aureooms/js-array

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

export function permutations(iterable, repeat, out) {
    // Permutations('ABCD', 2) --> AB AC AD BA BC BD CA CB CD DA DB DC
    // permutations(range(3), 3) --> 012 021 102 120 201 210

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

    if (repeat > len) {
        return out;
    }

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

    out.push(pick(pool, indices.slice(0, repeat), []));

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

    for (;;) {
        let i = repeat;

        while (i--) {
            --cycles[i];

            if (cycles[i] === 0) {
                const x = indices[i];

                indices.splice(i, 1);
                indices.push(x);

                cycles[i] = len - i;
            } else {
                const j = cycles[i];

                const tmp = indices[i];
                indices[i] = indices[len - j];
                indices[len - j] = tmp;

                out.push(pick(pool, indices.slice(0, repeat), []));
                break;
            }
        }

        if (i === -1) {
            return out;
        }
    }
}