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From: Stephen Leake <stephen_leake@stephe-leake.org>
Newsgroups: comp.lang.ada
Subject: Re: highest bit, statically determined
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Date: Thu, 04 Oct 2012 04:25:37 -0400
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Date: 2012-10-04T04:25:37-04:00
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Brian Drummond <brian@shapes.demon.co.uk> writes:

> On Mon, 01 Oct 2012 23:30:24 +0200, Georg Bauhaus wrote:
>
>> On 01.10.12 10:52, Anatoly Chernyshev wrote:
>>> (0|1=>0,2|3=>1,4..7=>2, 8..15=>3, 16..31=>4,32..63=>5, 64..127=>6,
>>> 128..255=>7);
>> 
>>     68580000 2.653934000 68580000 2.021029000 68580000 27.702262000
>>     68580000 1.173348000   -- first_1_bit_table
>> 
>> I guess the approaches can be used together for larger N-tets.
>
> Just for completeness,
>
> function first_bit_case(N:Octet) return natural is 
> begin 
>    case N is
>       when 0       => return 0;
>       when 1       => return 1;
>       when 2..3    => return 2;
>       when 4..7    => return 3;
>       when 8..15   => return 4;
>       when 16..31  => return 5;
>       when 32..63  => return 6;
>       when 64..127 => return 7;
>       when others  => return 8;
>    end case;
> end first_bit_case;

Even better; binary search if/then/else instead of a linear case
statement; that cuts the number of compares significantly, especially
for larger word sizes.

-- 
-- Stephe