Re: [DUG]: C Conversion

[Dennis Chuah]

Use Cardinal instead of Integer - because the C code uses "unsigned int".

Your for should be 0-based:

  for currByte := 0 to dataLength-1 do

^ maps to xor

      if ((dataStore xor accum) and $8000) > 0 then
        accum := (accum shl 1) xor CRCDIV;

Lastly, this does not affect your code, but for correctness:

    for currBit := 0 to 7 do

Interesting polynomial:

            CRCDIV = $8005;


----- Original Message -----
From: James Sugrue
To: Multiple recipients of list delphi
Sent: Tuesday, November 04, 2003 11:50 AM
Subject: [DUG]: C Conversion


I have the following code. I have tried to convert into Delphi but it isn't
100% correct. Could some kind soul show me where I've gone wrong?

#define CRCDIV 0x8005

unsigned int computeCRC (unsigned char *data, unsigned int dataLength)
{
  unsigned int accum;
  unsigned int currByte;
  unsigned int currBit;
  unsigned int dataStore;

  accum = 0;

  // repeat for all bytes in message from the header to the data
  for (currByte = 0;  currByte < dataLength;  currByte++) {
    dataStore = ((unsigned int) data[currByte]) << 8;

    for (currBit = 0;  currBit < 8;  currBit++) {
      if ((dataStore ^ accum) & 0x8000) {
        accum = (accum << 1) ^ CRCDIV;
      }
      else {
        accum <<= 1;
      }
      dataStore <<= 1;
    }
  }

  return (accum);
}


function computeCRC (data : PChar; dataLength : Integer) : Integer;
const
            CRCDIV = $8005;
var
  accum : Integer;
  currByte : Integer;
  currBit : Integer;
  dataStore : Integer;
begin
  accum := 0;

  // repeat for all bytes in message from the header to the data
  for currByte := 1 to dataLength do
  begin
    dataStore := Integer(data[currByte]) shl 8;

    for currBit := 1 to 8 do
    begin
      if ((dataStore or accum) and $8000) > 0 then
      begin
        accum := (accum shl 1) or CRCDIV;
      end
      else
      begin
        accum := accum shl 1;
      end;
      dataStore := dataStore shl 1;
    end;
  end;

  result := accum;
end;






James Sugrue

Software Developer
WA Systems
Timaru
Phone 03 688-1131

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