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From: Wilhelm Spickermann <wilhelm.spickermann@t-online.de>
Subject: RE: Newbie question : types , representation
Date: 1999/08/22
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Date: 1999-08-22T00:00:00+00:00
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On 22-Aug-99 Jos De Laender wrote:

> ...
> In base64 decoding , each ASCII character must be translated to 6
> 'bit'.
> 4 characters translate then to 3 bytes. The problem and algorithms
> ...

I think there are two different problems:
  - converting 8 bit characters to 6 Bit somethings
  - packing over the byte borders

That 6-bit-something is:
type Char_64 is mod 2**6;
for Char_64'Size use 6;

The Conversion may be done using a simple Table:
type Conversion_Table_Type is array (Character) of Char_64;
Conversion_Table: constant Conversion_Table_Type := {0,1,2,3,...};

The second problem may be solveable using the following parts
(look into Cohen: Ada as a second language, 2nd Ed.; Part 19.4.5
where he solved the problem of working with Microsofts FAT12 (12 Bit
numbers in a packed array))

type Char_64_Array is array (Sometype) of Char_64;
type Packed_Char_64_Array is new Char_64_Array;
for Packed_Char_64_Array'Component_Size use 6;
pragma Pack (Packed_Char_64_Array);

Now we can use both types - the packed and the unpacked
one - in the normal way. But access to the unpacked one is much more
efficient and conversion is easy:

Packed_One: Packed_Char_64_Array;
Unpacked_One: Char_64_Array;
...
-- now we can fill the unpacked array efficiently
...
-- and now we pack them:
Packed_One := Packed_Char_64_Array(Unpacked_One);

Wilhelm