#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <errno.h>
#include <unistd.h>
#include <arpa/inet.h>
#include <event2/event.h>

#define PORT 9999
#define BUF_SIZE 1024

void on_read(evutil_socket_t fd, short events, void *arg) {
  char buf[BUF_SIZE];
  struct sockaddr_in client_addr;
  socklen_t client_len = sizeof(client_addr);
  ssize_t len;

  // 1. 读取数据包
  // 注意：UDP 是保留消息边界的，一次 recvfrom 对应一个包
  len = recvfrom(fd, buf, BUF_SIZE - 1, 0, 
  (struct sockaddr *)&client_addr, &client_len);

  if (len < 0) {
  perror("recvfrom");
  return;
  }

  buf[len] = '\0';
  printf("Received %zd bytes from %s:%d: %s\n", 
  len, 
  inet_ntoa(client_addr.sin_addr), 
  ntohs(client_addr.sin_port), 
  buf);

  // 2. 回复数据 (Echo)
  sendto(fd, buf, len, 0, (struct sockaddr *)&client_addr, client_len);
}

int main() {
  struct event_base *base;
  struct event *udp_event;
  evutil_socket_t fd;
  struct sockaddr_in sin;

  // 1. 初始化 Libevent
  base = event_base_new();
  if (!base) return 1;

  // 2. 创建 UDP Socket
  fd = socket(AF_INET, SOCK_DGRAM, 0);
  if (fd < 0) {
  perror("socket");
  return 1;
  }

  // 设置为非阻塞 (这是 Libevent 的要求)
  evutil_make_socket_nonblocking(fd);

  // 3. 绑定端口
  memset(&sin, 0, sizeof(sin));
  sin.sin_family = AF_INET;
  sin.sin_addr.s_addr = INADDR_ANY;
  sin.sin_port = htons(PORT);

  if (bind(fd, (struct sockaddr *)&sin, sizeof(sin)) < 0) {
  perror("bind");
  return 1;
  }

  // 4. 创建并添加事件
  // 监听 EV_READ，并设置 EV_PERSIST 保持持续监听
  udp_event = event_new(base, fd, EV_READ | EV_PERSIST, on_read, NULL);
  event_add(udp_event, NULL);

  printf("UDP Server listening on port %d\n", PORT);

  // 5. 进入事件循环
  event_base_dispatch(base);

  // 清理
  event_free(udp_event);
  evutil_closesocket(fd);
  event_base_free(base);
  return 0;
}

int opt = 1;
setsockopt(fd, SOL_SOCKET, SO_REUSEPORT, &opt, sizeof(opt));
// 多个线程都执行这段 bind 代码
bind(fd, ...);