move map data tile cache code into it's own file and clase

code/map_data_cache.js

@@ -0,0 +1,65 @@
+// MAP DATA CACHE ///////////////////////////////////
+// cache for map data tiles. 
+
+window.DataCache = function() {
+  this.REQUEST_CACHE_FRESH_AGE = 90;  // if younger than this, use data in the cache rather than fetching from the server
+  this.REQUEST_CACHE_MAX_AGE = 60*60;  // maximum cache age. entries are deleted from the cache after this time
+  this.REQUEST_CACHE_MIN_SIZE = 200;  // if fewer than this many entries, don't expire any
+  this.REQUEST_CACHE_MAX_SIZE = 2000;  // if more than this many entries, expire early
+
+  this._cache = {};
+
+}
+
+window.DataCache.prototype.store = function(qk,data,date) {
+  // fixme? common behaviour for objects is that properties are kept in the order they're added
+  // this is handy, as it allows easy retrieval of the oldest entries for expiring
+  // however, this is not guaranteed by the standards, but all our supported browsers work this way
+
+  delete this._cache[qk];
+
+  if (date === undefined) date = new Date();
+  this._cache[qk] = { time: date.getTime(), data: data };
+}
+
+window.DataCache.prototype.get = function(qk) {
+  if (qk in this._cache) return this._cache[qk].data;
+  else return undefined;
+}
+
+window.DataCache.prototype.getTime = function(qk) {
+  if (qk in this._cache) return this._cache[qk].time;
+  else return 0;
+}
+
+window.DataCache.prototype.isFresh = function(qk) {
+  if (qk in this._cache) {
+    var d = new Date();
+    var t = d.getTime() - this.REQUEST_CACHE_FRESH_AGE*1000;
+    if (this._cache[qk].time >= t) return true;
+    else return false;
+  }
+
+  return undefined;
+}
+
+
+window.DataCache.prototype.expire = function() {
+  var d = new Date();
+  var t = d.getTime()-this.REQUEST_CACHE_MAX_AGE*1000;
+
+  var cacheSize = Object.keys(this._cache).length;
+
+  for(var qk in this._cache) {
+    // stop processing once we hit the minimum size
+    if (cacheSize < this.REQUEST_CACHE_MIN_SIZE) break;
+
+    // fixme? our MAX_SIZE test here assumes we're processing the oldest first. this relies
+    // on looping over object properties in the order they were added. this is true in most browsers,
+    // but is not a requirement of the standards
+    if (cacheSize > this.REQUEST_CACHE_MAX_SIZE || this._cache[qk].time < t) {
+      delete this._cache[qk];
+      cacheSize--;
+    }
+  }
+}