internal static unsafe int GetOccurrenceLength(byte* newPtr, int newLength, byte* oldPtr, int oldLength, out int disp)
{
disp = 0;
if (newLength == 0)
return 0;
int maxLength = 0;
// try every possible 'disp' value (disp = oldLength - i)
for (int i = 0; i < oldLength - 1; i++)
{
// work from the start of the old data to the end, to mimic the original implementation's behaviour
// (and going from start to end or from end to start does not influence the compression ratio anyway)
byte* currentOldStart = oldPtr + i;
int currentLength = 0;
// determine the length we can copy if we go back (oldLength - i) bytes
// always check the next 'newLength' bytes, and not just the available 'old' bytes,
// as the copied data can also originate from what we're currently trying to compress.
for (int j = 0; j < newLength; j++)
{
// stop when the bytes are no longer the same
if (*(currentOldStart + j) != *(newPtr + j))
break;
currentLength++;
}
// update the optimal value
if (currentLength > maxLength)
{
maxLength = currentLength;
disp = oldLength - i;
}
}
return maxLength;
}