iterable-iterator/zip

import {iter} from '@iterable-iterator/iter';
import {list} from '@iterable-iterator/list';
import {map} from '@iterable-iterator/map';
import {_next} from '@iterable-iterator/next';
import {repeat} from '@iterable-iterator/repeat';

import enumerate from './enumerate.js';

/**
 * Same as _zip, but continues to yield zipped tuples until the last iterable is
 * exhausted.
 *
 * @example
 * // returns [['A','x'],['B','y'],['C','-'],['D','-']]
 * list( _ziplongest( '-' , [ 'ABCD', 'xy' ] ) ) ;
 *
 * @param {any} fillvalue - The value to yield for iterators that are exhausted.
 * @param {Iterable[]} iterables - The iterables to zip.
 * @returns {IterableIterator}
 *
 */
export default function* _ziplongest(fillvalue, iterables) {
    let counter = iterables.length;

    if (counter === 0) {
        return;
    }

    const filler = repeat(fillvalue);

    const iterators = list(map(iter, iterables));

    while (true) {
        const buffer = [];

        for (const [i, event] of enumerate(map(_next, iterators))) {
            if (event.done) {
                --counter;
                if (counter === 0) {
                    return;
                }

                iterators[i] = filler;
                buffer.push(fillvalue);
            } else {
                buffer.push(event.value);
            }
        }

        yield buffer;
    }
}