#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <unistd.h>
#include <sys/epoll.h>
#include <sys/socket.h>
#include <netinet/in.h>
#include <fcntl.h>
#include <errno.h>

#define PORT 8080
#define MAX_EVENTS 64
#define BUFFER_SIZE 1024

void set_nonblocking(int32_t fd)
{
	int32_t flags = fcntl(fd, F_GETFL, 0);
	fcntl(fd, F_SETFL, flags | O_NONBLOCK);
}

int32_t main(void)
{
	int32_t server_fd = socket(AF_INET, SOCK_STREAM, 0);
	int32_t opt = 1;
	setsockopt(server_fd, SOL_SOCKET, SO_REUSEADDR, &opt, sizeof(opt));

	struct sockaddr_in address = {
		.sin_family = AF_INET,
		.sin_addr.s_addr = INADDR_ANY,
		.sin_port = htons(PORT)
	};
	bind(server_fd, (struct sockaddr *)&address, sizeof(address));
	listen(server_fd, SOMAXCONN);

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

	int32_t epoll_fd = epoll_create1(0);
	if (epoll_fd < 0) {
		perror("epoll_create1 failed");
		exit(EXIT_FAILURE);
	}

	struct epoll_event ev;
	ev.events = EPOLLIN;
	ev.data.fd = server_fd;
	if (epoll_ctl(epoll_fd, EPOLL_CTL_ADD, server_fd, &ev) < 0) {
		perror("epoll_ctl: server_fd failed");
		exit(EXIT_FAILURE);
	}

	struct epoll_event events[MAX_EVENTS];
	char buffer[BUFFER_SIZE];

	while (1) {
		int32_t nfds = epoll_wait(epoll_fd, events, MAX_EVENTS, -1);
		if (nfds < 0) {
			perror("epoll_wait failed");
			break;
		}

		for (int32_t i = 0; i < nfds; i++) {
			if (events[i].data.fd == server_fd) {
				struct sockaddr_in client_addr;
				socklen_t client_len = sizeof(client_addr);

				int32_t client_fd = accept(server_fd, (struct sockaddr *)&client_addr, &client_len);
				if (client_fd < 0) {
					perror("accept failed");
					continue;
				}

				printf("connection accepted (client fd: %d)\n", client_fd);
				set_nonblocking(client_fd);

				ev.events = EPOLLIN | EPOLLET;
				ev.data.fd = client_fd;
				if (epoll_ctl(epoll_fd, EPOLL_CTL_ADD, client_fd, &ev) < 0) {
					perror("epoll_ctl: client_fd failed");
					close(client_fd);
				}
			} else {
				int32_t client_fd = events[i].data.fd;

				while (1) {
					ssize_t bytes_read = read(client_fd, buffer, sizeof(buffer));

					if (bytes_read > 0) {
						write(client_fd, buffer, bytes_read);
					} else if (bytes_read == 0) {
						printf("client on fd %d closed connection.\n", client_fd);
						epoll_ctl(epoll_fd, EPOLL_CTL_DEL, client_fd, NULL);
						close(client_fd);
						break;
					} else {
						if (errno == EAGAIN || errno == EWOULDBLOCK) {
							break;
						}

						perror("read error");
						epoll_ctl(epoll_fd, EPOLL_CTL_DEL, client_fd, NULL);
						close(client_fd);
						break;
					}
				}
			}
		}
	}

	close(epoll_fd);
	close(server_fd);
	return 0;
}