#-------------------------------------------------------------------------------
#  Recursion.py
#  functions that call themselves
#-------------------------------------------------------------------------------

# factorial()
def factorial(n):
   """Returns the factorial of n if n>=0, returns None otherwise."""
   if n==0:
      return 1
   elif n>0:
      return n*factorial(n-1)
   # end if-elif
# end factorial()

# Fibonacci()
def Fibonacci(n):
   """Returns the nth Fibonacci number if n>=0, returns None otherwise."""
   if n==0:
      return 0
   elif n==1:
      return 1
   elif n>=2:
      return Fibonacci(n-1)+Fibonacci(n-2)
   # end if-elif
# end Fibonacci()

# Ackermann()
def Ackermann(m, n):
   """Returns values of the Ackermann function if m>=0, n>=0, None otherwise."""
   if m==0 and n>=0:
      return n+1
   elif m>0 and n==0:
      return Ackermann(m-1, 1)
   elif m>0 and n>0:
      return Ackermann(m-1,Ackermann(m, n-1))
   # end if-elif
# end Ackermann()

# T()
def T(m, n):
   if m==0 and n>=0:
      return n+1
   elif m>0 and n==0:
      return T(m-1, 1)
   elif m>0 and n>0:
      return T(m-1, n) + T(m, n-1)
   # end if-elif
# end Ackermann()


#-- main program --------------------------------------------------------------

if __name__ == '__main__':  # eliminate global variables

   print('100! =', factorial(100))
   print('Fibonacci(25) =', Fibonacci(25))
   print('Ackermann(3, 6) =', Ackermann(3, 6))
   print('T(10, 12) =', T(10, 12))

# end if

#-- end main program ----------------------------------------------------------