From 6e46c977e8573d4c3ac5f1297321497e57c030be Mon Sep 17 00:00:00 2001
From: Hidekazu Kobayashi <kobahide789@gmail.com>
Date: Sat, 7 Oct 2023 05:52:04 +0000
Subject: [PATCH] Implement DebounceLoader

---
 packages/backend/src/misc/loader.ts       | 49 +++++++++++++
 packages/backend/test/unit/misc/loader.ts | 88 +++++++++++++++++++++++
 2 files changed, 137 insertions(+)
 create mode 100644 packages/backend/src/misc/loader.ts
 create mode 100644 packages/backend/test/unit/misc/loader.ts

diff --git a/packages/backend/src/misc/loader.ts b/packages/backend/src/misc/loader.ts
new file mode 100644
index 000000000000..49aac5d883c9
--- /dev/null
+++ b/packages/backend/src/misc/loader.ts
@@ -0,0 +1,49 @@
+export type FetchFunction<K, V> = (key: K) => Promise<V>;
+type ResolveReject<V> = Parameters<ConstructorParameters<typeof Promise<V>>[0]>;
+type ResolverPair<V> = {
+	resolve: ResolveReject<V>[0];
+	reject: ResolveReject<V>[1];
+};
+export class DebounceLoader<K, V> {
+	private resolverMap = new Map<K, ResolverPair<V>>();
+	private promiseMap = new Map<K, Promise<V>>();
+	private resolvedPromise = Promise.resolve();
+	constructor(private loadFn: FetchFunction<K, V>) {}
+
+	public load(key: K): Promise<V> {
+		const promise = this.promiseMap.get(key);
+		if (typeof promise !== 'undefined') {
+			return promise;
+		}
+
+		const isFirst = this.promiseMap.size === 0;
+		const newPromise = new Promise<V>((resolve, reject) => {
+			this.resolverMap.set(key, { resolve, reject });
+		});
+		this.promiseMap.set(key, newPromise);
+
+		if (isFirst) {
+			this.enqueueDebouncedLoadJob();
+		}
+
+		return newPromise;
+	}
+
+	private runDebouncedLoad(): void {
+		const resolvers = [...this.resolverMap];
+		this.resolverMap.clear();
+		this.promiseMap.clear();
+
+		for (const [key, { resolve, reject }] of resolvers) {
+			this.loadFn(key).then(resolve, reject);
+		}
+	}
+
+	private enqueueDebouncedLoadJob(): void {
+		this.resolvedPromise.then(() => {
+			process.nextTick(() => {
+				this.runDebouncedLoad();
+			});
+		});
+	}
+}
diff --git a/packages/backend/test/unit/misc/loader.ts b/packages/backend/test/unit/misc/loader.ts
new file mode 100644
index 000000000000..fa3795095118
--- /dev/null
+++ b/packages/backend/test/unit/misc/loader.ts
@@ -0,0 +1,88 @@
+import { DebounceLoader } from '@/misc/loader.js';
+
+class Mock {
+	loadCountByKey = new Map<number, number>();
+	load = async (key: number): Promise<number> => {
+		const count = this.loadCountByKey.get(key);
+		if (typeof count === 'undefined') {
+			this.loadCountByKey.set(key, 1);
+		} else {
+			this.loadCountByKey.set(key, count + 1);
+		}
+		return key * 2;
+	};
+	reset() {
+		this.loadCountByKey.clear();
+	}
+}
+
+describe(DebounceLoader, () => {
+	describe('single request', () => {
+		it('loads once', async () => {
+			const mock = new Mock();
+			const loader = new DebounceLoader(mock.load);
+			expect(await loader.load(7)).toBe(14);
+			expect(mock.loadCountByKey.size).toBe(1);
+			expect(mock.loadCountByKey.get(7)).toBe(1);
+		});
+	});
+
+	describe('two duplicated requests at same time', () => {
+		it('loads once', async () => {
+			const mock = new Mock();
+			const loader = new DebounceLoader(mock.load);
+			const [v1, v2] = await Promise.all([
+				loader.load(7),
+				loader.load(7),
+			]);
+			expect(v1).toBe(14);
+			expect(v2).toBe(14);
+			expect(mock.loadCountByKey.size).toBe(1);
+			expect(mock.loadCountByKey.get(7)).toBe(1);
+		});
+	});
+
+	describe('two different requests at same time', () => {
+		it('loads twice', async () => {
+			const mock = new Mock();
+			const loader = new DebounceLoader(mock.load);
+			const [v1, v2] = await Promise.all([
+				loader.load(7),
+				loader.load(13),
+			]);
+			expect(v1).toBe(14);
+			expect(v2).toBe(26);
+			expect(mock.loadCountByKey.size).toBe(2);
+			expect(mock.loadCountByKey.get(7)).toBe(1);
+			expect(mock.loadCountByKey.get(13)).toBe(1);
+		});
+	});
+
+	describe('non-continuous same two requests', () => {
+		it('loads twice', async () => {
+			const mock = new Mock();
+			const loader = new DebounceLoader(mock.load);
+			expect(await loader.load(7)).toBe(14);
+			expect(mock.loadCountByKey.size).toBe(1);
+			expect(mock.loadCountByKey.get(7)).toBe(1);
+			mock.reset();
+			expect(await loader.load(7)).toBe(14);
+			expect(mock.loadCountByKey.size).toBe(1);
+			expect(mock.loadCountByKey.get(7)).toBe(1);
+		});
+	});
+
+	describe('non-continuous different two requests', () => {
+		it('loads twice', async () => {
+			const mock = new Mock();
+			const loader = new DebounceLoader(mock.load);
+			expect(await loader.load(7)).toBe(14);
+			expect(mock.loadCountByKey.size).toBe(1);
+			expect(mock.loadCountByKey.get(7)).toBe(1);
+			mock.reset();
+			expect(await loader.load(13)).toBe(26);
+			expect(mock.loadCountByKey.size).toBe(1);
+			expect(mock.loadCountByKey.get(13)).toBe(1);
+		});
+	});
+});