/*
The MIT License (MIT)

Copyright © 2020-2025 pacman64

Permission is hereby granted, free of charge, to any person obtaining a copy of
this software and associated documentation files (the “Software”), to deal
in the Software without restriction, including without limitation the rights to
use, copy, modify, merge, publish, distribute, sublicense, and/or sell copies
of the Software, and to permit persons to whom the Software is furnished to do
so, subject to the following conditions:

The above copyright notice and this permission notice shall be included in all
copies or substantial portions of the Software.

THE SOFTWARE IS PROVIDED “AS IS”, WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE
SOFTWARE.
*/

/*
You can build this command-line app by running

cc -Wall -s -O2 -o ./tcpecho ./tcpecho.c
*/

#include <arpa/inet.h>
#include <fcntl.h>
#include <netinet/in.h>
#include <stdbool.h>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <sys/socket.h>
#include <sys/types.h>
#include <unistd.h>

#ifdef _WIN32
#include <windows.h>
#endif

// info is the message shown when this app is given any of its help options
const char* info = ""
"tcpecho [port] [words...]\n"
"\n"
"\n"
"Start a TCP server which always replies with a plain-text line made from\n"
"the arguments given after the port number, joined by single spaces.\n"
"\n"
"\n"
"Options, all of which can start with either 1 or 2 dashes:\n"
"\n"
"\n"
"  -h          show this help message\n"
"  -help       show this help message\n"
"";

// is_help_option simplifies control-flow for func main
bool is_help_option(char* s) {
    return (s[0] == '-') && (
        strcmp(s, "-h") == 0 || strcmp(s, "-help") == 0 ||
        strcmp(s, "--h") == 0 || strcmp(s, "--help") == 0
    );
}

int main(int argc, char** argv) {
#ifdef _WIN32
    setmode(fileno(stdin), O_BINARY);
    // ensure output lines end in LF instead of CRLF on windows
    setmode(fileno(stdout), O_BINARY);
    setmode(fileno(stderr), O_BINARY);
#endif

    // handle any of the help options, if given
    if (argc > 1 && is_help_option(argv[1])) {
        puts(info);
        return 0;
    }

    if (argc < 2) {
        char* msg = "you forgot the port number as the leading argument";
        fprintf(stderr, "\x1b[31m%s\x1b[0m\n", msg);
        return 1;
    }

    char* end;
    int port = strtol(argv[1], &end, 10);
    if (*end != 0 || port < 1 || port > 65535) {
        char* msg;
        msg = "isn't a valid port number";
        fprintf(stderr, "\x1b[31m%s %s\x1b[0m\n", argv[1], msg);
        msg = "maybe you forgot the port number as the leading argument";
        fprintf(stderr, "\x1b[31m%s\x1b[0m\n", msg);
        return 1;
    }

    // count spaces between words
    size_t msglen = argc - 2;
    // count word lengths
    for (size_t i = 2; i < argc; i++) {
        msglen += strlen(argv[i]);
    }
    // remember the final line-feed
    msglen++;

    // also remember that strings are ended with an extra null byte
    char* msg = malloc(msglen + 1);
    if (msg == NULL) {
        const char* m = "can't get enough memory for the response message";
        fprintf(stderr, "\x1b[31m%s\x1b[0m\n", m);
        return 1;
    }

    for (size_t i = 2, pos = 0; i < argc; i++) {
        if (i > 2) {
            msg[pos] = ' ';
            pos++;
        }

        for (size_t j = 0; argv[i][j] != 0; j++) {
            msg[pos] = argv[i][j];
            pos++;
        }
    }

    msg[msglen - 1] = '\n';
    msg[msglen] = 0;

    struct sockaddr_in srv;
    memset(&srv, 0, sizeof(srv));

    int fd = socket(AF_INET, SOCK_STREAM, 0);
    srv.sin_family = AF_INET;
    srv.sin_addr.s_addr = htonl(INADDR_ANY);
    srv.sin_port = htons(port);

    bind(fd, (struct sockaddr*)&srv, sizeof(srv));
    listen(fd, 1);

    while (true) {
        int conn = accept(fd, NULL, NULL);
        send(conn, msg, msglen, 0);
        close(conn);
    }

    free(msg);
    return 0;
}