aureooms/js-array

import {pick} from '../utils/pick.js';

import {repeat} from './repeat.js';

export function combinationswithrepetition(iterable, r, out) {
    // Combinationswithrepetition('ABC', 2) --> AA AB AC BB BC CC

    const pool = iterable;
    const len = pool.length;

    if (len === 0 && r > 0) {
        return out;
    }

    const indices = repeat(0, r, []);

    out.push(pick(pool, indices, []));

    for (;;) {
        let i = r - 1;
        let next;
        for (; i >= 0; --i) {
            if (indices[i] !== len - 1) {
                next = indices[i] + 1;
                break;
            }
        }

        if (i < 0) {
            return out;
        }

        for (; i < r; ++i) {
            indices[i] = next;
        }

        out.push(pick(pool, indices, []));
    }
}