#include <grp.h>
#include <pwd.h>
#include <stdio.h>
#include <errno.h>
#include <errno.h>
#include <string.h>
#include <unistd.h>
#include <stdlib.h>
#include <limits.h>
#include <sys/types.h>

char g_flag;
char G_flag;
char n_flag;
char r_flag;
char u_flag;

int print_groups(const struct passwd *pwd, const char *fmt, const int flag) {
	gid_t groups[NGROUPS_MAX];
	int ngroups = NGROUPS_MAX;
	if (getgrouplist(pwd->pw_name, pwd->pw_gid, groups, &ngroups) < 0)
		return 1;

	for (int i = 0; i < ngroups; i++) {
		struct group *grp = getgrgid(groups[i]);
		if (grp && !n_flag)
			printf("%u", groups[i]);

		if ((!r_flag && n_flag) || flag)
			printf(fmt, grp->gr_name);

		if (g_flag && i == 0)
			break;

		putchar(' ');
	}

	return 0;
}

int ids(uid_t uid, struct passwd *pwd) {
	if (u_flag) {
		if (n_flag)
			printf("%s", pwd->pw_name);

		else
			printf("%u", uid);
	}

	else if (G_flag || g_flag)
		print_groups(pwd, "%s", 0);

	putchar('\n');
	return 0;
}

int def_ids(uid_t uid, struct passwd *pwd) {
	printf("uid=%d(%s) gid=%d", uid, pwd->pw_name, pwd->pw_gid);
	struct group *grp = getgrgid(pwd->pw_gid);
	if (grp)
		printf("(%s)", grp->gr_name);

	printf(" groups=");
	print_groups(pwd, "(%s)", 1);

	putchar('\n');
	return 0;
}

void usage(int sig) {
	puts("id [gGurn] [user]\n\t-u User ID\n\t-g Group ID\n\t-G Supplementary group IDs\n\t-n Print names instead of numbers\n\t-r Print real ID instead of effective ID");
	exit(sig);
}

int main(int argc, char **argv) {
	int opt;
	while ((opt = getopt(argc, argv, "gGnru")) != -1) {
		switch (opt) {
			case 'g':
				if (G_flag || u_flag)
					usage(1);

				g_flag = 1;
				break;

			case 'G':
				if (g_flag || u_flag)
					usage(1);

				G_flag = 1;
				break;

			case 'u':
				if (G_flag || g_flag)
					usage(1);

				u_flag = 1;
				break;

			case 'r':
				r_flag = 1;
				break;

			case 'n':
				n_flag = 1;
				break;

			default:
				usage(0);
		}
	}

	argv += optind;
	argc -= optind;

	uid_t uid = getuid();
	struct passwd *pwd = getpwuid(uid);
	if (argv[0] != NULL)
		pwd = getpwnam(argv[0]);

	if (!pwd) {
		fprintf(stderr, "id: %s\n", strerror(errno));
		return 1;
	}

	if (u_flag || G_flag || g_flag)
		return ids(uid, pwd);

	return def_ids(uid, pwd);
}