From cdeecf750f8a3f781459ed817815a537392993c1 Mon Sep 17 00:00:00 2001
From: David Majda <david@majda.cz>
Date: Fri, 7 Aug 2015 14:34:19 +0200
Subject: [PATCH] reportLeftRecursion: Change handling of the |visitedRules|
 array

Before this commit, the |reportLeftRecursion| pass was written in
functional style, passing the |visitedRules| array around as a parameter
and making a new copy each time a rule was visited. This apparently
caused performance problems in some deeply recursive grammars.

This commit makes it so that there is just one array which is shared
across all the visitor functions via a closure and modified as rules are
visited.

I don't like losing the functional style (it was elegant) but
performance is more important.

Fixes #203.
---
 lib/compiler/passes/report-left-recursion.js | 18 +++++++++++-------
 1 file changed, 11 insertions(+), 7 deletions(-)

diff --git a/lib/compiler/passes/report-left-recursion.js b/lib/compiler/passes/report-left-recursion.js
index 4bb12dc..8decd1b 100644
--- a/lib/compiler/passes/report-left-recursion.js
+++ b/lib/compiler/passes/report-left-recursion.js
@@ -18,22 +18,26 @@ var arrays       = require("../../utils/arrays"),
  * it can lead to left recursion.
  */
 function reportLeftRecursion(ast) {
+  var visitedRules = [];
+
   var check = visitor.build({
-    rule: function(node, visitedRules) {
-      check(node.expression, visitedRules.concat(node.name));
+    rule: function(node) {
+      visitedRules.push(node.name);
+      check(node.expression);
+      visitedRules.pop(node.name);
     },
 
-    sequence: function(node, visitedRules) {
+    sequence: function(node) {
       arrays.every(node.elements, function(element) {
         if (element.type === "rule_ref") {
-          check(element, visitedRules);
+          check(element);
         }
 
         return !asts.alwaysAdvancesOnSuccess(ast, element);
       });
     },
 
-    rule_ref: function(node, visitedRules) {
+    rule_ref: function(node) {
       if (arrays.contains(visitedRules, node.name)) {
         throw new GrammarError(
           "Left recursion detected for rule \"" + node.name + "\".",
@@ -41,11 +45,11 @@ function reportLeftRecursion(ast) {
         );
       }
 
-      check(asts.findRule(ast, node.name), visitedRules);
+      check(asts.findRule(ast, node.name));
     }
   });
 
-  check(ast, []);
+  check(ast);
 }
 
 module.exports = reportLeftRecursion;