#! /usr/bin/env python3
#
def collatz_test ( ):

#*****************************************************************************80
#
## collatz_test()
#
  import json
#
#  Create the dict.
#
  collatz_dict = {}
#
#  Store data for 1 <= n < 21.
#
  n_max = 21
  collatz_dict_initialize ( collatz_dict, n_max )

  print ( '' )
  print ( '  After initialization with n_max = ', n_max, 'the Collatz dict' )
  print ( '  has "length" (number of entries) = ', len ( collatz_dict ) )
#
#  Now add data for n = 27.
#
  n = 27
  collatz_add_to_dict ( collatz_dict, n )

  print ( '' )
  print ( '  After adding data for n = ', n, 'the Collatz dict' )
  print ( '  has "length" (number of entries) = ', len ( collatz_dict ) )
#
#  Now write dict to JSON file.
#
  filename = 'collatz_dict.json'
  output = open ( filename, 'w' )
  json.dump ( collatz_dict, output )
  output.close ( )

  print ( '' )
  print ( '  collatz_dict was written to "' + filename + '"' )

  return

def collatz_dict_initialize ( collatz_dict, n_max ):

#*****************************************************************************80
#
## collatz_dict_initialize()
#
  for n in range ( 1, n_max ):
    collatz_add_to_dict ( collatz_dict, n )

  return

def collatz_add_to_dict ( collatz_dict, n ):

#*****************************************************************************80
#
## collatz_add_to_dict()
#
  while ( not ( n in collatz_dict ) ):
    n2 = collatz ( n )
    collatz_dict[n] = n2
    n = n2

  return

def collatz ( n ):

#*****************************************************************************80
#
## collatz() applies one step of the Collatz iteration to an integer.
#
#  Licensing:
#
#    This code is distributed under the GNU LGPL license. 
#
#  Modified:
#
#    11 June 2022
#
#  Author:
#
#    John Burkardt
#
#  Input:
#
#    integer n: the starting value.
#
#  Output:
#
#    integer n2: the transformed value.
#
  if ( not isinstance ( n, int ) ):
    raise Exception ( 'collatz: type(n) = ', type ( n ), \
      ', but n must be an integer.' )
  elif ( n < 1 ):
    raise Exception ( 'collatz: n =', n, ', but 1 <= n required.' )
  elif ( n == 1 ):
    n2 = 1
  elif ( ( n % 2 ) == 0 ):
    n2 = n // 2
  else:
    n2 = 3 * n + 1

  return n2

if ( __name__ == '__main__' ):
  collatz_test ( )