Add FenwickTree class to src/App.tsx

src/App.tsx

@@ -80,6 +80,36 @@ type HistoryEntry = {
   baseVersionId?: string | null;
 };
 
+class FenwickTree {
+  private size: number;
+  private tree: Float64Array;
+
+  constructor(size: number) {
+    this.size = size;
+    this.tree = new Float64Array(size + 1);
+  }
+
+  reset(size: number) {
+    this.size = size;
+    this.tree = new Float64Array(size + 1);
+  }
+
+  add(index: number, delta: number) {
+    if (!Number.isFinite(delta) || delta === 0) return;
+    for (let i = index + 1; i <= this.size; i += i & -i) {
+      this.tree[i] += delta;
+    }
+  }
+
+  sum(index: number) {
+    let result = 0;
+    for (let i = index; i > 0; i -= i & -i) {
+      result += this.tree[i];
+    }
+    return result;
+  }
+}
+
 type SidebarEntry =
   | { kind: "folder"; item: Folder }
   | { kind: "text"; item: Text };