wie art der daten in blob erkennen?

[himitsu]

Zitat:

Naja, lesbar ist aber anders. Und ob es dabei wirklich auf die paar Millisekunden ankommt, die diese "Verbesserung" bringt, wage ich mal zu bezweifeln.

Wie oft kommt es denn vor, dass diese Prüfung 2 Millionen Mal pro Sekunde ausführen muß, um es nötig zu haben, das noch zu optimieren?

Delphi-Quellcode:

{$SCOPEDENUMS ON}
type TImageFormat = (Unknown, BMP, GIF87a, TIFF, JPEG, PNG, ICO);

{$POINTERMATH ON}
function GetBytesFormat(const AData: TBytes): TImageFormat;
var
  Signature: LongWord;
begin
  Result := TImageFormat.Unknown;
  if Length(AData) >= 2 then
    if PWord(AData)^ = $4D42 then
      Result := TImageFormat.BMP;
  if Length(AData) >= 4 then
    case PLongWord(AData)^ of
      $DBFFD8FF, $01004649, $EEFFD8FF, $00006669, $E0FFD8FF:
        Result := TImageFormat.JPEG;
      $E0FFD8FF:
        if (Length(AData) >= 8) and ((PLongWord(AData) + 1)^ = $464A1000) then
          Result := TImageFormat.JPEG;
      $E1FFD8FF:
        if (Length(AData) >= 6) and ((PWord(AData) + 2)^ = $7845) then
          Result := TImageFormat.JPEG;
      $474E5089:
        if (Length(AData) >= 8) and ((PLongWord(AData) + 1)^ = $0A1A0A0D) then
          Result := TImageFormat.PNG;
      $38464947:
        if Length(AData) >= 6 then
          case (PWord(AData) + 2)^ of
            $6137, $6139:
              Result := TImageFormat.GIF87a;
          end;
      $00010000:
        Result := TImageFormat.ICO;
      $002A4949, $2A004D4D:
        Result := TImageFormat.TIFF;
    end;
end;

Lesbarer wird es jedenfalls, wenn man Daten und Code noch etwas trennt.

Code:

type
  {$SCOPEDENUMS ON}
  TImageFormat = (Unknown, BMP, GIF87a, TIFF, JPEG, PNG, ICO);
  {$SCOPEDENUMS OFF}
  TImageSignature = array[-2..7] of Byte;

const
  cImageSignatures: array[0..13] of TImageSignature = (
    (Ord(TImageFormat.JPEG),    4, $FF, $D8, $FF, $DB,    0,    0,    0,    0),
    (Ord(TImageFormat.JPEG),    8, $FF, $D8, $FF, $E0, $00, $10, $4A, $46),
    (Ord(TImageFormat.JPEG),    4, $49, $46, $00, $01,    0,    0,    0,    0),
    (Ord(TImageFormat.JPEG),    4, $FF, $D8, $FF, $EE,    0,    0,    0,    0),
    (Ord(TImageFormat.JPEG),    6, $FF, $D8, $FF, $E1, $45, $78,    0,    0),
    (Ord(TImageFormat.JPEG),    4, $69, $66, $00, $00,    0,    0,    0,    0),
    (Ord(TImageFormat.JPEG),    4, $FF, $D8, $FF, $E0,    0,    0,    0,    0),
    (Ord(TImageFormat.PNG),     8, $89, $50, $4E, $47, $0D, $0A, $1A, $0A),
    (Ord(TImageFormat.GIF87a), 6, $47, $49, $46, $38, $37, $61,    0,    0),
    (Ord(TImageFormat.GIF87a), 6, $47, $49, $46, $38, $39, $61,    0,    0),
    (Ord(TImageFormat.BMP),     2, $42, $4D,    0,    0,    0,    0,    0,    0),
    (Ord(TImageFormat.ICO),     4, $00, $00, $01, $00,    0,    0,    0,    0),
    (Ord(TImageFormat.TIFF),    4, $49, $49, $2A, $00,    0,    0,    0,    0),
    (Ord(TImageFormat.TIFF),    4, $4D, $4D, $00, $2A,    0,    0,    0,    0)
  );

function GetBytesFormat(const Data: TBytes): TImageFormat;
begin
  for var Signature: TImageSignature in cImageSignatures do
    if (Length(Data) >= Signature[-1]) and CompareMem(@Data[0], @Signature[0], Signature[-1]) then
      Exit(TImageFormat(Signature[-2]));
  Result := TImageFormat.Unknown;
end;

Delphi-Quellcode:

{$SCOPEDENUMS ON}
type
  TImageFormat = (Unknown, BMP, GIF87a, TIFF, JPEG, PNG, ICO);
{$SCOPEDENUMS OFF}

var
  cImageSignatures: array of array of Byte = [
    [Ord(TImageFormat.JPEG),   $FF, $D8, $FF, $DB],
    [Ord(TImageFormat.JPEG),   $FF, $D8, $FF, $E0, $00, $10, $4A, $46],
    [Ord(TImageFormat.JPEG),   $49, $46, $00, $01],
    [Ord(TImageFormat.JPEG),   $FF, $D8, $FF, $EE],
    [Ord(TImageFormat.JPEG),   $FF, $D8, $FF, $E1, $45, $78],
    [Ord(TImageFormat.JPEG),   $69, $66, $00, $00],
    [Ord(TImageFormat.JPEG),   $FF, $D8, $FF, $E0],
    [Ord(TImageFormat.PNG),    $89, $50, $4E, $47, $0D, $0A, $1A, $0A],
    [Ord(TImageFormat.GIF87a), $47, $49, $46, $38, $37, $61],
    [Ord(TImageFormat.GIF87a), $47, $49, $46, $38, $39, $61],
    [Ord(TImageFormat.BMP),    $42, $4D],
    [Ord(TImageFormat.ICO),    $00, $00, $01, $00],
    [Ord(TImageFormat.TIFF),   $49, $49, $2A, $00],
    [Ord(TImageFormat.TIFF),   $4D, $4D, $00, $2A]
  ];

function GetBytesFormat(const Data: TBytes): TImageFormat;
begin
  for var Signature in cImageSignatures do
    if (Length(Data) >= Length(Signature)-1) and CompareMem(@Data[0], @Signature[1], Length(Signature)-1) then
      Exit(TImageFormat(Signature[0]));
  Result := TImageFormat.Unknown;
end;