#! /usr/bin/env python
#
def isbn_check_digit_calculate ( s ):

#*****************************************************************************80
#
## ISBN_CHECK_DIGIT_CALCULATE determines the check digit for an ISBN.
#
#  Discussion:
#
#    ISBN stands for International Standard Book Number.  A unique ISBN
#    is assigned to each new book.  The ISBN includes 10 digits.  There is
#    an initial digit, then a dash, then a set of digits which are a
#    code for the publisher, another digit, and then the check digit:
#
#      initial-publisher-book-check
#
#    The number of digits used for the publisher and book codes can vary,
#    but the check digit is always one digit, and the total number of
#    digits is always 10.
#
#    The check digit is interesting, because it is a way of trying to
#    make sure that an ISBN has not been incorrectly copied.  Specifically,
#    if the ISBN is correct, then its ten digits will satisfy
#
#       10 * A + 9 * B + 8 * C + 7 * D + 6 * E
#      + 5 * F * 4 * G * 3 * H + 2 * I +     J  = 0 mod 11.
#
#    Here, we've taken 'A' to represent the first digit and 'J' the
#    last (which is the check digit).  In order for the code to work,
#    the value of J must be allowed to be anything from 0 to 10.  In
#    cases where J works out to be 10, the special digit 'X' is used.
#    An 'X' digit can only occur in this last check-digit position
#    on an ISBN.
#
#  Example:
#
#    S  => 0-8493-9640-?
#    D <=  9
#
#    meaning the ISBN is 0-8493-9640-9
#
#  Licensing:
#
#    This code is distributed under the GNU LGPL license. 
#
#  Modified:
#
#    17 September 2015
#
#  Author:
#
#    John Burkardt
#
#  Parameters:
#
#    Input, string S, a string.  Dashes and spaces and other
#    nonnumeric data is ignored, but at least 9 digits are expected, and only
#    the first 9 digits will be examined.
#
#    Output, character ISBN_CHECK_DIGIT_CALCULATE, the ISBN 
#    check digit that should be appended to form the full 10 digit ISBN.
#
  from i4_to_isbn_digit import i4_to_isbn_digit
  from s_to_digits import s_to_digits
#
#  Extract first 9 digits.
#
  n = 9
  dvec = s_to_digits ( s, n )
#
#  Compute the check digit.
#
  d = 0
  for i in range ( 0, 9 ):
    d = d + ( 10 - i ) * dvec[i]

  d = ( ( 11 - ( d % 11 ) ) % 11 )
#
#  Convert digits 0 through 9, 10 to characters '0' through '9', 'X'.
#
  c = i4_to_isbn_digit ( d )

  return c

def isbn_check_digit_calculate_test ( ):

#*****************************************************************************80
#
## ISBN_CHECK_DIGIT_CALCULATE_TEST tests ISBN_CHECK_DIGIT_CALCULATE.
#
#  Licensing:
#
#    This code is distributed under the GNU LGPL license.
#
#  Modified:
#
#    17 September 2015
#
#  Author:
#
#    John Burkardt
#
  import platform

  print ( '' )
  print ( 'ISBN_CHECK_DIGIT_CALCULATE_TEST' )
  print ( '  Python version: %s' % ( platform.python_version ( ) ) )
  print ( '  ISBN_CHECK_DIGIT_CALCULATE calculates the 10-th digit' )
  print ( '  (the check digit) of a 10-digit ISBN.' )
  print ( '' )
#
#  Supply the full code, with dashes.
#
  s1 = '0-306-40615-2'
  c1 = '2'
  c2 = isbn_check_digit_calculate ( s1 )
  print ( '  Check digit of "%s" is "%c", expecting "%c"' % ( s1, c2, c1 ) )
#
#  Supply a partial code, with spaces.
#
  s1 = '0 8493 9640'
  c1 = '9'
  c2 = isbn_check_digit_calculate ( s1 )
  print ( '  Check digit of "%s" is "%c", expecting "%c"' % ( s1, c2, c1 ) )
#
#  Supply a partial code, no spaces.
#
  s1 = '158488059'
  c1 = '7'
  c2 = isbn_check_digit_calculate ( s1 )
  print ( '  Check digit of "%s" is "%c", expecting "%c"' % ( s1, c2, c1 ) )
#
#  Supply a partial code, no spaces.
#
  s1 = '246897531'
  c1 = '6'
  c2 = isbn_check_digit_calculate ( s1 )
  print ( '  Check digit of "%s" is "%c", expecting "%c"' % ( s1, c2, c1 ) )
#
#  Supply a partial code, no spaces.
#
  s1 = '135798642'
  c1 = '4'
  c2 = isbn_check_digit_calculate ( s1 )
  print ( '  Check digit of "%s" is "%c", expecting "%c"' % ( s1, c2, c1 ) )
#
#  Terminate.
#
  print ( '' )
  print ( 'ISBN_CHECK_DIGIT_CALCULATE_TEST:' )
  print ( '  Normal end of execution.' )
  return

if ( __name__ == '__main__' ):
  from timestamp import timestamp
  timestamp ( )
  isbn_check_digit_calculate_test ( )
  timestamp ( )