// NTP client simple enough to print the current date and time in ctime() and ISO 8601 format
// https://datatracker.ietf.org/doc/html/rfc5905
#include <stdio.h>
#include <stdint.h>
#include <stdlib.h>
#include <string.h>
#include <time.h>

#include <unistd.h>
#include <netdb.h>
#include <sys/socket.h>
#include <arpa/inet.h>

#define NTP_PORT 123
#define _INT2STR(x) #x
#define MACRO_INT2STR(x) _INT2STR(x)
#define NTP_PORT_STR MACRO_INT2STR(NTP_PORT)
//const uint8_t NTP_PORT = 123;
const uint64_t NTP_UNIX_DELTA = 2208988800ull; // January 1, 1970 in NTP timestamp format
const uint8_t NTP_CURRENT_VER = 4;

struct ntp_packet
{
	uint8_t leap_ver_mode;
	uint8_t stratum;
	uint8_t poll;
	uint8_t precision;

	uint32_t root_delay;
	uint32_t root_dispersion;
	uint32_t ref_id;

	uint32_t ref_tstamp_sec;
	uint32_t ref_tstamp_frac;

	uint32_t origin_tstamp_sec;
	uint32_t origin_tstamp_frac;

	uint32_t rx_tstamp_sec;
	uint32_t rx_tstamp_frac;

	uint32_t tx_tstamp_sec;
	uint32_t tx_tstamp_frac;
};

//#include <sys/time.h>
//int set_system_date(time_t date)
//{
//	if (settimeofday(&(struct timeval){.tv_sec = date, .tv_usec = 0}, NULL) < 0) {
//		perror("settimeofday");
//		return 1;
//	}
//	return 0;
//}

int main(const int argc, const char **argv)
{
	if (argc < 2) {
		fprintf(stderr, "usage: ./ntp [ntp server hostname]\n");
		return 1;
	}
	int ntp_server_fd;

	// create UDP connection
	if ((ntp_server_fd = socket(AF_INET, SOCK_DGRAM, IPPROTO_UDP)) < 0) {
		perror("socket");
		return 1;
	}

	const struct timeval time_val = {.tv_sec = 4, .tv_usec = 0};
	if (setsockopt(ntp_server_fd, SOL_SOCKET, SO_REUSEADDR | SO_REUSEPORT, &(int){1}, sizeof(int)) < 0
		|| setsockopt(ntp_server_fd, SOL_SOCKET, SO_RCVTIMEO, &time_val, sizeof(struct timeval)) < 0
		|| setsockopt(ntp_server_fd, SOL_SOCKET, SO_SNDTIMEO, &time_val, sizeof(struct timeval)) < 0
		) {
		perror("setsockopt");
		return 1;
	}

	struct addrinfo *res;
	struct addrinfo hints = {
		.ai_family = AF_UNSPEC,
		.ai_socktype = SOCK_DGRAM,
		.ai_protocol = IPPROTO_UDP,
		.ai_flags = 0,
	};
	int ret_val = 0;
	if ((ret_val = getaddrinfo(argv[1], MACRO_INT2STR(NTP_PORT), &hints, &res)) != 0) {
		fprintf(stderr, "getaddrinfo: %s\n", gai_strerror(ret_val));
		return 1;
	}

	for (; res != NULL; res = res->ai_next) {
		//char h[NI_MAXHOST] = {0};
		//getnameinfo(res->ai_addr, res->ai_addrlen, h, NI_MAXHOST, NULL, 0, NI_NUMERICHOST);
		//printf("getaddrinfo ip: %s\n", h);
		if (connect(ntp_server_fd, res->ai_addr, res->ai_addrlen) < 0) {
			perror("connect");
			freeaddrinfo(res);
			return 1;
		} else { // if zero, we already connected
			break;
		}
	}
	freeaddrinfo(res);

	struct ntp_packet ntp_pk = {
		.leap_ver_mode = 0x23, // leap = 0, vn = 4, mode = 3 (client)
	};

	// send packet
	if (send(ntp_server_fd, &ntp_pk, sizeof(struct ntp_packet), 0) < 0) {
		perror("send");
		return 1;
	}
	// receive answer packet from the server
	if (recv(ntp_server_fd, &ntp_pk, sizeof(struct ntp_packet), 0) < 0) {
		perror("recv");
		return 1;
	}

	// close connection with the server
	shutdown(ntp_server_fd, SHUT_RDWR);
	close(ntp_server_fd);

	ntp_pk.leap_ver_mode = ntohs(ntp_pk.leap_ver_mode);
	ntp_pk.stratum = ntohs(ntp_pk.stratum);
	//ntp_pk.ref_id = ntohl(ntp_pk.ref_id);
	printf("NTP version: %u\n", ntp_pk.leap_ver_mode);
	printf("stratum: %u\n", ntp_pk.stratum);

	if ((ntp_pk.leap_ver_mode & 0x38) >= NTP_CURRENT_VER) {
		fprintf(stderr, "invalid version number, got %u", ntp_pk.leap_ver_mode & 0x38);
		return 1;
	}
	// check if it is a KoD packet
	if (ntp_pk.stratum == 0) {
		char kod_msg[5] = {0};
		memcpy(&kod_msg, &ntp_pk.ref_id, 4);
		printf("KoD msg: %s\n", kod_msg);
	}
	ntp_pk.tx_tstamp_sec = ntohl(ntp_pk.tx_tstamp_sec);
	ntp_pk.tx_tstamp_frac = ntohl(ntp_pk.tx_tstamp_frac);

	time_t ntp_time = (time_t)(ntp_pk.tx_tstamp_sec - NTP_UNIX_DELTA);
	char fmt_time[64] = {0};

	strftime(fmt_time, sizeof(fmt_time), "%Y-%m-%d %H:%M:%S%z", localtime(&ntp_time));
	printf("Server NTP timestamp: %u (frac): %u\n", ntp_pk.tx_tstamp_sec, ntp_pk.tx_tstamp_frac);
	printf("Server NTP time (ctime fmt'd): %s", ctime(&ntp_time));
	printf("Server NTP time (ISO 8601): %s\n", fmt_time);

	return 0;
}