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From: Shark8 <onewingedshark@gmail.com>
Newsgroups: comp.lang.ada
Subject: Re: Using representation clauses in networking software
Date: Sun, 15 Aug 2010 12:11:18 -0700 (PDT)
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Date: 2010-08-15T12:11:18-07:00
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Here's what I've been using to swap endian-ness... as you can see
though it is the "programmer's responsibility" to use it and use it
correctly; a representation clause for endian-ness might not be a bad
thing, it would make implementing the protocols which do specify the
endian-ness easier though.

   -- 8-bit 'word' [Byte]
   Type Word8 is mod 2**8;
    For Word8'Size use 8;

   -- Array of some number of bits... I am assuming that a single unit-
size will not exceed 255-bits.
   Type Bit_Array is Array (Word8 range<>) of Boolean;
   Pragma Pack(Bit_Array);


---- Function for swapping big-endian to little-endian and vice versa.

   Procedure Swap( B : in out Bit_Array ) is

      Low, High : Bit_Array(1..B'Length/2);
      For Low'Address use B'Address;
      For High'Address use B(1+B'Length/2)'Address;
   begin
      Case B'Length is
         when 64 | 32 | 16 =>
		Swap( Low );
		Swap( High );
		B(1..B'Length):=  High & Low;
         when 8 | 4 | 2 | 1 | 0 =>
            null;
         when others => raise Program_Error;  -- Non powers of 2 are
not supported; and sizes in excess of 64-bits.
      end case;
   end Swap;