Re: Boolean Representation

[eachus@spectre.mitre.org (Robert I. Eachus)]

In article <6ue3uh$qjo$1@nnrp1.dejanews.com> matthew_snyder@my-dejanews.com writes:

  > The reason for my original question is that I'm trying to decide
  > the best (most portable) way to convert an UNSIGNED_8 to a record
  > type which provides access to the individual bits. The record
  > could either consist of booleans, or it could consist of my own
  > bit type (is there a predefined BIT type that I'm overlooking?).
  > Right now my record consists of booleans and I'm performing an
  > unchecked conversion from UNSIGNED_8 to the record type. The
  > option of doing a "for RECORD'ADDRESS use SOME_ADDRESS" is not
  > available in this situation. Is any use of UNCHECKED_CONVERSION
  > considered "junky coding"?  I certainly would not want to have to
  > do a 'POS on every bit to assure portability.  What's the best way
  > to do this?

   Hmmm, do you really need the record type, or is access to the bits
enough?

 with Interfaces;
 package Bit_Manipulation is
 
   subtype Index is Integer range 1..8;

   function Get(X: Interfaces.Unsigned_8; Bit: Integer) return Boolean;
 
   procedure Set(X: in out Interfaces.Unsigned_8; Bit: in Integer);

   procedure Unset(X: in out Interfaces.Unsigned_8; Bit: in Integer);
   procedure Set(X: in out Interfaces.Unsigned_8;
                 Bit: in Integer;
                 Value: in Boolean);

   pragma Inline(Get, Set, Unset);

 end Bit_Manipulation;

 package body Bit_Manipulation is

   use Interfaces;
 
   Mask: constant array(Index) of Interfaces.Unsigned_8 := 
                                       (1,2,4,8,16,32,64,128);

   function Get(X: Interfaces.Unsigned_8; Bit: Integer) return Boolean is
   begin return (X and Mask(Bit)) /= 0; end Get;
        
   procedure Set(X: in out Interfaces.Unsigned_8; Bit: in Integer) is
   begin X := X or Mask(Bit); end Set;

   procedure Unset(X: in out Interfaces.Unsigned_8; Bit: in Integer) is
   begin X := X and not Mask(Bit); end Unset;

   procedure Set(X: in out Interfaces.Unsigned_8;
                 Bit: in Integer;
                 Value: in Boolean) is
   begin 
     if Value then Set(X, Bit); else Unset(X, Bit); end if;
   end Set;

 end Bit_Manipulation;

    I haven't checked compilers to see if the resulting code is what
you would expect for these operations, but it should come pretty close
as long as Bit is static.
--

					Robert I. Eachus

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