From: Stefan Bellon <sbellon@sbellon.de>
Subject: Base64-Encoding
Date: Mon, 15 Oct 2007 16:12:29 +0200
Date: 2007-10-15T16:12:29+02:00 [thread overview]
Message-ID: <20071015161229.3f439230@cube.tz.axivion.com> (raw)
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
next reply other threads:[~2007-10-15 14:12 UTC|newest]
Thread overview: 17+ messages / expand[flat|nested] mbox.gz Atom feed top
2007-10-15 14:12 Stefan Bellon [this message]
2007-10-15 14:46 ` Base64-Encoding Jacob Sparre Andersen
2007-10-15 14:54 ` Base64-Encoding Stefan Bellon
2007-10-15 15:14 ` Base64-Encoding Jacob Sparre Andersen
2007-10-15 15:37 ` Base64-Encoding Robert A Duff
2007-10-15 15:40 ` Base64-Encoding Jean-Pierre Rosen
2007-10-15 16:39 ` Base64-Encoding Stefan Bellon
2007-10-16 10:42 ` Base64-Encoding Stephen Leake
2007-10-17 14:07 ` Base64-Encoding Stefan Bellon
2007-10-17 15:09 ` Base64-Encoding Adam Beneschan
2007-10-17 18:15 ` Base64-Encoding Larry Kilgallen
2007-10-15 18:24 ` Base64-Encoding Adam Beneschan
2007-10-19 2:43 ` Base64-Encoding anon
2007-10-19 4:33 ` Base64-Encoding anon
2007-10-19 7:35 ` Base64-Encoding Jean-Pierre Rosen
2007-10-19 6:59 ` Base64-Encoding Stefan Bellon
2007-10-19 19:40 ` Base64-Encoding anon
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