Replace Ember app with Mithril app

js/lib/utils/subtree-retainer.js

@@ -0,0 +1,33 @@
+/**
+  // constructor
+  this.subtree = new SubtreeRetainer(
+    () => this.props.post.freshness,
+    () => this.showing
+  );
+  this.subtree.add(() => this.props.user.freshness);
+
+  // view
+  this.subtree.retain() || 'expensive expression'
+ */
+export default class SubtreeRetainer {
+  constructor() {
+    this.old = [];
+    this.callbacks = [].slice.call(arguments);
+  }
+
+  retain() {
+    var needsRebuild = false;
+    this.callbacks.forEach((callback, i) => {
+      var result = callback();
+      if (result !== this.old[i]) {
+        this.old[i] = result;
+        needsRebuild = true;
+      }
+    });
+    return needsRebuild ? false : {subtree: 'retain'};
+  }
+
+  add() {
+    this.callbacks = this.callbacks.concat([].slice.call(arguments));
+  }
+}