Issue 6956 - Move extension's style sheet generation into core

lib/elemHide.js

Index: lib/elemHide.js
===================================================================
--- a/lib/elemHide.js
+++ b/lib/elemHide.js
@@ -20,16 +20,25 @@
 /**
  * @fileOverview Element hiding implementation.
  */
 
 const {ElemHideExceptions} = require("./elemHideExceptions");
 const {filterNotifier} = require("./filterNotifier");
 
 /**
+ * The maximum number of selectors in a CSS rule. This is used by
+ * <code>{@link createStyleSheet}</code> to split up a long list of selectors
+ * into multiple rules.
+ * @const {number}
+ * @default
+ */
+const selectorGroupSize = 1024;
+
+/**
  * Lookup table, active flag, by filter by domain.
  * (Only contains filters that aren't unconditionally matched for all domains.)
  * @type {Map.<string,Map.<Filter,boolean>>}
  */
 let filtersByDomain = new Map();
 
 /**
  * Lookup table, filter by selector. (Only used for selectors that are
@@ -236,8 +245,58 @@
     }
 
     if (!specificOnly)
       selectors = getUnconditionalSelectors().concat(selectors);
 
     return selectors;
   }
 };
+
+/**
+ * Splits a list of selectors into groups determined by the value of
+ * <code>{@link selectorGroupSize}</code>.
+ *
+ * @param {Array.<string>} selectors
+ * @yields {Array.<string>}
+ */
+function* splitSelectors(selectors)
+{
+  // Chromium's Blink engine supports only up to 8,192 simple selectors, and
+  // even fewer compound selectors, in a rule. The exact number of selectors
+  // that would work depends on their sizes (e.g. "#foo .bar" has a size of 2).
+  // Since we don't know the sizes of the selectors here, we simply split them
+  // into groups of 1,024, based on the reasonable assumption that the average
+  // selector won't have a size greater than 8. The alternative would be to
+  // calculate the sizes of the selectors and divide them up accordingly, but
+  // this approach is more efficient and has worked well in practice. In theory
+  // this could still lead to some selectors not working on Chromium, but it is
+  // highly unlikely.
+  // See issue #6298 and https://crbug.com/804179
+  for (let i = 0; i < selectors.length; i += selectorGroupSize)
+    yield selectors.slice(i, i + selectorGroupSize);
+}
+
+/**
+ * Creates element hiding CSS rules for a given list of selectors. Each rule
+ * contains no more than the maximum number of selectors as determined by the
+ * value of <code>{@link selectorGroupSize}</code>.
+ *
+ * @param {Array.<string>} selectors
+ * @yields {string}
+ */
+function* createRules(selectors)
+{
+  for (let selectorGroup of splitSelectors(selectors))
+    yield selectorGroup.join(", ") + " {display: none !important;}";
+}
+
+/**
+ * Creates an element hiding CSS style sheet from a given list of selectors.
+ * @param {Array.<string>} selectors
+ * @returns {string}
+ */
+function createStyleSheet(selectors)
+{
+  return [...createRules(selectors)].join("\n");
+}
+
+exports.createStyleSheet = createStyleSheet;