#include "packet.h"

// shamelessly copied from:
// http://www.roman10.net/how-to-calculate-iptcpudp-checksumpart-2-implementation/
unsigned short compute_checksum(unsigned short *addr, unsigned int count) {

  unsigned long sum = 0;

  while (count > 1) {
    sum += * addr++;
    count -= 2;
  }

  //if any bytes left, pad the bytes and add
  if(count > 0) {
    sum += ((*addr)&htons(0xFF00));
  }

  //Fold sum to 16 bits: add carrier to result
  while (sum>>16) {
      sum = (sum & 0xffff) + (sum >> 16);
  }

  //one's complement
  sum = ~sum;

  return ((unsigned short)sum);

}

void compute_ip_checksum(struct iphdr* pkt)
{
  pkt->check = 0x0000;
  pkt->check = compute_checksum( (unsigned short*) pkt, pkt->ihl * 4 );
}