diff --git a/README.md b/README.md
index f46b21f..c0a0dc6 100644
--- a/README.md
+++ b/README.md
@@ -2,7 +2,7 @@
 ===========
 
 [![.github/workflows/build.yml](https://github.com/eight04/store/actions/workflows/build.yml/badge.svg)](https://github.com/eight04/store/actions/workflows/build.yml)
-[![codecov](https://codecov.io/gh/eight04/store/branch/master/graph/badge.svg)](https://codecov.io/gh/eight04/store)
+[![Coverage Status](https://coveralls.io/repos/github/eight04/store/badge.svg?branch=main)](https://coveralls.io/github/eight04/store?branch=main)
 
 A reactive store library designed to work with large collection. Inspired by svelte/store.
 
diff --git a/index.mts b/index.mts
index 783b396..75c54d4 100644
--- a/index.mts
+++ b/index.mts
@@ -624,3 +624,69 @@ export function sort<Item>(
   }
 }
 
+
+/**
+ * Reindex a collection into a new set
+ */
+export function reindex<C extends KeyedCollection<any, any>, Index>(
+  $c: C,
+  indexFn: (item: ItemFromCollection<C>) => Index
+): SetStore<readonly [Index, ItemFromCollection<C>[]]> {
+  const $s = new SetStore<readonly [Index, ItemFromCollection<C>[]]>(t => t[0]);
+  const keyToIndexCache = new Map<ReturnType<typeof $c.key>, Index>();
+  onChange({
+    added: [...$c.get()],
+    updated: [],
+    removed: [],
+    ts: $c.ts
+  });
+  $c.on("change", onChange);
+  $s.addCleanup(() => $c.off("change", onChange));
+  return $s;
+
+  function onChange({added, updated, removed, ts}: CollectionDelta<ItemFromCollection<C>>) {
+    const modified = new Map<Index, ItemFromCollection<C>[]>();
+
+    for (const item of [...updated, ...removed]) {
+      // FIXME: updated items will be move to the last. is that intended?
+      const key = $c.key(item);
+      const index = keyToIndexCache.get(key)!;
+      if (!modified.has(index)) {
+        modified.set(index, $s.map.get(index)![1]);
+      }
+      modified.set(index, modified.get(index)!.filter(i => key !== $c.key(i)))
+      keyToIndexCache.delete(key);
+    }
+
+    for (const item of [...updated, ...added]) {
+      const index = indexFn(item);
+      const key = $c.key(item);
+      if (!modified.has(index)) {
+        modified.set(index, $s.map.get(index)?.[1] || []);
+      }
+      modified.set(index, [...modified.get(index)!, item]);
+      keyToIndexCache.set(key, index);
+    }
+
+    const toAdd = [];
+    const toUpdate = [];
+    const toRemove = [];
+
+    for (const [index, value] of modified.entries()) {
+      if (!$s.map.has(index)) {
+        toAdd.push([index, value] as const);
+      } else if (value.length) {
+        toUpdate.push([index, value] as const);
+      } else {
+        // FIXME: value is always an empty array here. which should be set to removed items
+        toRemove.push([index, value] as const);
+      }
+    }
+
+    $s.set({
+      added: toAdd,
+      updated: toUpdate,
+      removed: toRemove
+    }, ts);
+  }
+}
diff --git a/test/reindex.mts b/test/reindex.mts
new file mode 100644
index 0000000..3227095
--- /dev/null
+++ b/test/reindex.mts
@@ -0,0 +1,66 @@
+/* eslint-env mocha */
+import * as assert from "node:assert";
+import {ArrayStore, reindex} from "../index.mjs";
+
+class Item {
+  id: number;
+  value: number;
+  constructor(id: any, value: any) {
+    this.id = id;
+    this.value = value;
+  }
+}
+
+function cmp(a: Item, b: Item) {
+  return a.value - b.value;
+}
+
+describe("reindex", () => {
+  it("basic", () => {
+    const $s = new ArrayStore<Item>(i => i.id, cmp);
+    const added = [new Item(1, 2), new Item(2, 1)];
+    $s.set({ added });
+    const $r = reindex($s, i => i.value);
+    assert.deepStrictEqual($r.get(), new Set([
+      [1, [ new Item(2, 1)]],
+      [2, [ new Item(1, 2)]]
+    ]));
+
+    $s.set({ removed: [new Item(1, 2)] });
+    assert.deepStrictEqual($r.get(), new Set([
+      [1, [ new Item(2, 1)]],
+    ]));
+
+    $s.set({ updated: [new Item(2, 3)] });
+    assert.deepStrictEqual($r.get(), new Set([
+      [3, [ new Item(2, 3)]],
+    ]));
+  });
+
+  it("duplicated index", () => {
+    const $s = new ArrayStore<Item>(i => i.id, cmp);
+    const $r = reindex($s, i => i.value);
+    const added = [new Item(1, 2), new Item(2, 1), new Item(3, 1)];
+    $s.set({ added });
+    assert.deepStrictEqual($r.get(), new Set([
+      [1, [ new Item(2, 1), new Item(3, 1) ]],
+      [2, [ new Item(1, 2)]]
+    ]));
+  });
+
+  it("index doesn't match when removed", () => {
+    const $s = new ArrayStore<Item>(i => i.id, cmp);
+    const added = [new Item(1, 2), new Item(2, 1)];
+    $s.set({ added });
+    const $r = reindex($s, i => i.value);
+    assert.deepStrictEqual($r.get(), new Set([
+      [1, [ new Item(2, 1)]],
+      [2, [ new Item(1, 2)]]
+    ]));
+
+    $s.set({ removed: [new Item(1, 3)] });
+    assert.deepStrictEqual($r.get(), new Set([
+      [1, [ new Item(2, 1)]],
+    ]));
+  })
+});