#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <unistd.h>
#include <arpa/inet.h>
#include <time.h>
#include <sys/time.h>
#include <errno.h>

#define NTP_TIMESTAMP_DELTA 2208988800ull
#define NTP_PORT 123

typedef struct {
    uint8_t li_vn_mode;
    uint8_t stratum;
    uint8_t poll;
    int8_t precision;
    uint32_t rootDelay;
    uint32_t rootDispersion;
    uint32_t refId;
    uint32_t refTm_s;
    uint32_t refTm_f;
    uint32_t origTm_s;
    uint32_t origTm_f;
    uint32_t rxTm_s;
    uint32_t rxTm_f;
    uint32_t txTm_s;
    uint32_t txTm_f;
} ntp_packet;

void sntp_request(const char *server_ip) {
    int sockfd;
    struct sockaddr_in server_addr, reply_addr;
    ntp_packet packet;
    socklen_t addr_len = sizeof(reply_addr);

    memset(&packet, 0, sizeof(ntp_packet));
    packet.li_vn_mode = (0 << 6) | (4 << 3) | 3; // LI=0, VN=4, Mode=3 (Client)

    sockfd = socket(AF_INET, SOCK_DGRAM, IPPROTO_UDP);
    if (sockfd < 0) {
        perror("Socket creation failed");
        exit(1);
    }

    // Set a timeout for recvfrom (3 seconds)
    struct timeval timeout;
    timeout.tv_sec = 3;
    timeout.tv_usec = 0;
    setsockopt(sockfd, SOL_SOCKET, SO_RCVTIMEO, &timeout, sizeof(timeout));

    memset(&server_addr, 0, sizeof(server_addr));
    server_addr.sin_family = AF_INET;
    server_addr.sin_port = htons(NTP_PORT);
    if (inet_pton(AF_INET, server_ip, &server_addr.sin_addr) != 1) {
        perror("Invalid IP address");
        close(sockfd);
        exit(1);
    }

    if (sendto(sockfd, &packet, sizeof(ntp_packet), 0,
               (struct sockaddr *)&server_addr, sizeof(server_addr)) < 0) {
        perror("Send failed");
        close(sockfd);
        exit(1);
    }

    printf("Packet sent to %s. Waiting for response...\n", server_ip);

    ssize_t n = recvfrom(sockfd, &packet, sizeof(ntp_packet), 0,
                         (struct sockaddr *)&reply_addr, &addr_len);
    if (n < 0) {
        perror("Receive failed");
        close(sockfd);
        exit(1);
    }

    packet.txTm_s = ntohl(packet.txTm_s);
    time_t tx_time = packet.txTm_s - NTP_TIMESTAMP_DELTA;

    printf("Server Time from %s: %s", server_ip, ctime(&tx_time));
    close(sockfd);
}

int main(int argc, char *argv[]) {
    if (argc != 2) {
        printf("Usage: %s <NTP server IP or hostname>\n", argv[0]);
        return 1;
    }

    sntp_request(argv[1]);
    return 0;
}