From a1498bb57bbb5bb51507149a71a3ca9511327718 Mon Sep 17 00:00:00 2001
From: fallenoak <git@fallenoak.me>
Date: Mon, 25 Dec 2023 13:18:46 -0600
Subject: [PATCH] feat(util): add paced helper to break up heavy iterators (#2)

---
 src/lib/util.ts       | 25 ++++++++++++++++++++++++-
 src/spec/util.spec.ts | 41 ++++++++++++++++++++++++++++++++++++++++-
 2 files changed, 64 insertions(+), 2 deletions(-)

diff --git a/src/lib/util.ts b/src/lib/util.ts
index fee8c0e..504b511 100644
--- a/src/lib/util.ts
+++ b/src/lib/util.ts
@@ -8,4 +8,27 @@ const nextFrame = () => {
   });
 };
 
-export { nextFrame };
+/**
+ * Returns an iterator for the given iterable that pauses for an animation frame every time the
+ * given pace has elapsed. Useful for pacing synchronous work without excessively blocking the
+ * main thread.
+ *
+ * @param iterable - Any iterable
+ * @param paceMs - Length of time in milliseconds before waiting for a frame.
+ */
+async function* paced<T>(iterable: Iterable<T>, paceMs = 10) {
+  let lastFrameMs = Date.now();
+
+  for (const it of iterable) {
+    if (Date.now() - lastFrameMs >= paceMs) {
+      await nextFrame();
+      lastFrameMs = Date.now();
+    }
+
+    yield it;
+  }
+}
+
+const sleep = (timeMs: number) => new Promise((resolve) => setTimeout(resolve, timeMs));
+
+export { nextFrame, paced, sleep };
diff --git a/src/spec/util.spec.ts b/src/spec/util.spec.ts
index c28c3bc..17f7ede 100644
--- a/src/spec/util.spec.ts
+++ b/src/spec/util.spec.ts
@@ -1,4 +1,4 @@
-import { nextFrame } from '../lib';
+import { nextFrame, paced, sleep } from '../lib/util';
 import { describe, expect, test } from 'vitest';
 
 describe('util', () => {
@@ -7,4 +7,43 @@ describe('util', () => {
       await expect(nextFrame()).resolves.toEqual(undefined);
     });
   });
+
+  describe('paced', () => {
+    test('should iterate over array in multiple frames when work takes longer than a frame', async () => {
+      let frames = 0;
+      const countFrame = () => {
+        frames++;
+        requestAnimationFrame(countFrame);
+      };
+      requestAnimationFrame(countFrame);
+
+      const elements = [1, 2, 3];
+      const iterated = [];
+      for await (const element of paced(elements)) {
+        await sleep(30);
+        iterated.push(element);
+      }
+
+      expect(iterated).toEqual(elements);
+      expect(frames).toBeGreaterThan(0);
+    });
+
+    test('should iterate over array in zero frames when work takes less than a frame', async () => {
+      let frames = 0;
+      const countFrame = () => {
+        frames++;
+        requestAnimationFrame(countFrame);
+      };
+      requestAnimationFrame(countFrame);
+
+      const elements = [1, 2, 3];
+      const iterated = [];
+      for await (const element of paced(elements)) {
+        iterated.push(element);
+      }
+
+      expect(iterated).toEqual(elements);
+      expect(frames).toBe(0);
+    });
+  });
 });