javascript permutation generator with permutation length parameter

Question

I\'ve seen a few generators out there but they all make a squared matrix. For example, you give it a list of three items and it\'ll assume the output of the length is also t

Answer 1

You can imagine the length as representing the number of slots. Each slot has N possibilities, given that N is the number of elements in your initial list. So given three values [1,2,3], you will have a total of 3 x 3 x 3 = 27 permutations.

Here's my attempt. Comments included!

var list = [1,2,3];

var getPermutations = function(list, maxLen) {
    // Copy initial values as arrays
    var perm = list.map(function(val) {
        return [val];
    });
    // Our permutation generator
    var generate = function(perm, maxLen, currLen) {
        // Reached desired length
        if (currLen === maxLen) {
            return perm;
        }
        // For each existing permutation
        for (var i = 0, len = perm.length; i < len; i++) {
            var currPerm = perm.shift();
            // Create new permutation
            for (var k = 0; k < list.length; k++) {
                perm.push(currPerm.concat(list[k]));
            }
        }
        // Recurse
        return generate(perm, maxLen, currLen + 1);
    };
    // Start with size 1 because of initial values
    return generate(perm, maxLen, 1);
};

var res = getPermutations(list, 3);
console.log(res);
console.log(res.length); // 27

fiddle