#include <linux/bpf.h>
#include <bpf/bpf_helpers.h>
#include <bpf/bpf_endian.h>
#include <linux/if_ether.h>
#include <linux/ip.h>
#include <linux/in.h>
#include <stdlib.h>
#include <stdio.h>

char pkt[]={0x04, 0xf0, 0x21, 0x65, 0x18, 0x12, 0x50, 0x9a, 0x4c,
            0x24, 0x1a, 0x72, 0x08, 0x00, 0x45, 0x00, 0x00, 0x20,
            0x37, 0xb5, 0x40, 0x00, 0x40, 0x11, 0x70, 0x86, 0xc0, 
            0xa8, 0x00, 0xad, 0xd4, 0x2f, 0xfd, 0x0c, 0x9e, 0x1e,
            0x09, 0x44, 0x00, 0x0c, 0xfd, 0x72, 0x64, 0x64, 0x64,
            0x0a};

char pkt2[] = { 0x04,0xf0,0x21,0x65,0x18,0x12,0x50,0x9a,0x4c,0x24,0x1a,0x72,0x08,0x00,0x45,0x00,0x00,0x4e,0xf7,0xb9,0x00,0x00,0x40,0x11,0xb1,0x80,0xc0,0xa8,0x00,0xad,0x08,0x08,0x08,0x08,0x95,0xa2,0x00,0x35,0x00,0x3a,0xd1,0xb0,0x4a,0x8d,0x01,0x20,0x00,0x01,0x00,0x00,0x00,0x00,0x00,0x01,0x06,0x67,0x6f,0x6f,0x67,0x6c,0x65,0x02,0x66,0x72,0x00,0x00,0x01,0x00,0x01,0x00,0x00,0x29,0x10,0x00,0x00,0x00,0x00,0x00,0x00,0x0c,0x00,0x0a,0x00,0x08,0x32,0x1b,0xf3,0x55,0xed,0xd5,0xd5,0x6b };

char pkt3[] = { 0x50,0x9a,0x4c,0x24,0x1a,0x72,0x04,0xf0,0x21,0x65,0x18,0x12,0x08,0x00,0x45,0x00,0x00,0x64,0x7e,0x9c,0x40,0x00,0x40,0x11,0xfb,0xfe,0xc0,0xa8,0x00,0xfe,0xc0,0xa8,0x00,0xad,0x00,0x35,0xe6,0x4c,0x00,0x50,0x88,0x50,0xd1,0x70,0x81,0x80,0x00,0x01,0x00,0x01,0x00,0x00,0x00,0x01,0x0c,0x73,0x61,0x66,0x65,0x62,0x72,0x6f,0x77,0x73,0x69,0x6e,0x67,0x0a,0x67,0x6f,0x6f,0x67,0x6c,0x65,0x61,0x70,0x69,0x73,0x03,0x63,0x6f,0x6d,0x00,0x00,0x01,0x00,0x01,0xc0,0x0c,0x00,0x01,0x00,0x01,0x00,0x00,0x01,0x0d,0x00,0x04,0x8e,0xfa,0xba,0x8a,0x00,0x00,0x29,0x04,0xd0,0x00,0x00,0x00,0x00,0x00,0x00 };

char csDesired[] = { 0x38, 0xf1 };

static __always_inline __u16 csum_fold_helper(__u32 csum)
{
	return ~((csum & 0xffff) + (csum >> 16));
}
static __always_inline void ipv4_csum(void *data_start, int data_size, __u32 *csum) {
	*csum = bpf_csum_diff(0, 0, data_start, data_size, *csum);
	*csum = csum_fold_helper(*csum);
}

int checksum(__u16* ipbuf, __u32 len) {
	// https://tools.ietf.org/html/rfc1071
	__u32 sum = 0;
	for (int i = 0; i < sizeof(struct iphdr)/2; i++) {
		sum += ipbuf[i];
	}

	sum = (sum & 0xffff) + (sum >> 16);
	sum = (sum & 0xffff) + (sum >> 16);

	return ~sum;
}

// @TODO: Implement https://tools.ietf.org/html/rfc1624 instead


int main(void) {
	void *data = pkt3;
	void *data_end = pkt3+114;

	struct ethhdr *eth = data; //structure ethernet from if_ether.h
	struct iphdr *ip = (struct iphdr*)(eth + 1);
	printf("%#06x\n", bpf_ntohs(ip->check));
	ip->check = 0;
	ip->check = checksum((__u16*)ip, sizeof(*ip));
	printf("%#06x\n", bpf_ntohs(ip->check));
}

int main2(void) {
	void *data = pkt2; //pkt;
	void *data_end = pkt2+92;//pkt+46;

	struct ethhdr *eth = data; //structure ethernet from if_ether.h
	struct iphdr *ip = (struct iphdr*)(eth + 1);

	// Check that our packet ethernet headers is big enough, ie. at least the size of Ethernet's fixed headers
	if((void*)ip > data_end) {
		printf("eth headers too short\n");
		return EXIT_SUCCESS;
	}

	// Check that the ethernet header declares an IPv4 packet
	if(eth->h_proto == bpf_htons(ETH_P_IP)) {
		printf("this is an IPv4 packet according to ethernet headers\n");
		// Check that the IP packet is big enough, ie. at least the size of IP's fixed headers
		if((void*)(ip + 1) > data_end) {
			printf("IPv4 headers are too short \n");
			return EXIT_SUCCESS;
		}

		if(ip->protocol == IPPROTO_UDP) {
			printf("Protocol is UDP, flags are %#06x\n", ip->frag_off);
			printf("Total offset: %x\n", (void*)&(ip->frag_off) - data);
			ip->frag_off |= bpf_htons(0x4000); //Set the DF flag to 1 -> 0b 0100 0000 0000 0000
			printf("Protocol is UDP, flags are now %#06x\n", ip->frag_off);
		}
	}


}