. We have to live with this until the
+  // WebKit bug is fixed in all supported versions of Safari.
+  // https://bugs.webkit.org/show_bug.cgi?id=167423
+  //
+  // Note that as a result of this workaround we end up with a huge rule set in
+  // terms of the amount of memory used. This can cause Node.js to throw
+  // "JavaScript heap out of memory". To avoid this, call Node.js with
+  // --max_old_space_size=4096
+  let elemhideExceptionDomains = extractFilterDomains(this.elemhideExceptions);
+
+  let genericSelectorExceptionDomains =
+    extractFilterDomains(this.generichideExceptions);
+  elemhideExceptionDomains.forEach(name =>
+  {
+    genericSelectorExceptionDomains.add(name);
+  });
+
+  addCSSRules(cssRules, genericSelectors, null,
+              genericSelectorExceptionDomains);
+
+  // Filter out whitelisted domains.
+  elemhideExceptionDomains.forEach(domain =>
+    groupedElemhideFilters.delete(domain));
 
   groupedElemhideFilters.forEach((selectors, matchDomain) =>
   {
-    while (selectors.length)
-    {
-      let selector = selectors.splice(0, selectorLimit).join(", ");
-
-      // As of Safari 9.0 element IDs are matched as lowercase. We work around
-      // this by converting to the attribute format [id="elementID"]
-      selector = convertIDSelectorsToAttributeSelectors(selector);
-
-      rules.push({
-        trigger: {"url-filter": matchDomain,
-                  "url-filter-is-case-sensitive": true},
-        action: {type: "css-display-none",
-                 selector: selector}
+    addCSSRules(cssRules, selectors, matchDomain, elemhideExceptionDomains);
+  });
+
+  let requestFilterExceptionDomains = [];
+  for (let filter of this.genericblockExceptions)
+  {
+    let parsed = parseFilterRegexpSource(filter.regexpSource);
+    if (parsed.hostname)
+      requestFilterExceptionDomains.push(parsed.hostname);
+  }
+
+  for (let filter of this.requestFilters)
+  {
+    convertFilterAddRules(blockingRules, filter, "block", true,
+                          requestFilterExceptionDomains);
+  }
+
+  for (let filter of this.requestExceptions)
+  {
+    convertFilterAddRules(blockingExceptionRules, filter,
+                          "ignore-previous-rules", true);
+  }
+
+  return async(ruleGroups, (group, index) => () =>
+  {
+    let next = () =>
+    {
+      if (index == ruleGroups.length - 1)
+        return ruleGroups.reduce((all, rules) => all.concat(rules), []);
+    };
+
+    if (this.options.merge == "all" ||
+        (this.options.merge == "auto" &&
+         ruleGroups.reduce((n, group) => n + group.length, 0) > 50000))
+    {
+      return mergeRules(ruleGroups[index], this.options.merge == "all")
+      .then(rules =>
+      {
+        ruleGroups[index] = rules;
+        return next();
       });
     }
+
+    return next();
   });
-
-  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);
-
-  rules = rules.filter(rule => !hasNonASCI(rule));
-
-  if (merge)
-    rules = mergeCloselyMatchingRules(rules);
-
-  return rules;
 };