subroutine atbash ( plain, crypt )

!*****************************************************************************80
!
!! atbash () encrypts or decrypts a string using the ATBASH code.
!
!  Discussion:
!
!    Only the alphabetic characters will be encrypted.
!
!  Licensing:
!
!    This code is distributed under the MIT license.
!
!  Modified:
!
!    29 August 2021
!
!  Author:
!
!    John Burkardt
!
!  Input:
!
!    character ( len = * ) PLAIN, the string to be encrypted.
!
!  Output:
!
!    character ( len = * ) CRYPT, the encrypted version of the string.
!
  implicit none

  integer big_a
  integer big_z
  character ( len = * ) crypt
  integer i
  integer j
  integer n
  character ( len = * ) plain
  integer plain_i
  integer small_a
  integer small_z
!
!  How long is the plain text?
!
  n = len ( plain )
!
!  Initialize the encrypted string to the plain text.  This way,
!  nonalphabetic characters are passed through.
!
  crypt = plain
!
!  Determine the numeric equivalents of the endpoints of the A-Z and a-z intervals.
!
  small_a = ichar ( 'a' )
  small_z = ichar ( 'z' )
  big_a = ichar ( 'A' )
  big_z = ichar ( 'Z' )
!
!  Convert plain text characters to unsigned integers,
!  and reverse the index.
!
  do i = 1, n
    plain_i = ichar ( plain(i:i) )
    if ( small_a <= plain_i .and. plain_i <= small_z ) then
      j = small_a + small_z - plain_i
      crypt(i:i) = char ( j );
    else if ( big_a <= plain_i .and. plain_i <= big_z ) then
      j = big_a + big_z - plain_i
      crypt(i:i) = char ( j )
    end if
  end do
      
  return
end