[fpc-pascal] Converting big-endian signed 16bits values to fpc integer

[Mark Morgan Lloyd]

Jonas Maebe wrote:
> On 16 Apr 2012, at 00:38, Giuliano Colla wrote:
> 
>> I'm dealing with a large number of data coming from an external device.
>> They are big-endian 16 bits signed numbers, which must be converted to integer and to real to perform calculations.
>>
>> Besides the obvious solutions of treating them as isolated bytes, multipying by 256 the highest, adding the lower, and oring wit $FFFF0000, or whatever sizeof(Integer) suggests if the result is bigger than 32767, is there a more efficient and elegant way to achieve the result?
> 
> Call BEtoN() on them (Big Endian to Native byte order). Note that this routine is overloaded for all integer sizes > 1, and that FPC extends the size of any number < 32/64 bits (depending on the architecture) to 32/64 bits when performing almost any operation on it. It's therefore probably the safest to call it as BEtoN(smallint(value)) to ensure that the correct overload is selected.

I asked this a few weeks ago in the context of doing endianness-changes 
in-place in a data structure, and Martin (I think) suggested overloading 
the := operator.

I did however find that one has to be extremely careful inside the 
custom operator, I ended up using Move() rather than an assignment to 
the result since it was tending to go recursive- some way of inhibiting 
all automatic type conversions would be useful.


(* This type represents a little-endian word, irrespective of the 
platform. It  *)
(* is of known size (16 bits, tested by assertion) but is not directly 
         *)
(* compatible with any other type. 
         *)
//
type    WordLE= packed record b0, b1: byte; end;


(* Assign a little-endian word to an (unsigned) integer, with 
appropriate       *)
(* conversion. 
         *)
//
operator := (wle: WordLE): word;

begin
..

Type TAWSHeader=packed Record
        ThisSize: WordLE;
        PrevSize: WordLE;
        Flags:    Byte;
        Rsvd:     Byte;
      End;

-- 
Mark Morgan Lloyd
markMLl .AT. telemetry.co .DOT. uk

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