Ensure that the same grammar and start rule always generate exactly the same parser.

lib/compiler.js

@@ -190,6 +190,10 @@ PEG.Compiler = {
 
   _uniqueIdentifierCounters: {},
 
+  _resetUniqueIdentifierCounters: function() {
+    this._uniqueIdentifierCounters = {};
+  },
+
   /* Generates a unique identifier with specified prefix. */
   generateUniqueIdentifier: function(prefix) {
     this._uniqueIdentifierCounters[prefix]
@@ -201,6 +205,12 @@ PEG.Compiler = {
    * Generates a parser from a specified grammar and start rule.
    */
   compileParser: function(grammar, startRule) {
+    /*
+     * This ensures that the same grammar and start rule always generate exactly
+     * the same parser.
+     */
+    this._resetUniqueIdentifierCounters();
+
     var parseFunctionDefinitions = [];
     for (var key in grammar) {
       parseFunctionDefinitions.push(grammar[key].compile());

test/compiler-test.js

@@ -309,6 +309,13 @@ test("cache", function() {
   parses(parser, "ac", ["a", "c"]);
 });
 
+test("indempotence", function() {
+  var parser1 = PEG.buildParser('start: "abcd"');
+  var parser2 = PEG.buildParser('start: "abcd"');
+
+  strictEqual(parser1.toSource(), parser2.toSource());
+});
+
 test("error messages", function() {
   var literalParser = PEG.buildParser('start: "abcd"');
   doesNotParseWithMessage(