computational-combinatorics/n-multicombinations

import {list} from '@iterable-iterator/list';
import {nrepeat} from '@iterable-iterator/repeat';

/**
 * Yields all k-multicombinations of {0, 1, ..., n-1}.
 *
 * @param {number} n
 * @param {number} r
 * @returns {IterableIterator<number[]>}
 */
export default function* _multicombinations(n, r) {
    if (n === 0 && r > 0) return;

    const indices = list(nrepeat(0, r));

    yield indices;

    while (true) {
        let i = r - 1;

        // eslint-disable-next-line no-constant-condition
        while (true) {
            if (i < 0) return;

            if (indices[i] !== n - 1) {
                const pivot = ++indices[i];

                for (++i; i < r; ++i) indices[i] = pivot;

                break;
            }

            --i;
        }

        yield indices;
    }
}