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
[Opinions above are the author's, not those of his employers or colleagues]
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