#include <stdint.h>
#include <stdio.h>
#include <asm/byteorder.h>
#include <netinet/ip.h>

#define ipv4_addr(o1, o2, o3, o4) __constant_htonl( \
	(o1) << 24 | \
	(o2) << 16 | \
	(o3) <<  8 | \
	(o4))

static struct iphdr ip = {
	.version = 4,
	.ihl = 5,
	.id = __constant_htons(0xcda3),
	.frag_off = 0x40,
	.ttl = 64,
	.protocol = IPPROTO_UDP,
};

static uint16_t ip_checksum(uint16_t *buf, size_t len)
{
	unsigned long sum = 0;
	int i;

	for (i = 0; i < len; i++) {
		sum += htons(buf[i]);
		if (sum & 0x80000000)
			sum = (sum & 0xFFFF) + (sum >> 16);
	}

	while (sum >> 16)
		sum = (sum & 0xFFFF) + (sum >> 16);

	return htons(~sum);
}

static void calc_csum_print(struct iphdr *ip)
{
	ip->check = 0;
	ip->check = ip_checksum((uint16_t *)ip, ip->ihl * 2);
	printf("chk: %hx\n", ip->check);
}

int main(void)
{
	ip.saddr = ipv4_addr(192, 168, 0, 1);
	ip.daddr = ipv4_addr(10, 0, 20, 1);
	calc_csum_print(&ip);

	ip.saddr = ipv4_addr(0, 1, 192, 168);
	calc_csum_print(&ip);

	ip.daddr = ipv4_addr(20, 1, 10, 0);
	calc_csum_print(&ip);

	ip.saddr = ipv4_addr(10, 0, 20, 1);
	ip.daddr = ipv4_addr(192, 168, 0, 1);
	calc_csum_print(&ip);

	return 0;
}