#include <pwd.h>
#include <stdio.h>
#include <errno.h>
#include <string.h>
#include <unistd.h>
#include <stdlib.h>
#include <sys/resource.h>

int renice(int which, int who, int adj) {
	adj += getpriority(which, who);

	if (setpriority(which, who, adj)) {
		fprintf(stderr, "renice: %s\n", strerror(errno));
		return 1;
	}

	return 0;
}

int main(int argc, char **argv) {
	int adj = 10;
	int which = PRIO_PROCESS;

	int opt;
	while ((opt = getopt(argc, argv, "n:gup")) != -1) {
		switch (opt) {
			case 'n':
				adj = atoi(optarg);
				break;

			case 'g':
				which = PRIO_PGRP;
				break;

			case 'u':
				which = PRIO_USER;
				break;

			case 'p':
				which = PRIO_PROCESS;
				break;

			default:
				printf("renice [ngup] [id1 id2...]\n\t-n N Add N to the niceness\n\t-g Process group ids\n\t-u Process user names\n\t-p Process ids\n");
				return 0;
		}
	}

	argv += optind;
	argc -= optind;

	int who = 0;
	int ret = 0;
	for (int i = 0; i < argc; i++) {
		if (which == PRIO_USER) {
			struct passwd *pw = getpwnam(argv[i]);
			if (pw == 0) {
				fprintf(stderr, "renice: %s\n", strerror(errno));
				return 1;
			}

			who = pw->pw_uid;
		}

		else
			who = atoi(argv[i]);

		if (renice(which, who, adj))
			ret = 1;
	}

	return ret;
}