"""
 hanoi.py

  Towers of Hanoi in python.

  Based on perl program by DigitalKitty (Katie)
  on perlmonks.org, node 400359 on 10/20/2004

 Jim Mahoney, Nov 2008, GPL


          A             B             C      

          |             |             |
         ---   1        |             |
        -----  2        |             |
       ------- 3        |             |
    ============================================

 For this stack with 3 disks, the moves would be

   disk 1 moves A to B
   disk 2 moves A to C
   disk 1 moves B to C
   disk 3 moves A to B
   disk 1 moves C to A
   disk 2 moves C to B
   disk 1 moves A to B
"""

def move_disks(n, start='A', end='B', extra='C'):
  """Return string of moves moving n disks start one column to another.
     >>> move_disks(1)
     ' disk 1 moves A to B \\n'
     >>> move_disks(2)
     ' disk 1 moves A to C \\n disk 2 moves A to B \\n disk 1 moves C to B \\n'
  """
  format = " disk %i moves %s to %s \n"
  if n == 1:
    return  format % (1, start, end)               # At top? Just move it.
  else:
    # Here's the heart of it.  The idea is
    #  1) move the top n-1 disks to the extra column, then
    #  2) move the biggest disk n from 'start' to 'end', then
    #  3) move the top n-1 disks again, this time from 'extra' to 'end'.
    return move_disks(n-1, start, extra, end) + \
           format % (n, start, end) + \
           move_disks(n-1, extra, end, start)             

def hanoi():
  n_disks = input("Number of disks? ")
  print "The moves are: "
  print move_disks(n_disks),

if __name__=="__main__":
  from doctest import testmod
  testmod()
  hanoi()