Re: Bitmanipulation in Ada

[Ludovic Brenta <ludovic.brenta@insalien.org>]

 (Bernd Specht) writes:
> Hi, 
>
> i have some questions regarding bitmanipulations:
>
> 1. I found bitwise "and", "or" and "xor" operations, but as far as I 
> understand they are applicable only for modular types or packed boolean 
> arrays. Is this correct?

Yes.

> 2. I did not found shift and rotate operations. Did I miss something or are 
> there really none?

These operations are usually carried out in hardware for certain sizes
of modular integers only.  Consequently, these operations are defined
in package Interfaces (see RM 3.5.4(31) and B.2).  Package Interfaces
defines the following subprograms:

   function Shift_Left  (Value  : Unsigned_n;
                         Amount : Natural) return Unsigned_n;
   function Shift_Right (Value  : Unsigned_n;
                         Amount : Natural) return Unsigned_n;
   function Shift_Right_Arithmetic (Value  : Unsigned_n;
                                    Amount : Natural)
     return Unsigned_n;
   function Rotate_Left  (Value  : Unsigned_n;
                          Amount : Natural) return Unsigned_n;
   function Rotate_Right (Value  : Unsigned_n;
                          Amount : Natural) return Unsigned_n;

Each compiler provides Unsigned_n types for various values of n
corresponding to natural sizes of the target hardware.

Doing bit rotations in the general case does not necessarily make
sense, because the modulo of the type may not be a power of two.

> 3. When I want treat a value both as an integer and as a boolean array, how 
> can i do this? In pascal I would use a tagless record like 
>
> TYPE ov is record
>   case boolean of
>     true : i : integer;
>     false  : byte_array;
>   end;
>   end;
>
> Such overlay structures are not valid with Ada, so what do instead?

Have you considered a variant record?

type Ov (T : Boolean) is record
   case T is
      when True => I : Integer;
      when False => B : Byte_Array;
   end T;
end record;

If what you really want is to convert an integer to a byte array, use
Unchecked_Conversion:

with Ada.Unchecked_Conversion;
procedure P is
   type Byte_Array is array (1 .. Integer'Size) of Boolean;
   pragma Pack (Byte_Array);
   for Byte_Array'Size use Integer'Size;
   function To_Byte_Array is new
      Ada.Unchecked_Conversion (Source => Integer, Target => Byte_Array);
   I : Integer := 42;
   B : Byte_Array := To_Byte_Array (I);
begin
   null;
end P;


-- 
Ludovic Brenta.