def collatz_len ( n ):

#*****************************************************************************80
#
## collatz_len(): length of the Collatz sequence for a given starting point.
#
#  Discussion:
#
#    The Collatz sequence is defined recursively as follows:
#
#      Let T be the current entry of the sequence, and U the next:
#
#      If T = 1, the sequence terminates (or U = 1, your choice)
#      Else if T is even, U = T / 2
#      Else (if T is odd, and greater than 1) U = 3 * T + 1.
#
#  Licensing:
#
#    This code is distributed under the MIT license.
#
#  Modified:
#
#    12 June 2022
#
#  Author:
#
#    John Burkardt
#
#  Input:
#
#    integer n, the starting point for the sequence.
#
#  Output:
#
#    integer tlen: the length of the sequence.
#
  if ( n <= 0 ):
    tlen = -1
    return tlen

  tlen = 1
 
  while ( n != 1 ):

    if ( ( n % 2 ) == 0 ):
      n = n // 2
    else:
      n = 3 * n + 1

    tlen = tlen + 1

  return tlen

def collatz_len_test ( ):

  print ( '' )
  print ( 'collatz_len_test ( ):' )
  print ( '  Return the length of a Collatz sequence' )
  n = 7
  tlen = collatz_len ( n )
  print ( '' )
  print ( '  Starting at ', n, ' the sequence has length ', tlen )

  return

if ( __name__ == "__main__" ):
  collatz_len_test ( )