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Date: Sat, 29 Sep 2012 20:11:11 +0200
From: Pascal Obry <pascal@obry.net>
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To: Georg Bauhaus <rm.dash-bauhaus@futureapps.de>
Subject: Re: highest bit, statically determined
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Date: 2012-09-29T20:11:11+02:00
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Georg,

> Is there a shorter/better way of having the compiler
> find the highest bit = 1 in a static numeric constant?
> 
> If N is such a constant, e.g. Some_Type'Last where
> Some_Type'Size = 8 and its bound are static, then
> 
>    Highest_Bit_In_Octet : constant :=
>      Natural'Max
>      (8 * Boolean'Pos (N / 2**7 > 0),
>       Natural'Max
>       (7 * Boolean'Pos (N / 2**6 > 0),
>        Natural'Max
>        (6 * Boolean'Pos (N / 2**5 > 0),
>         Natural'Max
>         (5 * Boolean'Pos (N / 2**4 > 0),
>          Natural'Max
>          (4 * Boolean'Pos (N / 2**3 > 0),
>           Natural'Max
>           (3 * Boolean'Pos (N / 2**2 > 0),
>            Natural'Max
>            (2 * Boolean'Pos (N / 2**1 > 0),
>             Natural'Max
>             (1 * Boolean'Pos (N / 2**0 > 0), 0))))))));

if N > 128 then
   return 8;
elsif N > 64
   return 7;
elsif ...

elsif N > 0 then
   return 1;
end if;

Pascal.

-- 

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--| Pascal Obry                           Team-Ada Member
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