//  Decrypt.java  Copyright (c) 2005 Kari Laitinen

//  http://www.naturalprogramming.com

//  2005-05-09   File created.
//  2005-11-19   Last modification.

//  The encryption/decryption algorithm used by this program is
//  explained in "A Natural Introduction to Computer Programming
//  with C++"

//  The best way to execute this program is to run it in a
//  command prompt window where command line parameters can
//  be given. It may be difficult, and even impossible, to
//  supply the command line parameters if this program is 
//  executed with a modern development tool such as JCreator
//  or Eclipse.



import java.util.* ;
import java.io.* ;    // Classes for file handling.

class Decrypt
{
   public static void main( String[] command_line_parameters )
   {
      if ( command_line_parameters.length  ==  2 )
      {
         try
         {
            BufferedReader encrypted_file  =  new  BufferedReader(
                      new  FileReader( command_line_parameters[ 0 ] ) ) ;

            FileOutputStream decrypted_file  =
                      new  FileOutputStream( command_line_parameters[ 1 ] ) ;

            int number_of_blocks_decrypted  =  0 ;

            byte[]  decrypted_bytes  =  new  byte[ 30 ] ;

            String  encrypted_bytes_from_file  =  encrypted_file.readLine() ;

            while ( encrypted_bytes_from_file  !=  null )
            {
               int  number_of_decrypted_bytes  =
                              encrypted_bytes_from_file.length() / 2 ;

               for ( int byte_index  =  0 ;
                         byte_index  <  number_of_decrypted_bytes ;
                         byte_index  ++ )
               {
                  decrypted_bytes[ byte_index ]  =  (byte)
                         
                     ( ( encrypted_bytes_from_file.charAt( byte_index * 2 ) 
                              - 0x20 - byte_index  )   + 

                     ( ( encrypted_bytes_from_file.charAt( byte_index * 2 + 1 ) 
                               - 0x20 - byte_index  )  <<  4  ) ) ;

               }

               decrypted_file.write( decrypted_bytes, 0, 
                                     number_of_decrypted_bytes ) ;

               number_of_blocks_decrypted  ++  ;

               encrypted_bytes_from_file  =  encrypted_file.readLine() ;
            }

            encrypted_file.close() ;
            decrypted_file.close() ;

            System.out.print( "\n "  +  number_of_blocks_decrypted
                        +  " blocks were decrypted. \n" ) ;
         }
         catch ( FileNotFoundException caught_file_not_found_exception )
         {
            System.out.print( "\n Cannot open file "
                           +  command_line_parameters[ 0 ] ) ;
         }
         catch ( Exception caught_exception )
         {
            System.out.print( "\n File error. Probably cannot write to "
                           +  command_line_parameters[ 1 ] ) ;
         }
      }
      else
      {
         System.out.print( "\n You have to command this program as: \n"
               +  "\n  java Decrypt ecryptedfile.ext decryptedfile.ext\n") ;
      }
   }
}