// Initially based on https://stackoverflow.com/a/35570418

#include <errno.h>
#include <fcntl.h>
#include <netinet/in.h>
#include <poll.h>
#include <stdbool.h>
#include <stdint.h>
#include <stdio.h>
#include <string.h>
#include <sys/socket.h>
#include <unistd.h>

enum frame_type {
    FT_NONE = 0,
    FT_HELLO = 42,
};

struct frame {
    uint32_t addr;
    uint16_t port;
    enum frame_type type;
};

static unsigned char udp_buffer[UINT16_MAX];

static int ReceiveFrame(int const fd, struct frame *const frame)
{
    frame->type = FT_NONE;
    struct sockaddr_in source_address;
    socklen_t source_address_len = sizeof(source_address);
    int const length = recvfrom(
            fd,
            udp_buffer,
            sizeof(udp_buffer) - 1,
            0,
            (struct sockaddr*)&source_address,
            &source_address_len);
    if (length == 0) {
        return -EWOULDBLOCK;
    }
    if (length < 0) {
        int const err = errno;
        if (err != EWOULDBLOCK) {
            perror("recvfrom failed");
        }
        return -err;
    }
    uint32_t const addr = ntohl(source_address.sin_addr.s_addr);
    uint32_t const port = ntohs(source_address.sin_port);
    if (length) {
        unsigned char frame_type = udp_buffer[0];
        if (frame_type == FT_HELLO) {
            *frame = (struct frame){
                .addr = addr,
                .port = port,
                .type = frame_type,
            };
        }
    }
    uint8_t const a0 = addr >> 24;
    uint8_t const a1 = addr >> 16;
    uint8_t const a2 = addr >> 8;
    uint8_t const a3 = addr;
    printf("Received %u from %u.%u.%u.%u:%u\n", udp_buffer[0], a0, a1, a2, a3, port);
    return 0;
}

static int SendFrame(int const fd, struct frame const *const frame)
{
    udp_buffer[0] = frame->type;
    struct sockaddr_in const addr = {
        .sin_family = AF_INET,
        .sin_port = htons(frame->port),
        .sin_addr.s_addr = htonl(frame->addr),
    };
    ssize_t const ret = sendto(
            fd, udp_buffer, 1, 0, (struct sockaddr const *)&addr, sizeof(addr));
    if (ret < 0) {
        int const err = errno;
        perror("sendto failed");
        return -err;
    }
    printf("message sent\n");
    return 0;
}

int main(void)
{
    int const fd = socket(AF_INET, SOCK_DGRAM, 0);
    if (fd < 0) {
        perror("socket failed");
        return 1;
    }
    fcntl(fd, F_SETFL, fcntl(fd, F_GETFL, 0) | O_NONBLOCK);
    struct sockaddr_in const serveraddr = {
        .sin_family = AF_INET,
        .sin_port = htons(50037u),
        .sin_addr.s_addr = htonl(INADDR_ANY),
    };
    if (bind(fd, (struct sockaddr *)&serveraddr, sizeof(serveraddr)) < 0) {
        perror("bind failed");
        return 1;
    }
    struct pollfd fds[1] = { { .fd = fd, .events = POLLIN, }, };
    for (;;) {
        int const timeout_msecs = 500;
        int const pollret = poll(fds, sizeof(fds)/sizeof(*fds), timeout_msecs);
        if (pollret > 0) {
            if (fds[0].revents & POLLIN) {
                while (1) {
                    struct frame frame = { .type = FT_NONE, };
                    int const ret = ReceiveFrame(fd, &frame);
                    if (ret) {
                        break;
                    }
                    if (frame.type == FT_HELLO) {
                        SendFrame(fd, &frame);
                    }
                }
            }
            if (fds[0].revents & POLLRDNORM) {
                printf("POLLRDNORM, ignoring...\n");
            }
            if (fds[0].revents & POLLRDBAND) {
                printf("POLLRDBAND, ignoring...\n");
            }
            if (fds[0].revents & POLLPRI) {
                printf("POLLPRI, ignoring...\n");
            }
            if (fds[0].revents & POLLPRI) {
                printf("POLLPRI, ignoring...\n");
            }
            if (fds[0].revents & POLLHUP) {
                printf("POLLHUP, ignoring...\n");
            }
            if (fds[0].revents & POLLERR) {
                printf("POLLERR, ignoring...\n");
            }
            if (fds[0].revents & POLLNVAL) {
                printf("POLLNVAL, exiting...\n");
                return 1;
            }
        }
    }
    close(fd);
}