// Copyright 2018-2024 the Deno authors. All rights reserved. MIT license.
// This module is browser compatible.
/** Error message emitted from the thrown error while mapping. */ export const ERROR_WHILE_MAPPING_MESSAGE = "Threw while mapping.";
/**
 * pooledMap transforms values from an (async) iterable into another async
 * iterable. The transforms are done concurrently, with a max concurrency
 * defined by the poolLimit.
 *
 * If an error is thrown from `iterableFn`, no new transformations will begin.
 * All currently executing transformations are allowed to finish and still
 * yielded on success. After that, the rejections among them are gathered and
 * thrown by the iterator in an `AggregateError`.
 *
 * @example Usage
 * ```ts
 * import { pooledMap } from "@std/async/pool";
 * import { assertEquals } from "@std/assert/assert-equals";
 *
 * const results = pooledMap(
 *   2,
 *   [1, 2, 3],
 *   (i) => new Promise((r) => setTimeout(() => r(i), 1000)),
 * );
 *
 * assertEquals(await Array.fromAsync(results), [1, 2, 3]);
 * ```
 *
 * @typeParam T the input type.
 * @typeParam R the output type.
 * @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.
 * @returns The async iterator with the transformed values.
 */ export function pooledMap(poolLimit, array, iteratorFn) {
  // Create the async iterable that is returned from this function.
  const res = new TransformStream({
    async transform (p, controller) {
      try {
        const s = await p;
        controller.enqueue(s);
      } catch (e) {
        if (e instanceof AggregateError && e.message === ERROR_WHILE_MAPPING_MESSAGE) {
          controller.error(e);
        }
      }
    }
  });
  // Start processing items from the iterator
  (async ()=>{
    const writer = res.writable.getWriter();
    const executing = [];
    try {
      for await (const item of array){
        const p = Promise.resolve().then(()=>iteratorFn(item));
        // Only write on success. If we `writer.write()` a rejected promise,
        // that will end the iteration. We don't want that yet. Instead let it
        // fail the race, taking us to the catch block where all currently
        // executing jobs are allowed to finish and all rejections among them
        // can be reported together.
        writer.write(p);
        const e = 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();
    } catch  {
      const errors = [];
      for (const result of (await Promise.allSettled(executing))){
        if (result.status === "rejected") {
          errors.push(result.reason);
        }
      }
      writer.write(Promise.reject(new AggregateError(errors, ERROR_WHILE_MAPPING_MESSAGE))).catch(()=>{});
    }
  })();
  // Feature test until browser coverage is adequate
  return Symbol.asyncIterator in res.readable && typeof res.readable[Symbol.asyncIterator] === "function" ? res.readable[Symbol.asyncIterator]() : async function*() {
    const reader = res.readable.getReader();
    while(true){
      const { done, value } = await reader.read();
      if (done) break;
      yield value;
    }
    reader.releaseLock();
  }();
}
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