How to convert Big Endian and how to flip the highest bit?

Question

I am using a TStream to read binary data (thanks to this post: How to use a TFileStream to read 2D matrices into dynamic array?).

My next problem is that the data is

Answer 1

Well, I don't know too much about byte swapping, but flipping a single bit is easy. XOR your value with a mask whose binary representation is all 0s, except for a 1 for the bit you want to flip. If you want the most significant bit, in hex that works out to $80 for a single byte, $8000 for a word and, you guessed it, $80000000 for a double word.

Answer 2

Not tested:

function Flip16(const Value: Word): Word; inline;
begin
  Result:= Value xor $8000;
end;

function Flip32(const Value: LongWord): LongWord; inline;
begin
  Result:= Value xor $80000000;
end;

function Flip64(const Value: UInt64): UInt64; inline;
begin
  Result:= Value xor $8000000000000000;
end;

function SwapBytes(Value: LongWord): Single;
type
  Bytes = packed array[0..3] of Byte;

begin
  Bytes(Result)[0]:= Bytes(Value)[3];
  Bytes(Result)[1]:= Bytes(Value)[2];
  Bytes(Result)[2]:= Bytes(Value)[1];
  Bytes(Result)[3]:= Bytes(Value)[0];
end;

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Updated: Well if you about to optimize :)

function SwapBytes(Value: LongWord): Single; register; 
asm
  BSWAP  EAX
end;

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Updated once more:

A common practice in SO is one question per one post - it generally results in better answers.

1) Are IEEE affected by Big Endian?

Yes, floating-point values can be Big Endians, but it does not mean that your floating-point values are Big Endians - check your documentation.

2) flipping a most significant bit - the answer is already given, but since you probably have Big Endians you may need just to flip a most significant bit in a least significant byte, i.e. just xor $80;

3) converting big endian to little endian;

for 16-bit values use Swap function - it is hard to understand why delphi help says that the function is for backward compatibility only; it is also possible to use XCHG asm instruction as Remko noticed;

for 32-bit values your can use my code or code in Marco's comment;

for 64-bit values you can use a modification of Marco's code like this:

function Swap64(Value: UInt64): UInt64;
begin
  Result:= Swap32(LongWord(Value));
  Result:= (Result shl 32) or Swap32(LongWord(Value shr 32));
end;

4) Is it possible to use generics here?

I don't think it is a good idea.