/* flaneth - flash and ethernet version 0.2 date 2008-11-08 Copyright (C)
2007-2008 Stefan Schuermans <stefan@schuermans.info> Copyleft: GNU public
license V2 - http://www.gnu.org/copyleft/gpl.html a BlinkenArea project -
http://www.blinkenarea.org/ */
#include "checksum.h"
#include "macros.h"
#include "nethelp.h"
// generate a checksum
// also includes Pseudo1 and Pseudo2 in the checksum - set to 0x0000 for
// normal operation
// can also be used to check a checksum (returns 0 if correct)
unsigned short Checksum(unsigned char *pData, unsigned short Length, unsigned short Pseudo1, unsigned short Pseudo2) // (extern)
{
unsigned long sum;
// sum up data of pseudo header
sum = Pseudo1; // convert to 32 bit first
sum += Pseudo2;
// add full 16 bit words in header
for (; Length >= 2; pData += 2, Length -= 2)
sum += ntohs(*(unsigned short *)pData);
// add last byte
if (Length >= 1)
sum += (unsigned short)(*pData << 8);
// convert sum to one's complement sum
sum = (sum & 0x0000FFFF) + (sum >> 16); // add carries (bits 31..16)
// to sum (bits 15..0)
sum = (sum & 0x0000FFFF) + (sum >> 16); // still a carry possible
// (from last addition)
// return complement of one's complement sum
return ~(unsigned short)sum;
}