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69 lines
2.6 KiB
TypeScript
69 lines
2.6 KiB
TypeScript
// Copyright 2018-2022 the Deno authors. All rights reserved. MIT license.
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/**
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* pooledMap transforms values from an (async) iterable into another async
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* iterable. The transforms are done concurrently, with a max concurrency
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* defined by the poolLimit.
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*
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* If an error is thrown from `iterableFn`, no new transformations will begin.
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* All currently executing transformations are allowed to finish and still
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* yielded on success. After that, the rejections among them are gathered and
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* thrown by the iterator in an `AggregateError`.
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*
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* @param poolLimit The maximum count of items being processed concurrently.
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* @param array The input array for mapping.
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* @param iteratorFn The function to call for every item of the array.
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*/
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export function pooledMap<T, R>(
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poolLimit: number,
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array: Iterable<T> | AsyncIterable<T>,
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iteratorFn: (data: T) => Promise<R>,
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): AsyncIterableIterator<R> {
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// Create the async iterable that is returned from this function.
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const res = new TransformStream<Promise<R>, R>({
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async transform(
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p: Promise<R>,
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controller: TransformStreamDefaultController<R>,
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) {
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controller.enqueue(await p);
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},
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});
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// Start processing items from the iterator
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(async () => {
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const writer = res.writable.getWriter();
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const executing: Array<Promise<unknown>> = [];
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try {
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for await (const item of array) {
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const p = Promise.resolve().then(() => iteratorFn(item));
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// Only write on success. If we `writer.write()` a rejected promise,
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// that will end the iteration. We don't want that yet. Instead let it
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// fail the race, taking us to the catch block where all currently
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// executing jobs are allowed to finish and all rejections among them
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// can be reported together.
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p.then((v) => writer.write(Promise.resolve(v))).catch(() => {});
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const e: Promise<unknown> = p.then(() =>
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executing.splice(executing.indexOf(e), 1)
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);
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executing.push(e);
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if (executing.length >= poolLimit) {
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await Promise.race(executing);
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}
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}
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// Wait until all ongoing events have processed, then close the writer.
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await Promise.all(executing);
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writer.close();
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} catch {
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const errors = [];
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for (const result of await Promise.allSettled(executing)) {
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if (result.status == "rejected") {
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errors.push(result.reason);
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}
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}
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writer.write(Promise.reject(
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new AggregateError(errors, "Threw while mapping."),
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)).catch(() => {});
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}
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})();
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return res.readable[Symbol.asyncIterator]();
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}
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