substring optimization

src/context.js

@@ -1,20 +1,22 @@
 exports.create = function(raw, options, regex) {
-	var index = 0;
+	var index = 0,
+		substring = null;
+
 	var context = {
 		text: '',
 		peek: function(count) {
 			count = count || 1;
-			return this.raw.substr(index + 1, count);
+			return this.raw.substr(this.index + 1, count);
 		},
 		read: function(count) {
 			if (count === 0) {
 				return '';
 			}
 			count = count || 1;
 			var next = this.peek(count);
-			index += count;
-			if (index > this.length) {
-				index = this.length;
+			this.index += count;
+			if (this.index > this.length) {
+				this.index = this.length;
 			}
 			return next;
 		},
@@ -31,11 +33,11 @@ exports.create = function(raw, options, regex) {
 			return value;
 		},
 		isEof: function() {
-			return index >= this.length;
+			return this.index >= this.length;
 		},
 		readRegex: function(regex) {
 			var value = (regex.exec(this.raw.substring(this.index)) || [''])[0];
-			index += value.length;
+			this.index += value.length;
 			return value;
 		},
 		peekIgnoreWhitespace: function(count) {
@@ -67,8 +69,12 @@ exports.create = function(raw, options, regex) {
 	context.__defineGetter__('index', function() {
 		return index;
 	});
+	context.__defineSetter__('index', function(value) {
+		index = value;
+		substring = null;
+	});
 	context.__defineGetter__('substring', function() {
-		return this.raw.substring(this.index);
+		return substring === null ? (substring = this.raw.substring(this.index)) : substring;
 	});
 
 	context.callbacks = {};