@thi.ng/transducers-async/sync.js

Async versions of various highly composable transducers, reducers and iterators

import { __inflightIters, __iterNext, __iterRemove } from "./internal/iter.js";
async function* sync(src, opts) {
  let iters = Object.entries(src).map(([key, v], id) => ({
    key,
    id,
    iter: v[Symbol.asyncIterator]()
  }));
  const $initial = async () => {
    const res = await Promise.all(iters.map(({ iter }) => iter.next()));
    for (let i = 0, n = iters.length; i < n; ) {
      if (res[i].done) {
        res.splice(i, 1);
        if (__iterRemove(iters, i)) return;
        n--;
      } else i++;
    }
    return res.reduce(
      (acc, x, i) => (acc[iters[i].key] = x.value, acc),
      {}
    );
  };
  if (opts?.reset) {
    let tuple;
    let curr;
    while (curr = await $initial()) {
      tuple = { ...tuple, ...curr };
      yield tuple;
    }
  } else {
    let tuple;
    if (opts?.mergeOnly) {
      tuple = {};
    } else {
      tuple = await $initial();
      if (!tuple) return;
      yield { ...tuple };
    }
    const promises = __inflightIters(iters);
    while (true) {
      const { iter, res } = await Promise.race(promises);
      if (res.done) {
        promises.splice(iter.id, 1);
        if (__iterRemove(iters, iter.id)) return;
      } else {
        tuple[iter.key] = res.value;
        yield { ...tuple };
        __iterNext(promises, iter);
      }
    }
  }
}
export {
  sync
};