Speed up deduplication of expectations

The expectation deduplication algorithm called |Array.prototype.splice|
to eliminate each individual duplication, which was slow. This caused
problems with grammar/input combinations that generated a lot of
expecations (see #377 for an example).

This commit replaces the algorithm with much faster one, eliminating the
problem.

lib/compiler/passes/generate-js.js

@@ -1056,7 +1056,7 @@ function generateJS(ast, options) {
     '',
     '    function peg$buildException(message, expected, location) {',
     '      function cleanupExpected(expected) {',
-    '        var i = 1;',
+    '        var i, j;',
     '',
     '        expected.sort(function(a, b) {',
     '          if (a.description < b.description) {',
@@ -1073,12 +1073,14 @@ function generateJS(ast, options) {
      * expectation object exists only once, so it's enough to use |===| instead
      * of deeper structural comparison.
      */
-    '        while (i < expected.length) {',
-    '          if (expected[i - 1] === expected[i]) {',
-    '            expected.splice(i, 1);',
-    '          } else {',
-    '            i++;',
+    '        if (expected.length > 0) {',
+    '          for (i = 1, j = 1; i < expected.length; i++) {',
+    '            if (expected[i - 1] !== expected[i]) {',
+    '              expected[j] = expected[i];',
+    '              j++;',
+    '            }',
     '          }',
+    '          expected.length = j;',
     '        }',
     '      }',
     '',

lib/parser.js

@@ -492,7 +492,7 @@ module.exports = (function() {
 
     function peg$buildException(message, expected, location) {
       function cleanupExpected(expected) {
-        var i = 1;
+        var i, j;
 
         expected.sort(function(a, b) {
           if (a.description < b.description) {
@@ -504,12 +504,14 @@ module.exports = (function() {
           }
         });
 
-        while (i < expected.length) {
-          if (expected[i - 1] === expected[i]) {
-            expected.splice(i, 1);
-          } else {
-            i++;
+        if (expected.length > 0) {
+          for (i = 1, j = 1; i < expected.length; i++) {
+            if (expected[i - 1] !== expected[i]) {
+              expected[j] = expected[i];
+              j++;
+            }
           }
+          expected.length = j;
         }
       }