# reads in 2 integers, prints their greatest common divisor
# ECS 10, May 29, 2009
# Matt Bishop

# read in a number
# parameters: none
# returns: 2 values
#     1st: the number, if read successfully
#          0, on EOF
#          -1, if not EOF and not successful
#     2nd: True, if list read successfully; False, if not
def getnum():
    # read the input
    # catch any problems and bail out at once
    try:
        # read in ... something (a number, supposedly)
        n = input("Enter the number of disks: ")
        # this causes a TypeError if not a number
        n += 0
    except EOFError:
        # didn't enter anything
        return 0, False
    except (NameError, TypeError, SyntaxError):
        # entered something bogus
        print "Please enter a positive integer"
        return -1, False
    # need at least one disk, and it better be an integer
    if n < 1 or n != int(n):
        print "Please enter a positive integer"
        return -1, False
    # return the value read
    return n, True

# solve the Tower of Hanoi
# parameters: n, number of disks to move from fromtower to totower
#             fromtower, the name of the tower with the disks
#             totower, the name of the tower where the disks are to
#                 be moved to
#             temptower, the name of the tower to be used to hold disks
#                 temporarily
# returns: nothing
def hanoi(n, fromtower, totower, temptower):
    # base case: only one disk, so move it directly to totower
    if n == 1:
        print "Move disk from tower", fromtower, "to tower", totower
    # recursion here
    else:
        # move the top n-1 disks to the temporary tower
        hanoi(n-1, fromtower, temptower, totower)
        # move the leftover disk to the destination
        # note: for efficiency, we could just put a copy of the
        # base case here
        hanoi(1, fromtower, totower, temptower)
        # move the n-1 disks from the temporary tower to the destination
        hanoi(n-1, temptower, totower, fromtower)

# this puts it all together
def main():
    # print out a description of the program
    print """
The Tower of Hanoi puzzle solver will move disks from tower A to tower C.
Tower B is in the middle and is used as a temporary tower.
"""
    # get a positive integer
    n, okay = getnum()
    # got it!
    if okay:
        # solve the problem
        hanoi(n, "A", "C", "B")

main()