# Nim implementation of the (very fast) Go example.
# http://rosettacode.org/wiki/Permutations#Go
# implementing a recursive https://en.wikipedia.org/wiki/Steinhaus–Johnson–Trotter_algorithm

import algorithm

proc perm(s: openArray[int]; emit: proc(emit: openArray[int])) =
  var s = @s
  if s.len == 0:
    emit(s)
    return

  proc rc(np: int) =
    if np == 1:
      emit(s)
      return
    var
      np1 = np - 1
      pp = s.len - np1

    rc(np1) # Recurse prior swaps.

    for i in countDown(pp, 1):
      swap s[i], s[i-1]
      rc(np1) # Recurse swap.

    s.rotateLeft(0..pp, 1)

  rc(s.len)

var se = @[0, 1, 2, 3] #, 4, 5, 6, 7, 8, 9, 10]

perm(se, proc(s: openArray[int])= echo s)