#include <arpa/inet.h>
#include <netinet/in.h>
#include <stdio.h>
#include <sys/types.h>
#include <sys/socket.h>
#include <unistd.h>
#include <stdlib.h>
#include <string.h>
#include <sys/select.h>
#include <errno.h>
#include <fcntl.h>
#include <signal.h>

char buf[4096];
int alarm_time = 60;

void init_address(struct sockaddr_in *sa, const char* host, int port) {
    memset((char *) sa, 0, sizeof(*sa));
    sa->sin_family = AF_INET;
    sa->sin_port = htons(port);
    if (inet_aton(host, &sa->sin_addr)==0) {
        perror("inet_aton");
    }
}

int s1;
int s2;
struct sockaddr_in peer1;
struct sockaddr_in peer2;

int udp_socket() {
    int s2;
    struct sockaddr_in si_me;

    if ((s2=socket(AF_INET, SOCK_DGRAM, IPPROTO_UDP))==-1) {
        perror("socket");
        return -1;
    }
 
    // si_me stores our local endpoint. Remember that this program
    // has to be run in a network with UDP endpoint previously known
    // and directly accessible by all clients. In simpler terms, the
    // server cannot be behind a NAT.
    memset((char *) &si_me, 0, sizeof(si_me));
    si_me.sin_family = AF_INET;
    si_me.sin_port = 0;
    si_me.sin_addr.s_addr = htonl(INADDR_ANY);
    if (bind(s2, (struct sockaddr*)(&si_me), sizeof(si_me))==-1) {
        perror("bind");
        return -1;
    }
    return s2;
}

void alarm_handler() {
    sendto(s1, "", 0, 0, (struct sockaddr*)(&peer1), sizeof(peer1));
    //sendto(s2, "", 0, 0, (struct sockaddr*)(&peer2), sizeof(peer2));
    alarm(alarm_time);    
}

int main(int argc, char* argv[])
{
    if (argc<3) {
        fprintf(stderr, "Usage: udppair_client host1 port1 host2 port2\n");
        return 1;
    }
    char* host1 = argv[1];
    int port1 = atoi(argv[2]);
    char* host2 = argv[3];
    int port2 = atoi(argv[4]);

    if(getenv("UDPPAIR_PERIOD")) {
        alarm_time = atoi(getenv("UDPPAIR_PERIOD"));
    }

    s1 = udp_socket();
    s2 = udp_socket();

    init_address(&peer1, host1, port1);
    init_address(&peer2, host2, port2);

    fcntl(s1, F_SETFL, O_NONBLOCK);
    fcntl(s2, F_SETFL, O_NONBLOCK);


    struct sigaction siga = {{alarm_handler}};
    sigaction(SIGALRM, &siga, NULL);
    alarm_handler();

    fd_set rfds;

    for(;;) {
select_repeat:
        FD_ZERO(&rfds);
        FD_SET(s1, &rfds);
        FD_SET(s2, &rfds);
        int maxfd = (s1>s2)?s1:s2;
        int ret = select(maxfd+1, &rfds, NULL, NULL, NULL);

        if(ret==-1) {
            if(errno==EINTR  || errno==EAGAIN) goto select_repeat;
            perror("select");
            return 2;
        }
        
        if(FD_ISSET(s1, &rfds)) {
            socklen_t slen = sizeof(peer1);
            ret = recvfrom(s1, &buf, sizeof(buf), 0, (struct sockaddr*)(&peer1), &slen);
            if(ret==-1) {
                perror("recvfrom");
                return 4;
            }
            if(ret) {
                ret = sendto(s2, &buf, ret, 0, (struct sockaddr*)(&peer2), slen);
                if (!ret) {
                    perror("sendto");
                } else {
                    write(1, ">", 1);
                }
            }
        }
        if(FD_ISSET(s2, &rfds)) {
            socklen_t slen = sizeof(peer2);
            ret = recvfrom(s2, &buf, sizeof(buf), 0, (struct sockaddr*)(&peer2), &slen);
            if(ret==-1) {
                perror("recvfrom");
                return 4;
            }
            if(ret) {
                ret = sendto(s1, &buf, ret, 0, (struct sockaddr*)(&peer1), slen);
                if (!ret) {
                    perror("sendto");
                } else {
                    write(1, "<", 1);
                }
            }

        }
    }    
}