feat(std/async): add pooledMap utility (denoland#6898)

std/async/mod.ts

@@ -2,3 +2,4 @@
 export * from "./deferred.ts";
 export * from "./delay.ts";
 export * from "./mux_async_iterator.ts";
+export * from "./pool.ts";

std/async/pool.ts

@@ -0,0 +1,46 @@
+// Copyright 2018-2020 the Deno authors. All rights reserved. MIT license.
+
+/**
+ * pooledMap transforms values from an (async) iterable into another async
+ * iterable. The transforms are done concurrently, with a max concurrency
+ * defined by the poolLimit.
+ * 
+ * @param poolLimit The maximum count of items being processed concurrently. 
+ * @param array The input array for mapping.
+ * @param iteratorFn The function to call for every item of the array.
+ */
+export function pooledMap<T, R>(
+  poolLimit: number,
+  array: Iterable<T> | AsyncIterable<T>,
+  iteratorFn: (data: T) => Promise<R>,
+): AsyncIterableIterator<R> {
+  // Create the async iterable that is returned from this function.
+  const res = new TransformStream<Promise<R>, R>({
+    async transform(
+      p: Promise<R>,
+      controller: TransformStreamDefaultController<R>,
+    ): Promise<void> {
+      controller.enqueue(await p);
+    },
+  });
+  // Start processing items from the iterator
+  (async (): Promise<void> => {
+    const writer = res.writable.getWriter();
+    const executing: Array<Promise<unknown>> = [];
+    for await (const item of array) {
+      const p = Promise.resolve().then(() => iteratorFn(item));
+      writer.write(p);
+      const e: Promise<unknown> = p.then(() =>
+        executing.splice(executing.indexOf(e), 1)
+      );
+      executing.push(e);
+      if (executing.length >= poolLimit) {
+        await Promise.race(executing);
+      }
+    }
+    // Wait until all ongoing events have processed, then close the writer.
+    await Promise.all(executing);
+    writer.close();
+  })();
+  return res.readable.getIterator();
+}

std/async/pool_test.ts

@@ -0,0 +1,19 @@
+import { pooledMap } from "./pool.ts";
+import { assert } from "../testing/asserts.ts";
+
+Deno.test("[async] pooledMap", async function (): Promise<void> {
+  const start = new Date();
+  const results = pooledMap(
+    2,
+    [1, 2, 3],
+    (i) => new Promise((r) => setTimeout(() => r(i), 1000)),
+  );
+  for await (const value of results) {
+    console.log(value);
+  }
+  const diff = new Date().getTime() - start.getTime();
+  assert(diff >= 2000);
+  assert(diff < 3000);
+});
+
+export {};