/*
 * Copyright (c) 2023–2024 DTB <trinity@trinity.moe>
 * SPDX-License-Identifier: AGPL-3.0-or-later
 *
 * This program is free software: you can redistribute it and/or modify it under
 * the terms of the GNU Affero General Public License as published by the Free
 * Software Foundation, either version 3 of the License, or (at your option) any
 * later version.
 * 
 * This program is distributed in the hope that it will be useful, but WITHOUT
 * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or FITNESS
 * FOR A PARTICULAR PURPOSE. See the GNU Affero General Public License for more
 * details.
 *
 * You should have received a copy of the GNU Affero General Public License
 * along with this program. If not, see https://www.gnu.org/licenses/.
 */

#include <signal.h> /* sigaction(2), signal(2), struct sigaction, SIGINT */
#include <stdio.h> /* fprintf(3), fgetc(3), perror(3), fputc(3), stderr, stdin,
                    * stdout, EOF, NULL */
#include <stdlib.h> /* exit(3), EXIT_FAILURE */
#include <sysexits.h> /* EX_IOERR, EX_OK, EX_USAGE */
#include <termios.h> /* tcgetattr(3), tcsetattr(3), struct termios, ECHO */
#include <unistd.h> /* getopt(3), isatty(3), pledge(2), unveil(2),
                     * STDIN_FILENO */

char *program_name = "peek";

/* Restores terminal echo; otherwise when a user ^Cs the terminal would
 * continue to not display typed text. If sig isn't zero, this will terminate
 * the program. */
static void
restore_echo(int sig) {
	static struct termios t;

	/* Failure isn't reported because this is the termination routine anyway;
	 * errors will be obvious. */
	if (tcgetattr(STDIN_FILENO, &t) == 0) {
		t.c_lflag |= ECHO;
		(void)tcsetattr(STDIN_FILENO, TCSAFLUSH, &t);
	}

	if (sig != 0) { exit(EXIT_FAILURE); } /* Terminated by signal. */

	return;
}

static int
ioerr(char *argv0) {
	perror(argv0);
	restore_echo(0);
	return EX_IOERR;
}

static int
usage(char *argv0) {
	(void)fprintf(stderr, "Usage: %s [-i]\n", argv0);

	return EX_USAGE;
}

int main(int argc, char *argv[]){
#ifdef __OpenBSD__
	if (pledge("stdio tty unveil", "") != 0 || unveil(NULL, NULL) != 0) {
		/* This isn't fatal; these return values could be cast to void just as
		 * easily. */
		(void)perror(argv[0] == NULL ? argv[0] : program_name);
	}
#endif

	if (argc > 0) { /* option parsing */
		char allow_nonterminals;
		int c;

		program_name = argv[0];

		allow_nonterminals = 0;
		while ((c = getopt(argc, argv, "i")) != -1) {
			switch (c) {
				case 'i':	allow_nonterminals = 1; break;
				default:	return usage(argv[0]);
			}
		}

		if (argc > optind) { return usage(argv[0]); }

		if (!allow_nonterminals && isatty(STDIN_FILENO) != 1) {
			(void)fprintf(
			    stderr,
			    "%s: Must be run in a terminal (option -i skips this check)\n",
			    argv[0]
			);
			return EX_USAGE;
		}
	}

	{ /* Install signal handler */
		/* There isn't a difference in functionality between the signal(2) and
		 * sigaction(2) methods. sigaction(2) is vastly preferred for
		 * portability but some configurations can only use signal(2). */
		/* Errors aren't terminating because the worst that happens is some
		 * terminal phooeyness if things go awry. */
#if defined _POSIX_C_SOURCE
		struct sigaction act = { 0 };

		act.sa_handler = restore_echo;
		if (sigaction(SIGINT, &act, NULL) != 0) { perror(program_name); }
#else
		if (signal(SIGINT, restore_echo) == SIG_ERR) { perror(program_name); }
#endif
	}

	/* Banish terminal echo; this terminates when it fails, because it's the
	 * whole point of the program. */
	{
		struct termios t;

		if (tcgetattr(STDIN_FILENO, &t) != 0) { return ioerr(program_name); }
		t.c_lflag ^= ECHO;
		if (tcsetattr(STDIN_FILENO, TCSAFLUSH, &t) != 0) {
			return ioerr(program_name);
		}
	}

	{ /* Input loop */
		int c;

		while ((c = fgetc(stdin)) != EOF) {
			if (fputc(c, stdout) == EOF) { return ioerr(program_name); }
		}
	}

	restore_echo(0);

	return EX_OK;
}