问如何使用FreePascal(拉撒路)计算CRC32校验和？

EN

procedure TMyFrame.CRC32_Checksum();
var
  P : Pointer;
  Size, i : Integer;
  CRC, j : LongWord;
  B : ^Byte;
  flag : Boolean;
begin
  AssignFile (f, FileName);
  Reset(f, 1);
  Size := FileSize(f);
  GetMem(P, Size);
  BlockRead(f, P^, Size);
  B := P;
  //
  //
  CRC := $FFFFFFFF;
  for i := 1 to Size do
    begin
      CRC := CRC XOR B^;
      Inc(B);
      for j := 0 to 7 do
        begin
          flag := (CRC AND 1) > 0;
          if flag then
            CRC := (CRC SHR 1) XOR $04C11DB7
          else
            CRC := CRC SHR 1;
        end;
      end;
  LabeledEdit1.Text := IntToHEX(CRC, 8);
  //
  //
  Freemem(P);
  CloseFile(f);
end;