From 19eaa181a524f9a1ed9f4073f284a5dde271a6d0 Mon Sep 17 00:00:00 2001
From: Dan Abramov <dan.abramov@gmail.com>
Date: Wed, 14 Jan 2015 20:02:40 +0300
Subject: [PATCH] Speed up common case where consumer moves only forward

---
 lib/6to5/generation/whitespace.js | 21 +++++++++++++++++++--
 1 file changed, 19 insertions(+), 2 deletions(-)

diff --git a/lib/6to5/generation/whitespace.js b/lib/6to5/generation/whitespace.js
index 6d16536f26..5e356c2f27 100644
--- a/lib/6to5/generation/whitespace.js
+++ b/lib/6to5/generation/whitespace.js
@@ -3,9 +3,22 @@ module.exports = Whitespace;
 var _    = require("lodash");
 var util = require("../util");
 
+// a helper to iterate array from arbitrary index
+function getLookupIndex(i, base, max) {
+  i += base;
+
+  if (i >= max)
+    i -= max;
+
+  return i;
+}
+
 function Whitespace(tokens, comments) {
   this.tokens = _.sortBy(tokens.concat(comments), "start");
   this.used   = [];
+
+  // speed up common case where methods are called for next tokens
+  this._lastFoundIndex = 0;
 }
 
 Whitespace.prototype.getNewlinesBefore = function (node) {
@@ -14,13 +27,15 @@ Whitespace.prototype.getNewlinesBefore = function (node) {
   var tokens = this.tokens;
   var token;
 
-  for (var i = 0; i < tokens.length; i++) {
+  for (var j = 0; j < tokens.length; j++) {
+    var i = getLookupIndex(j, this._lastFoundIndex, this.tokens.length);
     token = tokens[i];
 
     // this is the token this node starts with
     if (node.start === token.start) {
       startToken = tokens[i - 1];
       endToken = token;
+      this._lastFoundIndex = i;
       break;
     }
   }
@@ -34,13 +49,15 @@ Whitespace.prototype.getNewlinesAfter = function (node) {
   var tokens = this.tokens;
   var token;
 
-  for (var i = 0; i < tokens.length; i++) {
+  for (var j = 0; j < tokens.length; j++) {
+    var i = getLookupIndex(j, this._lastFoundIndex, this.tokens.length);
     token = tokens[i];
 
     // this is the token this node ends with
     if (node.end === token.end) {
       startToken = token;
       endToken = tokens[i + 1];
+      this._lastFoundIndex = i;
       break;
     }
   }