#define _XOPEN_SOURCE
#include <stdio.h>
#include <string.h>
#include <unistd.h>
#include <stdlib.h>
#include <errno.h>
#include <time.h>

const char *fmts[] = {
	"%R",
	"%T",
	"%d-%m",
	"%d-%m %H",
	"%d-%m %R",
	"%d-%m %T",
	"%d-%m-%Y",
	"%d-%m-%Y %H",
	"%d-%m-%Y %R",
	"%d-%m-%Y %T"
};

time_t parse_date(char *str) {
	time_t local = time(NULL);
	struct tm *tm = localtime(&local);

	char flag = 0;
	for (size_t i = 0; i < sizeof(fmts) / sizeof(char *); i++) {
		char *res = strptime(str, fmts[i], tm);
		if (res && *res == '\0') {
			flag = 1;
			break;
		}
	}

	if (flag == 0) {
		fprintf(stderr, "date: parsing: invalid format\n");
		exit(1);
	}

	time_t rt = mktime(tm);
	if (rt < 0) {
		if (rt == -1)
			fprintf(stderr, "date: parsing: %s\n", strerror(errno));

		else
			fprintf(stderr, "date: parsing: invalid date\n");

		exit(1);
	}

	return rt;
}

int main(int argc, char **argv) {
	time_t t = time(NULL);
	char *fmt = "%a %b %e %H:%M:%S %Z %Y";

	/* For -s flag */
	struct timespec ts;

	int opt;
	while ((opt = getopt(argc, argv, "s:d:u")) != -1) {
		switch (opt) {
			case 's':
				ts.tv_sec = parse_date(optarg);
				if (clock_settime(CLOCK_REALTIME, &ts) < 0) {
					fprintf(stderr, "date: settime: %s\n", strerror(errno));
					return 1;
				}

				return 0;

			case 'd':
				t = parse_date(optarg);
				break;

			case 'u':
				if (setenv("TZ", "UTC0", 1) < 0) {
					fprintf(stderr, "date: %s\n", strerror(errno));
					return 1;
				}

				break;

			default:
				puts("date [rsdu] [+\"fmt\"]\n\t-s TIME Set new date\n\t-d TIME Print new date\n\t-u Work in UTC\n\nRecognized TIME formats:");
				for (size_t i = 0; i < sizeof(fmts) / sizeof(char *); i++)
					puts(fmts[i]);

				return 0;
		}
	}

	argv += optind;
	argc -= optind;

	if (argc != 0)
		if (argv[0][0] == '+')
			fmt = argv[0] + 1;

	struct tm *tm = localtime(&t);

	char buf[256];
	strftime(buf, sizeof(buf), fmt, tm);

	puts(buf);
	return 0;
}