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From: Ludovic Brenta <ludovic@ludovic-brenta.org>
Newsgroups: comp.lang.ada
Subject: Re: Hexadecimal and stream element arrays
Date: Mon, 19 Apr 2010 22:50:35 +0200
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tonyg writes on comp.lang.ada:
> Changed the hex function to
> function To_Hex (E : in Ada.Streams.Stream_Element) return String
> is
>    -- Warning: not compiled and not tested...
>    X : constant array (0 .. 15) of Character :=

(1) I should have written:

    X : constant array (Ada.Streams.Stream_Element range 0 .. 15) of Character :=

>      ('0', '1', '2', '3', '4', '5', '6', '7', '8', '9', 'A', 'B',
>       'C', 'D', 'E', 'F');
>    Result : String (1 .. Ada.Streams.Stream_Element'Size / 4); -- 1 hex digits = 4 bits
>    Working_Copy : Ada.Streams.Stream_Element := E;
>    use type Ada.Streams.Stream_Element;
>    First_Character : Natural := 0;
>    Base : constant  := 16;
> begin
>    for K in reverse Result'First .. Result'Length loop

(2) and this should be:

     for K in reverse Result'Range loop

>       Result (K) := X (integer(Working_Copy) mod integer (Base) );

(3) and thanks to (1), this can come back to the simpler:

       Result (K) := X (Working_Copy mod Base);

>       Working_Copy := Working_Copy / Base;
>       if Working_Copy = 0 then
>          First_Character := K;
>          exit;
>       end if;
>    end loop;
>    return Result (First_Character .. Result'Last);
> end To_Hex;
>
> It still seems to be dropping a few zeros though and I'm stumped where
> its going wrong

I tested it with 42 and 0 and correctly got 2A and 0, so I don't know
what you mean by that.

Maybe the fact that the result has a variable width is a problem?  If
so, here is a fixed-width variant which is actually a bit simpler:

function To_Hex (E : in Ada.Streams.Stream_Element) return String is
   X : constant array (Ada.Streams.Stream_Element range 0 .. 15)
     of Character :=
     ('0', '1', '2', '3', '4', '5', '6', '7', '8', '9', 'A', 'B', 'C',
      'D', 'E', 'F');
   Result : String
     (1 .. Ada.Streams.Stream_Element'Size / 4) -- 1 hex digits = 4 bits
     := (others => '0');
   Working_Copy : Ada.Streams.Stream_Element := E;
   use type Ada.Streams.Stream_Element;
   Base : constant := 16;
begin
   for K in reverse Result'Range loop
      Result (K) := X (Working_Copy mod Base);
      Working_Copy := Working_Copy / Base;
   end loop;
   return Result;
end To_Hex;

and this yields 2A and 00 for my "test vector".

-- 
Ludovic Brenta.