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From: "jimmaureenrogers@worldnet.att.net" <jimmaureenrogers@worldnet.att.net>
Newsgroups: comp.lang.ada
Subject: Re: help with string splitting
Date: 8 Mar 2006 16:44:39 -0800
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Date: 2006-03-08T16:44:39-08:00
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isaac2004 wrote:
> hello how would I create an Ada program to take an encrypted string
> from a txt file and decrypt it. i have a so so program but it doesnt
> work i cant get the algorithm right.
>
> an example is this
> Mother would be Mtoehr encrypted
>
> so its just like cutting the string in half and rearranging the
> chaacters to fix the decryption
>
> my example uses a txt file which the contents are
>
> encrypted version
> aaabbb
>
> real code
> ababab

Your two examples above do not agree.
Following the encrypted example, Mother would encrypt to
Mteohr. I suspect that may have been a slip of the fingers during
typing, but it does add confusion to your requirements.

>
> here is the program i have so  far
>
> with Ada.Text_Io;
> use Ada.Text_Io;
> procedure Assignment_4 is
>    ---------------------------------------------------------
>    --|This program takes in a railfence encryption and deciphers it
>    --|Author: Isaac Levin
>    -------------------------------------------------------
>    subtype Message is String (1..1024);
>
>    Codedtext : Message;   --input - string of coded text
>    Plaintext : Message;   --output - plain text message
>    Indata    : File_Type;
>
>    Length : Natural := 1; --total size of the encrypted message
>    J      : Integer := 0; --adjusts midpoint for odd and even strings
>    K      : Integer := 2; --counter for the encrypted string
>
> begin -- Assingment_4
>
>    --Open the proper text file and read the code into the string
> CodedText
>
>    Open(
>       File => Indata,
>       Mode => In_File,
>       Name => "test.txt");
>
>    while not End_Of_File(File => Indata) loop
>
>       Get(
>          File => Indata,
>          Item => Codedtext (Length));
>       Length := Length + 1;
>
>    end loop;
>
>    --Reconfigures the length to account for odd or even string lengths
>
>    if Length REM 2 = 0 then
>       J := 0;
>    else
>       J := 1;
>    end if;
>
>    --Stores the first charcter of the coded message into the decoded
> one
>
>    if Length > 1 then
>       Plaintext(1) := Codedtext(1);
>    end if;
>
>    --Stores every character of the encoded string up to the halfway
> point
>    --into every odd position in the decoded one
>
>    for I in 2..(Length - 1) loop
>
>       if I REM 2 = 0 then
>          Plaintext(I + 1) := Codedtext(K);
>          K := K + 1;
>       else
>          null;
>       end if;
>
>    end loop;
>
>    K := 1;
>
>    --Stores every character of the encoded string past the halfway
> point
>    --into every even position in the decoded one
>
>    for I in 2..(Length - 1) loop
>
>       if I REM 2 = 0 then
>          Plaintext(I) := Codedtext((Length/2) + J + K);
>          K := K + 1;
>       else
>          null;
>       end if;
>
>    end loop;
>

In both loops you  are placing characters only into
even positions in Plaintext. What about the odd positions?

It also appears that you can do all the work in a single loop,
cutting the time in half during which you traverse the strings.

First_Half_Index  : Positive := 1;
Second_Half_Index :  Positive := 2;

for Code_Index in Codetext'Length loop
    if Code_Index <= Codetext'Length / 2 then
       Plaintext(First_Half_index) := Codetext(Code_Index);
       First_Half_Index := First_Half_Index + 2;
    else
       Plaintext(Second_Half_Index) := Codetext(Code_Index);
        Second_Half_Index := Second_Half_Index + 2;
   end if;
end loop;


I am not sure how the encryption deals with odd sized strings.
Does the First Half or the Second Half get the extra character?
Adjust the algorithm above accordingly.

Jim Rogers