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diff --git a/ui/lib/iterator.test.js b/ui/lib/iterator.test.js
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+++ b/ui/lib/iterator.test.js
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+import * as iter from './iterator.js';
+import { beforeEach, expect, test, describe, it, vi } from 'vitest';
+
+describe('map', async () => {
+  it('applies the mapping function to each item', async () => {
+    const seq = Iterator.from([1, 2, 3, 5, 8]);
+    const mapped = iter.map(seq, (x) => 2 * x);
+    expect(Array.from(mapped)).toStrictEqual([2, 4, 6, 10, 16]);
+  });
+});
+
+describe('reduce', async () => {
+  it('accumulates results and returns them', async () => {
+    const seq = Iterator.from([1, 2, 3, 4]);
+    const reduced = iter.reduce(seq, (sum, x) => sum + x, 0);
+
+    // Good ol' triangle numbers.
+    expect(reduced).toStrictEqual(10);
+  });
+});
+
+describe('chunkBy', async () => {
+  describe('with trivial operand functions', async () => {
+    it('yields nothing for an empty input', async () => {
+      const chunks = iter.chunkBy(
+        [],
+        (val) => val,
+        (last, next) => last === next
+      );
+
+      expect(Array.from(chunks)).toStrictEqual([]);
+    });
+
+    it('yields one chunk input for a singleton input', async () => {
+      const chunks = iter.chunkBy(
+        [37],
+        (val) => val,
+        (last, next) => last === next
+      );
+
+      expect(Array.from(chunks)).toStrictEqual([{ key: 37, chunk: [37] }]);
+    });
+
+    it('yields chunks of successive inputs', async () => {
+      const chunks = iter.chunkBy(
+        [37, 37, 28, 37],
+        (val) => val,
+        (last, next) => last === next
+      );
+
+      expect(Array.from(chunks)).toStrictEqual([
+        { key: 37, chunk: [37, 37] },
+        { key: 28, chunk: [28] },
+        {
+          key: 37,
+          chunk: [37]
+        }
+      ]);
+    });
+  });
+
+  describe('with a complex key function', async () => {
+    it('returns the key with each chunk', async () => {
+      const chunks = iter.chunkBy(
+        [37, 37, 28, 37],
+        (val) => val >> 1,
+        (last, next) => last === next
+      );
+
+      expect(Array.from(chunks)).toStrictEqual([
+        { key: 18, chunk: [37, 37] },
+        { key: 14, chunk: [28] },
+        { key: 18, chunk: [37] }
+      ]);
+    });
+  });
+
+  describe('with a complex coalesce function', async () => {
+    it('continues the chunk when specified', async () => {
+      const chunks = iter.chunkBy(
+        [36, 37, 28, 29, 30, 38],
+        (val) => val,
+        (last, next) => last + 1 === next
+      );
+
+      expect(Array.from(chunks)).toStrictEqual([
+        { key: 37, chunk: [36, 37] },
+        { key: 30, chunk: [28, 29, 30] },
+        { key: 38, chunk: [38] }
+      ]);
+    });
+  });
+});