feat(async): add repeatable helper function

Author: lambdalisue

.github/FUNDING.yml

@@ -1,6 +1,8 @@
 # These are supported funding model platforms
 
-github: [lambdalisue] # Replace with up to 4 GitHub Sponsors-enabled usernames e.g., [user1, user2]
+github: [
+  lambdalisue,
+] # Replace with up to 4 GitHub Sponsors-enabled usernames e.g., [user1, user2]
 patreon: # Replace with a single Patreon username
 open_collective: # Replace with a single Open Collective username
 ko_fi: # Replace with a single Ko-fi username

README.md

@@ -1115,6 +1115,27 @@ const iter = pipe(
 console.log(await Array.fromAsync(iter)); // [1, 2, 3, 1, 2, 3]
 ```
 
+### repeatable
+
+Transform an async iterable into a repeatable async iterable. It caches the
+values of the original iterable so that it can be replayed. Useful for replaying
+the costly async iterable.
+
+```ts
+import { repeatable } from "@core/iterutil/async/repeatable";
+import { assertEquals } from "@std/assert";
+
+const origin = (async function* () {
+  yield 1;
+  yield 2;
+  yield 3;
+})();
+const iter = repeatable(origin);
+assertEquals(await Array.fromAsync(iter), [1, 2, 3]);
+assertEquals(await Array.fromAsync(iter), [1, 2, 3]); // iter can be replayed
+assertEquals(await Array.fromAsync(origin), []); // origin is already consumed
+```
+
 ### some
 
 Returns true if at least one element in the iterable satisfies the provided

async/mod.ts

@@ -21,6 +21,7 @@ export * from "./pairwise.ts";
 export * from "./partition.ts";
 export * from "./reduce.ts";
 export * from "./repeat.ts";
+export * from "./repeatable.ts";
 export * from "./some.ts";
 export * from "./take.ts";
 export * from "./take_while.ts";

async/repeatable.ts

@@ -0,0 +1,46 @@
+const done = Symbol("done");
+
+export function repeatable<T>(iterable: AsyncIterable<T>): AsyncIterable<T> {
+  const cache: T[] = [];
+  let buildingCache: Promise<void> | undefined = undefined;
+  let pendingResolvers: ((value: T | typeof done) => void)[] = [];
+  let finished = false;
+
+  return {
+    [Symbol.asyncIterator]: async function* () {
+      yield* cache;
+
+      if (!finished) {
+        if (!buildingCache) {
+          buildingCache = (async () => {
+            try {
+              for await (const item of iterable) {
+                cache.push(item);
+                pendingResolvers.forEach((resolve) => resolve(item));
+                pendingResolvers = [];
+              }
+            } finally {
+              finished = true;
+              pendingResolvers.forEach((resolve) => resolve(done));
+              pendingResolvers = [];
+            }
+          })();
+        }
+      }
+      let index = cache.length;
+      while (!finished || index < cache.length) {
+        if (index < cache.length) {
+          yield cache[index++];
+        } else {
+          const nextItem = await new Promise<T | typeof done>((resolve) => {
+            pendingResolvers.push(resolve);
+          });
+          if (nextItem !== done) {
+            yield nextItem;
+            index++;
+          }
+        }
+      }
+    },
+  };
+}

async/repeatable_test.ts

@@ -0,0 +1,82 @@
+import { test } from "@cross/test";
+import { delay } from "@std/async/delay";
+import { assertEquals } from "@std/assert";
+import { repeatable } from "./repeatable.ts";
+
+async function* delayedGenerator(sideEffect?: () => void) {
+  yield 1;
+  await delay(100);
+  yield 2;
+  await delay(100);
+  yield 3;
+  sideEffect?.();
+}
+
+await test("repeatable should return the same sequence on multiple iterations", async () => {
+  const input = delayedGenerator();
+  const it = repeatable(input);
+
+  const result1 = await Array.fromAsync(it);
+  const result2 = await Array.fromAsync(it);
+
+  assertEquals(result1, [1, 2, 3], "First iteration");
+  assertEquals(result2, [1, 2, 3], "First iteration");
+});
+
+await test("repeatable should call internal iterator only once", async () => {
+  let called = 0;
+  const input = delayedGenerator(() => called++);
+  const it = repeatable(input);
+
+  const result1 = await Array.fromAsync(it);
+  const result2 = await Array.fromAsync(it);
+
+  assertEquals(result1, [1, 2, 3], "First iteration");
+  assertEquals(result2, [1, 2, 3], "First iteration");
+  assertEquals(called, 1, "Internal iterator called only once");
+});
+
+await test("repeatable should work correctly when consumed partially and then fully", async () => {
+  const input = delayedGenerator();
+  const it = repeatable(input);
+
+  const result1: number[] = [];
+  const firstIter = it[Symbol.asyncIterator]();
+
+  result1.push((await firstIter.next()).value); // 1
+
+  const result2 = await Array.fromAsync(it);
+
+  result1.push((await firstIter.next()).value); // 2
+  result1.push((await firstIter.next()).value); // 3
+
+  assertEquals(result1, [1, 2, 3], "First iteration");
+  assertEquals(result2, [1, 2, 3], "First iteration");
+});
+
+await test("repeatable should cache values and return them immediately on subsequent iterations", async () => {
+  const input = delayedGenerator();
+  const it = repeatable(input);
+
+  const start = performance.now();
+  const result1 = await Array.fromAsync(it);
+  const end1 = performance.now();
+  const timeTaken1 = end1 - start;
+
+  const start2 = performance.now();
+  const result2 = await Array.fromAsync(it);
+  const end2 = performance.now();
+  const timeTaken2 = end2 - start2;
+
+  assertEquals(result1, [1, 2, 3], "First iteration");
+  assertEquals(result2, [1, 2, 3], "Second iteration");
+
+  console.debug("Time taken for first consume:", timeTaken1);
+  console.debug("Time taken for second consume (with cache):", timeTaken2);
+
+  if (timeTaken2 > timeTaken1 / 10) {
+    throw new Error(
+      "Second consume took too long, cache might not be working.",
+    );
+  }
+});

deno.jsonc

@@ -27,6 +27,7 @@
     "./async/partition": "./async/partition.ts",
     "./async/reduce": "./async/reduce.ts",
     "./async/repeat": "./async/repeat.ts",
+    "./async/repeatable": "./async/repeatable.ts",
     "./async/some": "./async/some.ts",
     "./async/take": "./async/take.ts",
     "./async/take-while": "./async/take_while.ts",
@@ -261,6 +262,7 @@
     "@core/unknownutil": "jsr:@core/unknownutil@^4.0.1",
     "@cross/test": "jsr:@cross/test@^0.0.9",
     "@std/assert": "jsr:@std/assert@^1.0.2",
+    "@std/async": "jsr:@std/async@^1.0.6",
     "@std/jsonc": "jsr:@std/jsonc@^1.0.0",
     "@std/path": "jsr:@std/path@^1.0.2",
     "@std/testing": "jsr:@std/testing@^1.0.0"