/*
 * 02_epoll_lt_server.c —— epoll LT (水平触发) 模式 echo 服务器
 *
 * 编译: gcc 02_epoll_lt_server.c -o lt_srv
 * 运行: ./lt_srv 9001
 * 测试: nc localhost 9001
 *
 * LT 模式特点：
 *   - 不需要 fd 设非阻塞
 *   - 不需要循环 read 到 EAGAIN（少读一次也没事，下次 epoll_wait 还会通知）
 *   - 写法接近 select/poll，最容易上手
 */

#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 <arpa/inet.h>
#include <errno.h>

#define MAX_EVENTS 64
#define BUF_SIZE   1024

static int make_listen_socket(int port) {
    int fd = socket(AF_INET, SOCK_STREAM, 0);
    int yes = 1; setsockopt(fd, SOL_SOCKET, SO_REUSEADDR, &yes, sizeof(yes));
    struct sockaddr_in addr = {0};
    addr.sin_family = AF_INET;
    addr.sin_port = htons(port);
    addr.sin_addr.s_addr = htonl(INADDR_ANY);
    if (bind(fd, (struct sockaddr*)&addr, sizeof(addr)) < 0 || listen(fd, 64) < 0) {
        perror("bind/listen"); exit(1);
    }
    printf("🚀 epoll [LT] 服务器已启动，端口 %d\n", port);
    return fd;
}

int main(int argc, char *argv[]) {
    int port = (argc > 1) ? atoi(argv[1]) : 9001;
    int lfd  = make_listen_socket(port);

    int epfd = epoll_create1(EPOLL_CLOEXEC);

    /* 注册监听 socket（LT 模式不加 EPOLLET） */
    struct epoll_event ev = {.events = EPOLLIN, .data.fd = lfd};
    epoll_ctl(epfd, EPOLL_CTL_ADD, lfd, &ev);

    struct epoll_event events[MAX_EVENTS];
    long wait_count = 0;

    while (1) {
        int n = epoll_wait(epfd, events, MAX_EVENTS, -1);
        if (n < 0) {
            if (errno == EINTR) continue;
            perror("epoll_wait"); break;
        }
        wait_count++;
        printf("🔔 [LT] epoll_wait 第 %ld 次返回, 就绪 %d 个 fd\n", wait_count, n);

        for (int i = 0; i < n; i++) {
            int fd = events[i].data.fd;
            uint32_t evs = events[i].events;

            if (fd == lfd) {
                /* 新连接 */
                struct sockaddr_in cli;
                socklen_t cl = sizeof(cli);
                int conn = accept(lfd, (struct sockaddr*)&cli, &cl);
                if (conn >= 0) {
                    ev.events  = EPOLLIN | EPOLLRDHUP;     /* LT 模式 */
                    ev.data.fd = conn;
                    epoll_ctl(epfd, EPOLL_CTL_ADD, conn, &ev);
                    printf("    ➕ 新客户端 fd=%d, 来自 %s:%d\n",
                           conn, inet_ntoa(cli.sin_addr), ntohs(cli.sin_port));
                }
            } else {
                if (evs & (EPOLLERR | EPOLLHUP | EPOLLRDHUP)) {
                    printf("    🗑  fd=%d 错误/挂断, 关闭\n", fd);
                    close(fd);
                    /* close 会自动从 epoll 摘除，无需 EPOLL_CTL_DEL */
                    continue;
                }
                if (evs & EPOLLIN) {
                    char buf[BUF_SIZE];
                    /* LT 模式：读一次就行，剩下的下次 epoll_wait 还会通知 */
                    ssize_t r = read(fd, buf, sizeof(buf));
                    if (r <= 0) {
                        if (r == 0) printf("    👋 fd=%d 对端关闭\n", fd);
                        close(fd);
                    } else {
                        write(fd, buf, r);
                        printf("    📨 fd=%d echo %ld 字节\n", fd, r);
                    }
                }
            }
        }
    }
    close(epfd);
    close(lfd);
    return 0;
}