1 files changed, 45 insertions, 0 deletions
diff --git a/js/util/memoize.js b/js/util/memoize.js
new file mode 100644
index 0000000..796c060
--- /dev/null
+++ b/js/util/memoize.js
@@ -0,0 +1,45 @@
+/*
+ * Helper function for memoization
+ *
+ * The passed labeller function must generate unique and exact names for entries that are naturally equal.
+ * IDs are persistent.
+ *
+ * names, idByName, and entries are accessible fields from the memoizer.
+ * Behaviour on external modification of these fields is undefined.
+ *
+ * An entry object passed to the memoizer has its field "id" set to the memoized id by default.
+ * The name of the ID field can be modified by calling the getter/setter idField() from the memoizer.
+ * Behaviour on changing the ID field used while the memoizer has already performed memoization is undefined.
+ */
+function memoize(labeller) {
+  var idField = "id";
+  var names = [];
+  var idByName = {};
+  var entries = [];
+  var _memoizer = function(entry) {
+    var name = labeller(entry);
+    if (name in idByName) {
+        /* We already have a suitable entry. */
+        var id = idByName[name];
+        return entries[id];
+    }
+    /* New entry. */
+    /* Set the entry's ID. */
+    entry[idField] = entries.length;
+    /* Index the new entry. */
+    idByName[name] = entry[idField];
+    names[entry[idField]] = name;
+    entries.push(entry);
+
+    return entry;
+  }
+  _memoizer.names = function() { return names; };
+  _memoizer.idByName = function() { return idByName; };
+  _memoizer.entries = function() { return entries; };
+  _memoizer.idField = function(x) {
+    if (!arguments.length) return idField;
+    idField = x;
+    return _memoizer;
+  };
+  return _memoizer;
+}