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From: =?UTF-8?Q?Bj=c3=b6rn_Lundin?= <b.f.lundin@gmail.com>
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Subject: Re: Examining individual bytes of an integer
Date: Mon, 15 Oct 2018 12:52:27 +0200
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Xref: reader02.eternal-september.org comp.lang.ada:54585
Date: 2018-10-15T12:52:27+02:00
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On 2018-10-14 23:04, Niklas Holsti wrote:
> 
> (I have become very sensitive to endianness problems because my daily
> work involves writing Ada programs targeted to big-endian SPARC
> machines, but tested initially on little-endian PCs.)


I usually decide how to work with the bytes (little- or bigEndian)
and implement something like this


  type Byte is range 0..255;
  for  Byte'Size use 8;

  type Byte_Array is array (Positive range <>) of Byte;

  subtype Byte_Array_2 is Byte_Array(1..2);
  subtype Byte_Array_4 is Byte_Array(1..4);


 procedure Swap4 (Bytes: in out Byte_Array_4) is
    Result : Byte_Array_4;
  begin
    Result := (1 => Bytes(4), 2 => Bytes(3),
               3 => Bytes(2), 4 => Bytes(1));
    Bytes := Result;
  end Swap4;
  -------------------------------------------------

 procedure Swap2 (Bytes: in out Byte_Array_2) is
    Result : Byte_Array_2;
  begin
    Result := (1 => Bytes(2), 2 => Bytes(1));
    Bytes := Result;
  end Swap2;
  -------------------------------------------------
  function Mirror (Data : Byte_Array) return Byte_Array is
  begin
    case Data'Length is
    when 2 =>
       declare
         Tmp : Byte_Array_2 := Data;
       begin
         case System.Default_Bit_Order is
           when System.High_Order_First => null;       --ppc
           when System. Low_Order_First => Swap2(Tmp); --x86
         end case;
         return Tmp;
       end;
    when 4 =>
       declare
         Tmp : Byte_Array_4 := Data;
       begin
         case System.Default_Bit_Order is
           when System.High_Order_First => null;       --ppc
           when System. Low_Order_First => Swap4(Tmp); --x86
         end case;
         return Tmp;
       end;
    when others => raise Constraint_Error with
           "not implemented lenght" & Data'Length'img;
    end case;
  end Mirror;
  -------------------------------------------------


  Then I call Mirror once (at input),
  and have the bytes in the same order no matter endianess.
  The I can modify the bytes as I please.

  I also call Mirror again at output, so I can write the
  modifications in a machine friendly way.


  (code is taken from project, but altered here without compilation,
   especially the 4byte part)

-- 
--
Björn