#include <sys/socket.h>
#include <netinet/in.h>
#include <unistd.h>
#include <string.h>

int tcp_server(uint16_t port) {
    int lfd = socket(AF_INET6, SOCK_STREAM, 0);

    /* SO_REUSEADDR: allow bind to port even if in TIME_WAIT */
    int opt = 1;
    setsockopt(lfd, SOL_SOCKET, SO_REUSEADDR, &opt, sizeof(opt));
    /* SO_REUSEPORT: multiple processes can bind same port (load balance) */
    setsockopt(lfd, SOL_SOCKET, SO_REUSEPORT, &opt, sizeof(opt));

    struct sockaddr_in6 addr = {0};
    addr.sin6_family = AF_INET6;
    addr.sin6_port   = htons(port);
    addr.sin6_addr   = in6addr_any;   /* :: = any interface */

    bind(lfd, (struct sockaddr *)&addr, sizeof(addr));
    listen(lfd, 128);  /* backlog: max pending connections in accept queue */

    while (1) {
        struct sockaddr_in6 client;
        socklen_t clen = sizeof(client);
        int cfd = accept(lfd, (struct sockaddr *)&client, &clen);
        /* cfd is a NEW socket for this connection; lfd still listens */

        /* Handle client — in production: fork() or thread */
        handle_client(cfd);
        close(cfd);
    }
}

void handle_client(int fd) {
    char buf[4096];
    ssize_t n;
    /* CRITICAL: recv may return LESS than requested — MUST loop */
    while ((n = recv(fd, buf, sizeof(buf), 0)) > 0) {
        /* n bytes received; process buf[0..n-1] */
        send(fd, buf, n, 0);  /* echo back */
        /* send may also return less than n → must loop send too */
    }
    /* n == 0: peer closed connection (FIN received) */
    /* n == -1: error (check errno: EAGAIN, ECONNRESET, etc.) */
}

/* TCP client */
int tcp_connect(const char *host, uint16_t port) {
    struct addrinfo hints = {0}, *res;
    hints.ai_family   = AF_UNSPEC;
    hints.ai_socktype = SOCK_STREAM;
    char portstr[8]; snprintf(portstr, sizeof(portstr), "%u", port);
    getaddrinfo(host, portstr, &hints, &res);

    int fd = socket(res->ai_family, res->ai_socktype, 0);
    connect(fd, res->ai_addr, res->ai_addrlen);
    freeaddrinfo(res);
    return fd;
}