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From: Stefan Bellon <sbellon@sbellon.de>
Newsgroups: comp.lang.ada
Subject: Base64-Encoding
Date: Mon, 15 Oct 2007 16:12:29 +0200
Organization: Comp.Center (RUS), U of Stuttgart, FRG
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Date: 2007-10-15T16:12:29+02:00
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Hi all,

I've been looking through the previous postings of the group and found
two major threads where this topic has already been discussed. But the
proposed solutions were all different to what I was thinking about. I
have thought about the following:


package body Base64 is

   type Six_Bits is mod 2**6;
   for Six_Bits'Size use 6;

   type Six_Bits_Array is array (Natural range <>) of Six_Bits;
   for Six_Bits_Array'Alignment use 1;  --  To overlay over String type.
   for Six_Bits_Array'Component_Size use Six_Bits'Size;
   pragma Pack (Six_Bits_Array);

   Base64_Chars : constant array (Six_Bits) of Character :=
     "ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789+/";

   function Encode
     (Data : in String)
     return String
   is
      Padded_Length : constant Natural := ((Data'Length + 2) / 3) * 3;
      --  Pad input data to 3-Byte boundary.

      Base64_Length : constant Natural := Padded_Length / 3 * 4;
      --  Number of six-bit tokens necessary (including padding).

      Six_Bits_Length : constant Natural := (Data'Length * 4 + 2) / 3;
      --  Number of six-bit tokens necessary (without padding).

      Padded_Data : String (1 .. Padded_Length) := (others => ASCII.NUL);
      --  Padded input data.

      Base64_Data : Six_Bits_Array (1 .. Base64_Length);
      for Base64_Data'Address use Padded_Data'Address;
      --  Overlay array of six-bit tokens over the padded input data.

      Result : String (1 .. Base64_Length) := (others => '=');
      --  Output buffer, initialized with '=' tokens for unfilled
      --  end-markers.
   begin
      Padded_Data (1 .. Data'Length) := Data;
      --  Initialize data into padded-data (can't be done with aggregate
      --  in elaboration part, sadly).

      --  Do the actual encoding ...
      for I in 1 .. Six_Bits_Length loop
         Result (I) := Base64_Chars (Base64_Data (I));
      end loop;

      return Result;
   end Encode;

end Base64;


However it looks like this solution has a problem with endianness, in a
way that the wrong 6 bits of the Bytes are used in the conversion.

Is there an easy way to fix this (as I think the rest would be pretty
neat) or is this way of trying to do it, doomed to fail anyway?

-- 
Stefan Bellon