in_cksum.c

#include "unp.h"

uint16_t
in_cksum(uint16_t *addr, int len)
{
	int				nleft = len;
	uint32_t		sum = 0;
	uint16_t		*w = addr;
	uint16_t		answer = 0;

	/*
	 * Our algorithm is simple, using a 32 bit accumulator (sum), we add
	 * sequential 16 bit words to it, and at the end, fold back all the
	 * carry bits from the top 16 bits into the lower 16 bits.
	 */
	while (nleft > 1)  {
		sum += *w++;
		nleft -= 2;
	}

		/* 4mop up an odd byte, if necessary */
	if (nleft == 1) {
		*(unsigned char *)(&answer) = *(unsigned char *)w ;
		sum += answer;
	}

		/* 4add back carry outs from top 16 bits to low 16 bits */
	sum = (sum >> 16) + (sum & 0xffff);	/* add hi 16 to low 16 */
	sum += (sum >> 16);			/* add carry */
	answer = ~sum;				/* truncate to 16 bits */
	return(answer);
}