#include <iostream>
#include <iomanip>  //for the iomanipulator setw
#include <cstdlib>
#include <string>   //for class string
using namespace std;

int n {10};    //at most n-1 people can be on line simultaneously
int f(int i);  //The function declaration announces the name of the function.

//if f(vp + 1) == qp, the queue is empty.
//if f(qp) == vp, the queue is full.

int main()
{
	string a[n];  //The queue is stored in an array of n strings.

	int qp {0};   //"queue pointer": subscript of 1st unoccupied element
	int vp {n-1}; //"vacated pointer": subscript of most recently freed el

	cout    << "Type a name (one word) to put that person on line.\n"
		<< "Type serve to serve the one who's been waiting longest.\n"
		<< "Type list to print the queue.\n"
		<< "Type exit to exit the program.\n";

	for (;;) {   //Infinite loop, until we break out below.
		cout << "\nGo ahead: ";
		string word;
		cin >> word;

		if (word == "exit") {
			cout << "Goodbye.\n";
			break;   //out of the for loop.
		} else if (word == "list") {
			int j {1};   //number the people who are waiting
			for (int i {f(vp+1)}; i != qp; i = f(++i)) {
				cout << setw(2) << j << " " << a[i];
				if (f(vp + 2) != qp) { //queue contains > 1
					if (i == f(vp+1)) {
						cout << " (oldest)";
					} else if (i == f(qp - 1)) {
						cout << " (newest)";
					}
				}
				++j;
				cout << "\n";
			}
		} else if (word == "serve") {
			if (f(vp + 1) == qp) {
				cerr << "Sorry, the queue is already empty.\n";
			} else {
				cout << "Serving " << a[f(vp + 1)] << ".\n";
				vp = f(++vp);   //End by making the queue smaller.
			}
		} else {   //If none of the above, assume the word is a name.
			if (f(qp) == vp) {
				cerr << "Sorry, the queue aleady contains "
					<< n-1 << " people.\n";
			} else {
				qp = f(++qp);   //Begin by making the queue bigger.
				cout << "Puting " << word << " online.\n";
				a[f(qp-1)] = word;
			}
		}
	}

	return EXIT_SUCCESS;   //Arrive here when the user types exit.
}

//This function takes an arbitrary integer i of any size, positive or negative,
//and returns an integer in the range 0 to n-1 inclusive.
//It would be legal to use this number i as a subscript in the array a.

int f(int i)   //The function definition creates the function.
{
	i %= n;         //means i = i % n

	if (i < 0) {    //If i is negative,
		i += n; //make it non-negative.
	}

	return i;
}