locutus/esm/ruby/Array/permutation.js

Locutus other languages' standard libraries to JavaScript for fun and educational purposes

export function permutation(arr, r) {
    // parity verified: Ruby 3.3
    //      discuss at: https://locutus.io/ruby/Array/permutation/
    //     original by: Kevin van Zonneveld (https://kvz.io)
    //          note 1: Returns all r-length permutations without repetition (default r = array length).
    //       example 1: permutation([1, 2, 3], 2)
    //       returns 1: [[1, 2], [1, 3], [2, 1], [2, 3], [3, 1], [3, 2]]
    //       example 2: permutation(['a', 'b'])
    //       returns 2: [['a', 'b'], ['b', 'a']]
    //       example 3: permutation([1, 2, 3], 0)
    //       returns 3: [[]]
    if (!Array.isArray(arr)) {
        return [];
    }
    const target = r === undefined ? arr.length : Math.trunc(Number(r));
    if (!Number.isFinite(target) || target < 0 || target > arr.length) {
        return [];
    }
    if (target === 0) {
        return [[]];
    }
    const out = [];
    const used = new Array(arr.length).fill(false);
    const current = [];
    const walk = () => {
        if (current.length === target) {
            out.push([...current]);
            return;
        }
        for (let i = 0; i < arr.length; i += 1) {
            if (used[i]) {
                continue;
            }
            used[i] = true;
            current.push(arr[i]);
            walk();
            current.pop();
            used[i] = false;
        }
    };
    walk();
    return out;
}