CoffeeScript Array permutation generator

ArrayPermutation.coffee

# internal: generate permutation recursively
generatePermutation = (perm, pre, post, n) ->
  if n > 0
    for i in [0...post.length]
      rest = post.slice 0
      elem = rest.splice i, 1
      generatePermutation perm, pre.concat(elem), rest, n - 1
  else
    perm.push pre
  return

###
extend Array.prototype
e.g. [0, 1, 2].permutation(2)
=> [[0, 1], [0, 2], [1, 0], [1, 2], [2, 0], [2, 1]]
###
Array::permutation = (n = @.length) ->
  perm = []
  generatePermutation perm, [], @, n
  perm