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// 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();
}
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