#include<unistd.h>
#include<time.h>
#include<stdio.h>
#include<fcntl.h>
#include<stdlib.h>
#include<vector>
#include<time.h>
#include<string>
#include<string.h>
using namespace std;

char* getoutput(const char* command);

vector<string> get_userlist(){
	char* userlist = getoutput("who | cut -d ' ' -f 1 | sort -u");
	char* current_beginning = userlist;
	vector<string> list;
	for(int i = 0; userlist[i]; i++)
		if(userlist[i] == '\n'){
			userlist[i] = 0;
			list.push_back(current_beginning);
			current_beginning = userlist + i;
			while(!isalpha(*current_beginning)){
				current_beginning++;
				if(!*current_beginning)
					break;
			}
		}
	free(userlist);	
	return list;
}

bool is_in_vector(vector<string> haystack, string target){
	for(auto s : haystack)
		if(s == target)
			return true;
	return false;
}

int main(){
	pid_t pid = fork();
	if(pid)
		return 0;
	setsid();
	int logfd = open("daemon_log", O_WRONLY | O_CREAT | O_APPEND, 0644);
	if(logfd == -1){
		perror("open");
		return 1;
	}
	dup2(logfd, 1);
	vector<string> userlist = get_userlist();
	while(1){
		vector<string> new_userlist = get_userlist();
		time_t now = time(0);
		char *nowstr = ctime(&now);
		nowstr[strlen(nowstr) - 1] = 0;
		for(string &new_user : new_userlist)
			if(!is_in_vector(userlist, new_user))
				printf("New User:  %s (%s)\n", new_user.c_str(), nowstr);
		for(string &old_user : userlist)
			if(!is_in_vector(new_userlist, old_user))
				printf("User logged out:  %s (%s)\n", old_user.c_str(), nowstr);
		userlist = new_userlist;
		fflush(stdout);
		sleep(1);
	}
	
	return 0;
}

char* getoutput(const char* command){
	int pipe_fds[2];
	char *buffer = (char*)malloc(16);
	pipe(pipe_fds);
	pid_t child = fork();

	if(!child){
		dup2(pipe_fds[1], 1);
		execl("/bin/sh", "sh", "-c", command, (char *) 0);
		perror("exec");
		exit(1);
	} else {
		size_t sofar = 0;
		close(pipe_fds[1]);
		for(;;) {
			ssize_t readlen = read(pipe_fds[0], buffer + sofar, 16);
			if(readlen <= 0)
				break;
			sofar += readlen;
			buffer = (char*)realloc(buffer, sofar + 16);
		}
		buffer[sofar] = 0;
		return buffer;
	}
}