#include <stdio.h>
#include <stdint.h>

#include "checksum.h"

/* http://locklessinc.com/articles/tcp_checksum/ */
unsigned short ip_checksum_1(const char *buf, size_t usize)
{
  unsigned size = (unsigned) usize;
	unsigned sum = 0;
	int i;

	/* Accumulate checksum */
	for (i = 0; i < size - 1; i += 2)
    {
      unsigned short word16 = *(unsigned short *) &buf[i];
      sum += word16;
    }

	/* Handle odd-sized case */
	if (size & 1)
    {
      unsigned short word16 = (unsigned char) buf[i];
      sum += word16;
    }

	/* Fold to get the ones-complement result */
	while (sum >> 16) sum = (sum & 0xFFFF)+(sum >> 16);

	/* Invert to get the negative in ones-complement arithmetic */
	return ~sum;
}