update IdentifierCache.js so that cache is persisted to disk only if …

…it is modified (homebridge#523)

lib/model/IdentifierCache.js

@@ -2,6 +2,7 @@
 
 var util = require('util');
 var storage = require('node-persist');
+var crypto = require('crypto');
 
 module.exports = {
   IdentifierCache: IdentifierCache
@@ -21,6 +22,7 @@ function IdentifierCache(username) {
   this.username = username;
   this._cache = {}; // cache[key:string] = id:number
   this._usedCache = null; // for usage tracking and expiring old keys
+  this._savedCacheHash = ""; // for checking if new cache neeed to be saved
 }
 
 IdentifierCache.prototype.startTrackingUsage = function() {
@@ -105,6 +107,7 @@ IdentifierCache.load = function(username) {
   if (saved) {
     var info = new IdentifierCache(username);
     info._cache = saved.cache;
+    info._savedCacheHash = crypto.createHash('sha1').update(JSON.stringify(info._cache)).digest('hex'); //calculate hash of the saved hash to decide in future if saving of new cache is neeeded
     return info;
   }
   else {
@@ -113,14 +116,18 @@ IdentifierCache.load = function(username) {
 };
 
 IdentifierCache.prototype.save = function() {
-  var saved = {
-    cache: this._cache
-  };
+  var newCacheHash = crypto.createHash('sha1').update(JSON.stringify(this._cache)).digest('hex'); //calculate hash of new cache
+  if (newCacheHash != this._savedCacheHash) {  //check if cache need to be saved and proceed accordingly
+    var saved = {
+      cache: this._cache
+    };
 
-  var key = IdentifierCache.persistKey(this.username);
+    var key = IdentifierCache.persistKey(this.username);
 
-  storage.setItemSync(key, saved);
-  storage.persistSync();
+    storage.setItemSync(key, saved);
+    storage.persistSync();
+    this._savedCacheHash = newCacheHash;  //update hash of saved cache for future use
+  }
 };
 
 IdentifierCache.prototype.remove = function () {