/* Decode.java allows its users to decode a message stored in a file
 *  using the Caesar cipher.
 *
 *  The encoded message is stored in a file.
 *
 *  Begun by: Charles Hoot, for Hands On Java.
 *  Adapted from code by: Joel C. Adams, for Hands On C++.
 *  Completed by:
 *  Date: 
 *
 *  Specification:
 *     input(input file):   an encoded sequence of characters.
 *     output(output file): the sequence of decoded input characters.
 **********************************************************************/

import java.io.*;

public class Decode
{
   public static void main(String Args[])
   {
      BufferedReader  inStream = null;
      BufferedWriter outStream = null;
	
      // 0. Create theKeyboard
 
      // 1. Display introductory message
      System.out.println( "\nThis program uses the Caesar cipher to decode the contents of a"
		+ "\nfile and writes the decoded characters to another file.");
  
      // 2. Prompt for and read name of the input file.
	

      // 3. Open a BufferedReader named inStream for input from inFile
      // 4. If inStream failed to open, display an error message and quit

      // 5. Prompt for and read name of the input file.
      System.out.print( "\nEnter the name of the output file: ");
      String outFile=""; 

      // 6. Open an ofstream named outStream for output to outFile
      // 7. If outStream failed to open, display an error message and quit

      int  inValue = 0;
      char outChar=' ';
  
      // 8. Loop
      while(true)
      {
         // a. read a character from the input file via inStream into inValue
		
         // b. if end-of-file was reached, terminate repetition

         // c. decode the character using the Caesar cipher
         outChar = caesarDecode((char) inValue, 3);
      
         // d. write the decoded character to the output file via outStream

      }
  
      // 9a. close the connection to the input file

      // 9b. close the connection to the output file
  	
      // 10. display a 'successful completion' message
      System.out.println("\nProcessing complete.\n Decoded message is in "
			+ outFile);
   }

   /********************************************************************
    * caesarDecode implements the Caesar cipher encoding scheme.       *
    *                                                                  * 
    * Receive: ch, a character,                                        *
    *          key, an integer.                                        *
    * Return:  The character that is key positions before ch,          *
    *          with "wrap-around" to the end of the sequence.          *
    ********************************************************************/

   public static char caesarDecode(char ch, int key)
   {
      final int FIRST_UPPER = 65,
                FIRST_LOWER = 97,
                NUM_CHARS = 26;
	  
      if (key <= 0 || key >= 26)
      {
         System.err.println("\n*** CaesarDecode: key must be between 1 and 25!\n");
         System.exit(1);
      }

      if (Character.isUpperCase(ch))
         return (char) ((ch - FIRST_UPPER + NUM_CHARS - key) % NUM_CHARS + FIRST_UPPER);
      else if (Character.isLowerCase(ch))
         return (char) ((ch - FIRST_LOWER + NUM_CHARS - key) % NUM_CHARS + FIRST_LOWER);
      else
         return ch;
   }

} //end class