computational-combinatorics/n-combinations

import {list} from '@iterable-iterator/list';
import {forwardRangeIterator} from '@iterable-iterator/range';

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

    const indices = list(forwardRangeIterator(0, k, 1));

    yield indices;

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

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

            if (indices[i] !== i + n - k) {
                let pivot = ++indices[i];

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

                break;
            }

            --i;
        }

        yield indices;
    }
}