Layout checkout, fix TODO in Collection

Layout.js

@@ -0,0 +1,38 @@
+class Layout {
+  constructor(graph) {
+    this.nodesByPos = new Map();
+
+    this.graph_ = graph;
+
+    this.setInitialPositions();
+  }
+
+  setInitialPositions() {
+    const SPACING = 4;
+
+    let maxRankNodes = 0;
+    for (let nodes of this.graph_.nodesByPageRank.values()) {
+      maxRankNodes = Math.max(maxRankNodes, nodes.length);
+    }
+
+    let ranks = Array.from(this.graph_.nodesByPageRank.keys());
+    ranks.sort((a, b) => a - b);
+    for (let r = 0; r < ranks.length; ++r) {
+      let nodes = this.graph_.nodesByPageRank.get(ranks[r]);
+      for (let n = 0; n < nodes.length; ++n) {
+        let node = nodes[n];
+        let pos = [
+            r * SPACING,
+            Math.floor((nodes.length / 2) * SPACING) + (n * SPACING) +
+                (node.subgraph * SPACING * maxRankNodes),
+        ];
+        node.pos = pos;
+        this.setNodePos(node, pos);
+      }
+    }
+  }
+
+  setNodePos(node, pos) {
+    this.nodesByPos.set(pos.toString(), node);
+  }
+}