Optionally allow `pMapIterable` to yield results out of order

[tgfisher4]

pMapIterable currently yields mapper results in the order they were obtained from the input iterable. In other words, pMapIterable always waits for, and returns, the first promise in the queue. In my use case, the order of my input iterable is insignificant: it is essentially an unordered stream. In this use case, I want to asyncly map, with bounded concurrency, each element of the iterable and treat the results as another unordered stream: the output order does not need to match the input order. Forcing the output order to match the input order introduces head-of-line blocking: mapper results later in the promises queue that are currently available are blocked from further processing by pending results earlier in the queue. Additionally, new promises cannot be started even though old promises have fulfilled, underutilizing the allowed concurrency.

I'd like to propose adding a new pMapIterable option that treats the promises buffer like a pool rather than a queue. In other words, the returned async iterable would yield results from the promises buffer as soon as they are available, rather than in FIFO order. Perhaps it could be called preserveOrder and default to true.

I'd be happy to work on a PR if you're interested in accepting such a feature: I think it should be relatively straightforward to devise an implementation based around the conditional use of await Promise.race(promises) instead of await promises[0].

P.S.: Thanks for all these nifty p-utils!

[sindresorhus]

This would indeed be useful. I agree with the option name. PR welcome 🙏

[lacherogwu]

It is a great use case, but I'm trying to approach it differently. It should be a much simpler solution @sindresorhus take a look here https://github.com/lacherogwu/p-map/tree/bug/pMapIterable-concurrency

I haven't touched the backpressure yet, and for some reason, I couldn't get the tests working with my implementation, and I found it hard to debug

this is the concept, I implemented this in a project I was needed it

export async function* mapIterable(items: any[], cb: (item: any) => Promise<any>, options?: { concurrency: number }) {
    type P = { done: boolean; value: any; promise: Promise<any> };

    const concurrency = options?.concurrency ?? 1;
    const next = () => {
        const item = items.shift();
        const promise = cb(item).then(value => ({ value, done: items.length === 0, promise })) as Promise<P>;
        return promise;
    };

    const promises = Array.from<undefined>({ length: Math.min(concurrency, items.length) }) //
        .reduce(set => set.add(next()), new Set<Promise<P>>());

    while (promises.size > 0) {
        const { value, done, promise } = await Promise.race(promises);
        promises.delete(promise);
        yield value;

        if (!done) {
            promises.add(next());
        }
    }
}