next_permutation = (x) ->
  l = x.length - 1;
  while (l >= 1 and x[l] < x[l - 1])
    l--;

  #console.log(l);

  if (l != 0)
    m = x.length - 1;
    while(m > l - 1 and x[m] < x[l - 1])
      m--;
    [x[m], x[l - 1]] = [x[l - 1], x[m]];
    #console.log(m);
  
  r = x.length - 1;
  while(l < r)
    [x[l], x[r]] = [x[r], x[l]];
    l++;
    r--;
  return x;