firmware/checksum.c (2342395) - flaneth · BlinkenArea

checksum.c

/* flaneth - flash and ethernet
   Copyright (C) 2007-2012 Stefan Schuermans <stefan@blinkenarea.org>
   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;
}

flaneth

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