"Вычисление простой контрольной суммы в цикле по каждому байту записи в Delphi и Pascal"

Delphi , Синтаксис , Циклы

Delphi: Loop through bytes in record to calculate a simple checksum

If you have a record in Delphi or Pascal that contains multiple bytes or arrays of bytes, and you need to loop over the bytes inside to create a simple checksum by adding each byte to the checksum, you can do this in a loop without having to go over each element inside the record individually.

Here's a simple way to achieve this using pointers and the SizeOf function:

var
  sum: Byte;
  ptr: PByte;
  i: Integer;
begin
  sum := 0;
  ptr := PByte(@rdmPacket);

  for i := 0 to SizeOf(TRdmPacket) - 1 do
  begin
    sum := sum xor ptr^;
    Inc(ptr);
  end;
end;

In this code, we first initialize a variable sum to 0, which will hold the checksum. We then declare a pointer ptr of type PByte and assign it the address of the rdmPacket variable using the @ symbol.

We then loop through the record using a for loop, where the loop counter i goes from 0 to SizeOf(TRdmPacket) - 1. In each iteration, we perform an XOR operation between the current value of sum and the byte pointed to by ptr, and store the result back in sum. We then increment the pointer using the Inc function to point to the next byte in the record.

This method works correctly because all fields in the TRdmPacket record are bytes or arrays of bytes, and they are not aligned. However, it's worth noting that the alignment of fields in a record can affect the internal layout of the record, so you should be careful when using this method with other types of records. You can read more about the Packed attribute and the $Align directive in the Delphi documentation.

Alternatively, you can use the Absolute directive to achieve the same result:

type
  TRdmPacket = record
    sc: byte;
    subSc: byte;
    msgLength: byte;
    destUID: array[0..5] of byte;
    srcUID: array[0..5] of byte;
    transNum: byte;
    portID: byte;
    msgCount: byte;
    subDevice: array[0..1] of byte;
    cc: byte;
    pid: array[0..1] of byte;
    pdl: byte;
  end;

Function GetPackChecksum( pack:TRdmPacket):Integer;
var
  BArray:Array [0..SizeOf(TRdmPacket) - 1] of Byte absolute pack;
  i:Integer;
begin
  Result := 0;
  for I := Low (BArray)to High(BArray) do
  begin
    Result := Result +  BArray[i];
  end;
end;

procedure TForm2.Button1Click(Sender: TObject);
var
  pack:TRdmPacket;
begin
  ZeroMemory(@pack,SizeOf(Pack));
  pack.sc := 100;
  pack.destUID[1] := 123;
  Showmessage(IntToStr(GetPackChecksum(pack)));
end;

In this code, we declare an array BArray of bytes using the Absolute directive, which allows us to access the bytes of the pack record directly. We then loop through the array using a for loop, adding each byte to the Result variable, which holds the checksum.

Both of these methods allow you to loop through the bytes of a record in Delphi or Pascal and calculate a simple checksum without having to access each field individually.

Создано по материалам из источника по ссылке.

Описание 'Context': В данном контексте рассматривается задача прохода по байтам в записи (record) на языке Delphi или Паскаль для расчета простого контрольной суммы (checksum) путем сложения каждого байта с суммой. Для этого можно использовать цикл (loop)

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