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Newsgroups: comp.lang.ada
Subject: Re: shifting bits
References: <ba2f9c57.0408281738.61a48643@posting.google.com>
From: Ludovic Brenta <ludovic.brenta@insalien.org>
Date: Sun, 29 Aug 2004 13:29:44 +0200
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Date: 2004-08-29T13:31:03+02:00
List-Id: <comp.lang.ada>

Fabio de Francesco writes:
> Hello.
>
> I had to pack/unpack some variables into/from one. In order to get the
> result I chose to shift the variables' bits using multiplications and
> divisions by powers of two.

Others have responded to the modular type part of your question, but I
suspect that modular types and shifting is not the best way to solve
your problem.  In Ada, you can express your problem much more cleanly,
using representation clauses:

procedure Packed_Date is
   type Day_Type is range 1 .. 31;
   for Day_Type'Size use 5;

   type Month_Type is range 1 .. 12;
   for Month_Type'Size use 4;

   type Year_Type is range 1900 .. 2099;
   for Year_Type'Size use 11;

   type Date_Type is record
      Day : Day_Type;
      Month : Month_Type;
      Year : Year_Type;
   end record;

   for Date_Type use record
      Day at 0 range 0 .. 4;
      Month at 0 range 5 .. 8;
      Year at 0 range 9 .. 19;
   end record;

   for Date_Type'Size use 20;

   Year, Month, Day : Integer;
   Date : Date_Type;
begin
   Get (Year);
   Date.Year := Year_Type (Year);
   Get (Month);
   Date.Month := Month_Type (Month);
   Get (Day);
   Date.Day := Day_Type (Day);
end Packed_Date;


The philosophy in Ada is: say what you want, and let the compiler do
the dirty work for you.

-- 
Ludovic Brenta.