#include <sys/socket.h>     //socket, inet_aton
#include <netinet/in.h>     //socket, inet_aton
#include <netinet/udp.h>    //socket
#include <arpa/inet.h>      //htons, ntohs, htonl, ntohl, inet_aton
#include <signal.h>         //signal
#include <stdio.h>          //fprintf
#include <errno.h>          //perror
#include <stdlib.h>         //strtol
#include <unistd.h>         //close

/*PROTOTYPES*/
void bail(int code);
void sig_handler(int signum);
void usage(char* name);

/*DEFINES*/
//Magic Number used to distinguish our UDP Packets (First 8 Bytes of Payload)
#define MAGIC           0xDEADBEEFCAFEFEED
//Receive Buffer Size
#define BUFFER_SIZE     1000

/*GLOBAL VARIABLES*/
long port = 0;
int sock_fd = -1;
struct sockaddr_in src_addr;

int main(int argc, char **argv){
    
    /*Parse Arguments*/
    if (argc < 3) {
        fprintf(stderr, "Insufficient arguments\n");
        usage(argv[0]);
    }
    
    //Source Address
    in_addr_t src_ip_addr = inet_addr(argv[1]);
    if (src_ip_addr == INADDR_NONE) {
        fprintf(stderr, "Invalid SRC IP Address\n");
        usage(argv[0]);
    }
    
    //Port Number
    errno = 0;
    port = strtol(argv[2], NULL, 10);
    if (errno != 0) {
        fprintf(stderr, "Invalid Port Number\n");
        usage(argv[0]);
    }
    
    /*MISC MAIN THREAD*/
    //Register Signal Handling
    signal(SIGHUP, sig_handler);
    signal(SIGINT, sig_handler);
    signal(SIGQUIT, sig_handler);
    signal(SIGABRT, sig_handler);
    
    /*SOCKET OPERATIONS*/
    src_addr = {AF_INET, htons(port), {src_ip_addr}};
    
    //Open Socket
    sock_fd = socket(AF_INET, SOCK_DGRAM, 0);
    //Bind Socket
    if (bind(sock_fd, (const struct sockaddr*) &src_addr, sizeof(src_addr)) < 0) {
        perror("Bind failed");
        bail(EXIT_FAILURE);
    }
    //Set Receive Timeout
    const struct timeval tv = {.tv_usec = 1000}; //1 ms
    setsockopt(sock_fd, SOL_SOCKET, SO_RCVTIMEO, &tv, sizeof(tv));
    
    char buffer[BUFFER_SIZE];
    uint64_t* p = (uint64_t*) buffer;
    socklen_t addrlen;
    struct sockaddr_in src;
    int n;
    
    printf("Entering Main loop\n");
    
    while(true){
        //Receive Packet
        n = recvfrom(sock_fd, buffer, BUFFER_SIZE, MSG_TRUNC, (struct sockaddr*) &src, &addrlen);
        if (n < 0){
            // Error
            if (errno != EAGAIN && errno != EWOULDBLOCK) {
                perror("rcvfrom() error");
            }
            continue;
        }
        //Packet not relevant
        if (src.sin_family != src_addr.sin_family  && src.sin_port != src_addr.sin_port && n >= 8 && p[0] != MAGIC){
            continue;
        }
        //Sent Packet Back
        if (sendto(sock_fd, buffer, n, 0, (const struct sockaddr*) &src, addrlen) < 0) {
            perror("sendto failed");
        }
    }
    
}

void bail(int code){
    //Close Sockets
    if (sock_fd != -1) {
        close(sock_fd);
    }
    //DONE
    exit(code);
}

void sig_handler(int signum){
    fprintf(stderr, "Received Signal %d.\n Exiting...\n", signum);
    bail(EXIT_SUCCESS);
}

void usage(char* name){
    printf("USAGE: %s SRC_ADDRESS PORT\n", name);
    printf("SRC_ADDRESS     The local source IPv4 Address from which to send and listen to (In x.x.x.x format)\n");
    printf("PORT            The port number of the Addresses\n");
    
    bail(EXIT_SUCCESS);
}