diff --git a/index.html b/index.html
index b905071..f9051d1 100644
--- a/index.html
+++ b/index.html
@@ -4,9 +4,10 @@
         <meta charset="UTF-8">
         <meta name="viewport" content="width=device-width, initial-scale=1.0">
         <title>BLE-mesh SIM</title>
-        <script type="text/javascript" src="vis-network.min.js"></script>
-        <script type="text/javascript" src="simhelper.js"></script>
-        <script type="text/javascript" src="CompleteMoveOnEdge.js"></script>
+        <script type="text/javascript" src="js/vis-network.min.js"></script>
+        <script type="text/javascript" src="js/simhelper.js"></script>
+        <script type="text/javascript" src="js/CompleteMoveOnEdge.js"></script>
+        <script type="text/javascript" src="js/GridIndex.js"></script>
         <link rel="stylesheet" id="styleSheet" href="style.css">
     </head>
     <body>
diff --git a/CompleteMoveOnEdge.js b/js/CompleteMoveOnEdge.js
similarity index 100%
rename from CompleteMoveOnEdge.js
rename to js/CompleteMoveOnEdge.js
diff --git a/js/GridIndex.js b/js/GridIndex.js
new file mode 100644
index 0000000..e377f44
--- /dev/null
+++ b/js/GridIndex.js
@@ -0,0 +1,60 @@
+/*
+    Grid index for quickly looking up neighbor candidates.
+*/
+class GridIndex {
+    constructor(cellSize) {
+        this.cellSize = cellSize;
+        this.cells = new Map();
+        this.nodeCell = new Map();
+    }
+    keyFor(x, y) {
+        const cx = Math.floor(x / this.cellSize);
+        const cy = Math.floor(y / this.cellSize);
+        return `${cx},${cy}`;
+    }
+    add(nodeId, x, y) {
+        const key = this.keyFor(x, y);
+        if (!this.cells.has(key)) {
+            this.cells.set(key, new Set());
+        }
+        this.cells.get(key).add(nodeId);
+        this.nodeCell.set(nodeId, key);
+    }
+    update(nodeId, x, y) {
+        const newKey = this.keyFor(x, y);
+        const oldKey = this.nodeCell.get(nodeId);
+        if (oldKey == newKey) {
+            return;
+        }
+        if (oldKey && this.cells.has(oldKey)) {
+            this.cells.get(oldKey).delete(nodeId);
+        }
+        if (!this.cells.has(newKey)) {
+            this.cells.set(newKey, new Set());
+        }
+        this.cells.get(newKey).add(nodeId);
+        this.nodeCell.set(nodeId, newKey);
+    }
+    remove(nodeId) {
+        const key = this.nodeCell.get(nodeId);
+        if (key && this.cells.has(key)) {
+            this.cells.get(key).delete(nodeId);
+        }
+        this.nodeCell.delete(nodeId);
+    }
+    getNeighborCandidates(x, y) {
+        const cx = Math.floor(x / this.cellSize);
+        const cy = Math.floor(y / this.cellSize);
+        const output = [];
+        for (let dx = -1; dx <= 1; dx++) {
+            for (let dy = -1; dy <= 1; dy++) {
+                const key = `${cx + dx},${cy + dy}`;
+                const bucket = this.cells.get(key);
+                if (bucket) {
+                    output.push(...bucket);
+                }
+            }
+        }
+        return output;
+    }
+}
\ No newline at end of file
diff --git a/simhelper.js b/js/simhelper.js
similarity index 100%
rename from simhelper.js
rename to js/simhelper.js
diff --git a/vis-network.min.js b/js/vis-network.min.js
similarity index 100%
rename from vis-network.min.js
rename to js/vis-network.min.js