Re: Base64-Encoding

[anon@anon.org (anon)]

that should say 

  type Six_Bits is new Integer mod 2**6 ; -- or use range 0 .. 63 ; 


In <eRURi.236438$ax1.167125@bgtnsc05-news.ops.worldnet.att.net>, anon@anon.org (anon) writes:
>with Interfaces ;
>use  Interfaces ;
>with System ;
>use  System ;
>with Ada.Unchecked_Conversion ;
>
>package b64 is
>
>  --
>  -- Uses the system default on endian
>  --
>
>  type Six_Bits is new Integer range mod 2**6 ; -- or use 0 .. 63 ; 
>
>
>  --
>  -- base64_descriptor is a record that insures the endian is set 
>  -- by the program ( in this case "little-endian format" ) in 
>  -- architectures that have switchable endianness.
>  -- Such as ARM, PowerPC (but not the PPC970/G5), DEC Alpha, SPARC V9, 
>  -- MIPS, PA-RISC and IA64 
>  --
>  type base64_descriptor is record
>                              Unused : Unsigned_8 range 0..3 ;
>                              Bit_5  : Unsigned_8 range 0..1 ;
>                              Bit_4  : Unsigned_8 range 0..1 ;
>                              Bit_3  : Unsigned_8 range 0..1 ;
>                              Bit_2  : Unsigned_8 range 0..1 ;
>                              Bit_1  : Unsigned_8 range 0..1 ;
>                              Bit_0  : Unsigned_8 range 0..1 ;
>                            end record ;
>  --
>  -- set bit order
>  --
>  for base64_descriptor use record
>                              Unused at 0 range 6..7 ;
>                              Bit_5  at 0 range 5..5 ;
>                              Bit_4  at 0 range 4..4 ;
>                              Bit_3  at 0 range 3..3 ;
>                              Bit_2  at 0 range 2..2 ;
>                              Bit_1  at 0 range 1..1 ;
>                              Bit_0  at 0 range 0..0 ;
>                            end record ;
>
>  --
>  -- or you could use the following statement which 
>  -- forces endian to little-endian format
>  --
>  for base64_descriptor'Bit_Order use Low_Order_First ;
>
>  --
>  -- insure 1 byte usage for base64_descriptor ;
>  --
>  pragma pack ( base64_descriptor ) ; 
>
>
>
>  function To_Base64 is new Ada.Unchecked_Conversion 
>                             ( Source => Unsigned_8, 
>                               Target => base64_descriptor ) ;
>
>  function From_Base64 is new Ada.Unchecked_Conversion 
>                             ( Target => Unsigned_8, 
>                               Source => base64_descriptor ) ;
>
>
>end ;
>
>In 2001 Tom Moran created a Ada BASE64 package which is archived at 
>
>http://www.adapower.com/index.php?Command=Class&ClassID=Algorithms&CID=257
>
>have a look at it. I a quick look it kind of suggest that the endian for 
>Six_Bits is not important.
>
>
>In <20071015161229.3f439230@cube.tz.axivion.com>, Stefan Bellon <sbellon@sbellon.de> writes:
>>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
>