aureooms/js-array

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

export function combinations(iterable, repeat, out) {
    // Combinations('ABCD', 2) --> AB AC AD BC BD CD
    // combinations(range(4), 3) --> 012 013 023 123

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

    if (repeat > len) {
        return out;
    }

    const indices = range(0, repeat, 1, []);

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

    if (repeat === 0 || len === 0) {
        return out;
    }

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

        if (i < 0) {
            return out;
        }

        ++indices[i];

        for (let j = i + 1; j < repeat; ++j) {
            indices[j] = indices[j - 1] + 1;
        }

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