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From: Nick Roberts <nickroberts@callnetuk.com>
Subject: Re: Binary files vs Portablity vs Ada
Date: 1999/11/08
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Newsgroups: comp.lang.ada
Date: 1999-11-08T00:00:00+00:00
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John Halleck wrote:
> [how to manipulate file bits portably]

I'd have thought you could go a long way towards a portable
implementation if you could code an algorithm that can work on words of
a parametric size (number of bits), and then use
Ada.Streams.Stream_Element'Size as the parameter.

Then you can use Ada.Streams.Stream_IO to do input and output directly
on (arrays of) stream elements. I think this would bypass any
implementation-specific characteristic of reading or writing the data,
in practice.

To get the bits out of a mod type (portably), you have to resort to
arithmetic (X and 2**N), and hope the compiler optimises correctly.

I think it's a subject that has been mentioned before in comp.lang.ada,
but possibly another suggestion for the next Ada revision might be two
new predefined types:

   type Bit_Integer is mod 2;

   type Bit_Array is array (Natural range <>) of Bit_Integer;

and then a new conversion introduced between Bit_Array (or types derived
from it) and any mod type whose modulus was a power of two (and
static?). This conversion would stipulate that the bit with the lowest
index (in the array) was the bit with the least significance (in the
integer). 

-- 
Nick Roberts
Computer Consultant (UK)
http://www.callnetuk.com/home/nickroberts
http://www.adapower.com/lab/adaos