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From: Niklas Holsti <niklas.holsti@tidorum.invalid>
Newsgroups: comp.lang.ada
Subject: Re: Representation clauses for base-64 encoding
Date: Thu, 22 Dec 2011 14:24:38 +0200
Organization: Tidorum Ltd
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Date: 2011-12-22T14:24:38+02:00
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On 11-12-22 13:37 , Georg Bauhaus wrote:
> On 22.12.11 10:41, Natasha Kerensikova wrote:
>> Hello,
>>
>> the recent discussion about representation clauses vs explicit shifting
>> made me wonder about what is the Right Way of performing base-64
>> encoding (rfc 1421).
>
>
>> My first thoughts were along the following lines:
>>
>> type Octet is mod 256;
>> -- or Character or Storage_Element or Stream_Element
>> -- or whatever 8-bit type relevant for the appliication
>>
>> for Octet'Size use 8;
>> for Octet'Component_Size use 8;
>
> Here I would stop.
>
> The RFC says that a value from the range 0 .. 63 is associated with
> a character from a specific set of characters, for encoding it:
> 'A' .. 'Z', 'a' .. 'z', '+', '/'.
> And there is a "pad", '='.
> Since the characters shall stand for 0 .. 25, 26 .. 51, 52 .. 63,
> this specifies a range, actually.
> In Ada, the 1:1 translation into a type can be:
>
> type Repertoire is (
> 'A','B','C','D','E','F','G','H','I','J','K','L','M',
> 'N','O','P','Q','R','S','T','U','V','W','X','Y','Z',
> 'a','b','c','d','e','f','g','h','i','j','k','l','m',
> 'n','o','p','q','r','s','t','u','v','w','x','y','z',
> '0','1','2','3','4','5','6','7','8','9',
> '+','/','=');
> subtype Base_64_Character is Repertoire range 'A' .. '/';
> subtype Padding is Repertoire range '=' .. '=';

This defines a nice Ada representation of the six-bit codes. But this 
was not Natasha's question; the question was about the slicing of a 
sequence of bits, composed from 8-bit groups, into a sequence of 6-bit 
groups.

> Let the compiler choose the best representation for Repertoire
> subtypes when encoding.

The point of base-64 encoding is to emit the six-bit groups as ordinary 
Characters (using whatever character encoding is standard, for example 
Latin-1). The compiler's internal representation of Repertoire elements 
is not suitable; the Repertoire literal 'A' should be emitted as the 
Character 'A', not as six zero bits. Natasha did not mention that, of 
course, since the focus was on the mapping between 8-bit and 6-bit 
slices of the bit-string.

> Only if you need some representation in memory or other storage
> that has 'Size /= Character'Size, or 'Size /= Repertoire'Size
> etc, derive new types as needed, and add representation clauses:

That was the point, but the problem is the difficulty of making 
representation clauses portable.

>
> http://www.adacore.com/2008/03/03/gem-27/

I am surprised and disappointed that there is *no* mention of 
portability problems in that "gem". This is marketing hype for GNAT, not 
sound programming advice.

> For streaming encoded text over the wire, a subtype of String should
> serve the job just fine, convert as necessary.

Huh? It is not possible to convert between Repertoire and Character, 
except through Unchecked_Conversion, which does not work as desired in 
this case. The best method would be an array indexed by Repertoire and 
containing Character elements, mapping the Repertoire literal 'A' to the 
Character 'A', etc.

-- 
Niklas Holsti
Tidorum Ltd
niklas holsti tidorum fi
       .      @       .