-- The program accepts as input the encoded message  and produces
-- the decoded message as output.
-- It is known that each message begins with the word 'Agent'.


  with text_io; use text_io;
  procedure agent is

  Input_File    : File_Type; -- The input File with encoded message.
  Result_File   : File_Type; -- The output File with decoded message.
  c             : CHARACTER; -- The character being read.
  shift         : INTEGER;   -- The value of shift.
  bigapos       : constant INTEGER := CHARACTER'POS('A');-- POS value of A
  smlapos       : constant INTEGER := CHARACTER'POS('a');-- POS value of a
  bigzpos       : constant INTEGER := CHARACTER'POS('Z');-- POS value of Z
  smlzpos       : constant INTEGER := CHARACTER'POS('z');-- POS value of z


  -- This function accepts the character and returns the decoded character
  function DECODE( N : in INTEGER; C : in CHARACTER) return CHARACTER is

  -- Preconditions : N contains the shift value used to encode and
  --                 C is the character to be decoded.

  -- Postconditions: The decoded character for C is returned.

  CH            : CHARACTER := C; -- A character variable to be returned.

  begin

    -- Check if C is a vowel and do the rotation specific to vowels.
    case CH is

      when 'A' => CH := 'U';

      when 'E' => CH := 'A';

      when 'I' => CH := 'E';

      when 'O' => CH := 'I';

      when 'U' => CH := 'O';

      when 'a' => CH := 'e';

      when 'e' => CH := 'i';

      when 'i' => CH := 'o';

      when 'o' => CH := 'u';

      when 'u' => CH := 'a';

      when others => CH := CH;
    end case;

    -- Now do the rotation for upper case letter.
    if ( CH in 'A'..'Z' ) then
      if ( (CHARACTER'POS(CH) - N) < bigapos ) then
        CH := CHARACTER'VAL(bigzpos- ( bigapos- (CHARACTER'POS(CH) - N) - 1 ));
      else
        CH := CHARACTER'Val( CHARACTER'POS(CH) - N );
      end if;
    end if;

    -- Now do the rotation for lower case letter.
    if ( CH in 'a'..'z' ) then
      if ( (CHARACTER'POS(CH) - N) < smlapos ) then
        CH := CHARACTER'VAL(smlzpos - ( smlapos- (CHARACTER'POS(CH) - N) - 1 ));
      else
        CH := CHARACTER'Val( CHARACTER'POS(CH) - N );
      end if;
    end if;

    return CH;
  end DECODE;


  begin

    -- setup files
    -- OPEN (Input_File,In_File,"encoded1.txt"); -- Open File
    -- SET_INPUT(Input_File); -- set as default input file
    -- OPEN (Result_File,Out_File,"decoded1.txt"); -- Open File
    -- SET_OUTPUT(Result_File); -- set as default input file

    -- The first character is treated  differently in order to derive
    -- the value of N. We know that the first word is always Agent.
    get(c);

    -- Check if c is a vowel and rotate back accordingly.
    case c is

      when 'A' => c := 'U';

      when 'E' => c := 'A';

      when 'I' => c := 'E';

      when 'O' => c := 'I';

      when 'U' => c := 'O';

      when others => c := c;
    end case;

    -- Now find the difference between this character and A.
    -- This is the amount by which other letters will be shifted.
    shift := CHARACTER'POS(c) - CHARACTER'POS('A');
    put(DECODE(shift, c));

    -- read and decode the file till it is over.
    while( not END_OF_FILE )
    loop
      if ( END_OF_LINE ) then
        new_line;
        skip_line;
      else
        get(c);
        put(DECODE(shift, c));
      end if;
    end loop;
  end agent;