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

1 files changed, 46 insertions, 0 deletions

diff --git a/std/async/pool.ts b/std/async/pool.ts
new file mode 100644
index 000000000..fc8517041
--- /dev/null
+++ b/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();
+}