Replace hashCode with checksum

Hash codes are not collision free, so what we
actually need is a checksum.

src/ol/style/fillstyle.js

@@ -1,8 +1,7 @@
 goog.provide('ol.style.Fill');
 
-goog.require('goog.string');
 goog.require('ol.color');
-goog.require('ol.structs.IHashable');
+goog.require('ol.structs.IHasChecksum');
 
 
 
@@ -12,7 +11,7 @@ goog.require('ol.structs.IHashable');
  *
  * @constructor
  * @param {olx.style.FillOptions=} opt_options Options.
- * @implements {ol.structs.IHashable}
+ * @implements {ol.structs.IHasChecksum}
  * @api
  */
 ol.style.Fill = function(opt_options) {
@@ -24,6 +23,12 @@ ol.style.Fill = function(opt_options) {
    * @type {ol.Color|string}
    */
   this.color_ = goog.isDef(options.color) ? options.color : null;
+
+  /**
+   * @private
+   * @type {?ol.structs.Checksum}
+   */
+  this.checksum_ = null;
 };
 
 
@@ -44,17 +49,18 @@ ol.style.Fill.prototype.getColor = function() {
  */
 ol.style.Fill.prototype.setColor = function(color) {
   this.color_ = color;
+  this.checksum_ = null;
 };
 
 
 /**
  * @inheritDoc
  */
-ol.style.Fill.prototype.hashCode = function() {
-  var hash = 17;
+ol.style.Fill.prototype.getChecksum = function() {
+  if (goog.isNull(this.checksum_)) {
+    this.checksum_ = 'f' + (!goog.isNull(this.color_) ?
+        ol.color.asString(this.color_) : '-');
+  }
 
-  hash = hash * 23 + (!goog.isNull(this.color_) ?
-      goog.string.hashCode(ol.color.asString(this.color_)) : 0);
-
-  return hash;
+  return this.checksum_;
 };