PremutationsN

premutationsN.js

// [a] -> [[a]] -> [[a]]
var iter = function(arr, parents) {
  if (R.isEmpty(arr)) {
    return parents;
  }
  
  return R.addIndex(R.chain)(function(a, index) {
    return iter(
      R.remove(index, 1, arr), 
      R.map(R.append(a), parents));    
  }, arr);
};

// [a] -> [[a]]
var premutations = function(arr) {
  return iter(arr, [[]]);
};

// Number -> [[Number]]
var premutationsN = function(n) {
  return premutations(R.range(0, n));
};

premutationsN(3);
// [[0, 1, 2], [0, 2, 1], [1, 0, 2], [1, 2, 0], [2, 0, 1], [2, 1, 0]]