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

#define PORT 123
#define NTP_TIMESTAMP_DELTA 2208988800ull

int main() {
    int sockfd;
    struct sockaddr_in server_addr, client_addr;
    unsigned char packet[48];
    socklen_t addr_len = sizeof(client_addr);

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

    memset(&server_addr, 0, sizeof(server_addr));
    server_addr.sin_family = AF_INET;
    server_addr.sin_addr.s_addr = INADDR_ANY;
    server_addr.sin_port = htons(PORT);

    if (bind(sockfd, (struct sockaddr *)&server_addr, sizeof(server_addr)) < 0) {
        perror("Bind failed");
        close(sockfd);
        return 1;
    }

    printf("SNTP server listening on port %d...\n", PORT);

    while (1) {
        memset(packet, 0, sizeof(packet));

        if (recvfrom(sockfd, packet, sizeof(packet), 0,
                     (struct sockaddr *)&client_addr, &addr_len) < 0) {
            perror("Receive failed");
            continue;
        }

        // Build SNTP server response
        memset(packet, 0, sizeof(packet));
        packet[0] = 0b00100100; // LI = 0, VN = 4, Mode = 4 (server)

        time_t now = time(NULL);
        uint32_t transmit_time = htonl((uint32_t)(now + NTP_TIMESTAMP_DELTA));
        memcpy(&packet[40], &transmit_time, sizeof(transmit_time));

        sendto(sockfd, packet, sizeof(packet), 0,
               (struct sockaddr *)&client_addr, addr_len);
        printf("Replied to client at %s", inet_ntoa(client_addr.sin_addr));
    }

    close(sockfd);
    return 0;
}