add stress test to repo

packages/svelte/src/internal/client/dom/blocks/each.js

@@ -43,6 +43,7 @@ import { current_batch } from '../../reactivity/batch.js';
 
 // When making substantive changes to this file, validate them with the each block stress test:
 // https://svelte.dev/playground/1972b2cf46564476ad8c8c6405b23b7b
+// This test also exists in this repo, as `packages/svelte/tests/manual/each-stress-test`
 
 /**
  * @param {any} _

packages/svelte/tests/manual/each-stress-test/main.svelte

@@ -0,0 +1,194 @@
+<script lang="ts">
+	import { tick } from 'svelte';
+
+	const VALUES = Array.from('abcdefghijklmnopqrstuvwxyz');
+
+	const presets = [
+		// b is never destroyed
+		[
+			"ab",
+			"",
+			"a",
+			"abc"
+		],
+		// the final state is 'abc', not 'cba'
+		[
+			"abc",
+			"",
+			"cba"
+		],
+		// the case in https://github.com/sveltejs/svelte/pull/17240
+		[
+			"abc",
+			"adbc",
+			"adebc"
+		],
+		[
+			"ab",
+			"a",
+			"abc"
+		],
+		[
+			"a",
+			"bc",
+			"bcd"
+		],
+		// add more presets by hitting 'party' and copying from the console
+	];
+
+	function shuffle() {
+		const values = VALUES.slice();
+		const number = Math.floor(Math.random() * VALUES.length);
+		let shuffled = '';
+		for (let i = 0; i < number; i++) {
+			shuffled += (values.splice(Math.floor(Math.random() * (number - i)), 1))[0];
+		}
+
+		return shuffled;
+	}
+
+	function mark(node) {
+		let prev = -1;
+
+		return {
+			duration: transition ? (slow ? 5000 : 500) : 0,
+			tick(t) {
+				const direction = t >= prev ? 'in' : 'out';
+				node.style.color = direction === 'in' ? '' : 'grey';
+
+				prev = t;
+			}
+		}
+	}
+
+	const record = [];
+
+	const sleep = (ms = slow ? 1000 : 100) => new Promise((f) => setTimeout(f, ms));
+
+	async function test(x: string) {
+		console.group(JSON.stringify(x));
+		error = null;
+
+		list = x;
+		record.push(list);
+		if (transition) {
+			await sleep();
+		} else {
+			await tick();
+			await tick();
+		}
+		check('reconcile');
+
+		n += 1;
+		await tick();
+		check('update');
+		console.groupEnd();
+	}
+
+	function check(task: string) {
+		const expected = list.split('').map((c) => `(${c}:${n})`).join('') || '(fallback)';
+
+		const children = Array.from(container.children);
+		const filtered = children.filter((span: HTMLElement) => !span.style.color);
+		const received = filtered.map((span) => span.textContent).join('');
+
+		if (expected !== received) {
+			console.log('expected:', expected);
+			console.log('received:', received);
+			console.log(JSON.stringify(record, null, '  '));
+
+			error = `failed to ${task}`;
+			throw new Error(error);
+		}
+	}
+
+	let list = $state('');
+	let n = $state(0);
+	let error = $state(null);
+	let slow = $state(false);
+	let transition = $state(true);
+	let partying = $state(false);
+
+	let container: HTMLElement;
+</script>
+
+<h1>each block stress test</h1>
+
+<label>
+	<input type="checkbox" bind:checked={transition} />
+	transition
+</label>
+
+<label>
+	<input type="checkbox" bind:checked={slow} />
+	slow
+</label>
+
+<fieldset>
+	<legend>random</legend>
+
+	<button onclick={() => test(shuffle())}>test</button>
+	<button onclick={async () => {
+		if (partying) {
+			partying = false;
+		} else {
+			partying = true;
+			while (partying) await test(shuffle());
+		}
+	}}>{partying ? 'stop' : 'party'}</button>
+</fieldset>
+
+<fieldset>
+	<legend>presets</legend>
+
+	{#each presets as preset, index}
+		<button onclick={async () => {
+			for (let i = 0; i < preset.length; i += 1) {
+				await test(preset[i]);
+			}
+		}}>{index + 1}</button>
+	{/each}
+</fieldset>
+
+<form onsubmit={(e) => {
+	e.preventDefault();
+	test(e.currentTarget.querySelector('input').value);
+}}>
+	<fieldset>
+		<legend>input</legend>
+		<input />
+	</fieldset>
+</form>
+
+<div id="output" bind:this={container}>
+	{#each list as c (c)}
+		<span transition:mark>({c}:{n})</span>
+	{:else}
+		<span transition:mark>(fallback)</span>
+	{/each}
+</div>
+
+{#if error}
+	<p class="error">{error}</p>
+{/if}
+
+<style>
+	fieldset {
+		display: flex;
+		gap: 0.5em;
+		border-radius: 0.5em;
+		corner-shape: squircle;
+		margin: 0 0 1em 0;
+		padding: 0.2em 0.8em 0.8em;
+	}
+
+	legend {
+		padding: 0.2em 0.5em;
+		left: -0.2em;
+		position: relative;
+	}
+
+	.error {
+		color: red;
+	}
+</style>