#include <algorithm>
#include <iostream>
#include <string>
#include <vector>
#include <assert.h>
using namespace std;

void erase_from_vector(unsigned int i, vector<string>& v) {  
  /* EXERCISE: IMPLEMENT THIS FUNCTION */ 
}

// Enroll a student if there is room and the student is not already in course or on waiting list.
void enroll_student(const string& id, unsigned int max_students, 
		    vector<string>& enrolled, vector<string>& waiting) {
  // Check to see if the student is already enrolled. 
  unsigned int i;
  for (i=0; i < enrolled.size(); ++i) {
    if (enrolled[i] == id) {
      cout << "Student " << id << " is already enrolled." << endl;
      return;
    }
  }
  // If the course isn't full, add the student.
  if (enrolled.size() < max_students) {
    enrolled.push_back(id);
    cout << "Student " << id << " added.\n" 
	 << enrolled.size() << " students are now in the course." << endl;
    return;
  }  
  // Check to see if the student is already on the waiting list.
  for (i=0; i < waiting.size(); ++i) {
    if (waiting[i] == id) {
      cout << "Student " << id << " is already on the waiting list." << endl;
      return;
    }
  }  
  // If not, add the student to the waiting list.
  waiting.push_back(id);
  cout << "The course is full.  Student " << id << " has been added to the waiting list.\n" 
       << waiting.size() << " students are on the waiting list." << endl;
}

// Remove a student from the course or from the waiting list.  If removing the student from the 
// course opens up a slot, then the first person on the waiting list is placed in the course.
void remove_student(const string& id, unsigned int max_students, 
		    vector<string>& enrolled, vector<string>& waiting) {
  // Check to see if the student is on the course list.
  bool found = false;
  unsigned int loc=0;
  while (!found && loc < enrolled.size()) {
    found = enrolled[loc] == id;
    if (!found) ++loc;
  }
  if (found) {
    // Remove the student and see if a student can be taken from the waiting list.
    erase_from_vector(loc, enrolled);
    cout << "Student " << id << " removed from the course." << endl;
    if (waiting.size() > 0) {
      enrolled.push_back(waiting[0]);
      cout << "Student " << waiting[0] << " added to the course from the waiting list." << endl;
      erase_from_vector(0, waiting);
      cout << waiting.size() << " students remain on the waiting list." << endl;
    } else {
      cout << enrolled.size() << " students are now in the course." << endl;
    }
  } else {
    // Check to see if the student is on the waiting list
    found = false;
    loc = 0;
    while (!found && loc < waiting.size()) {
      found = waiting[loc] == id;
      if (!found) ++loc;
    }
    if (found) {
      erase_from_vector(loc, waiting);
      cout << "Student " << id << " removed from the waiting list.\n"
           << waiting.size() << " students remain on the waiting list." << endl;
    } else {
      cout << "Student " << id << " is in neither the course nor the waiting list" << endl;
    }
  }
}

int main() {
  // Read in the maximum number of students in the course
  unsigned int max_students;
  cout << "\nEnrollment program for CSCI 1200\nEnter the maximum number of students allowed\n";
  cin >> max_students;  

  // Initialize the vectors
  vector<string> enrolled;
  vector<string> waiting;

  // Invariant:
  // (1) enrolled contains the students already in the course, 
  // (2) waiting contains students who will be admitted (in the order of request) if a spot opens up
  // (3) enrolled.size() <= max_students, 
  // (4) if the course is not filled (enrolled.size() != max_students) then waiting is empty
  do {
    // check (part of) the invariant
    assert (enrolled.size() <= max_students);
    assert (enrolled.size() == max_students || waiting.size() == 0);
    cout << "\nOptions:\n"
         << "  To enroll a student type 0\n"
         << "  To remove a student type 1\n"
         << "  To end type 2\n"
         << "Type option ==> ";
    int option;
    if (!(cin >> option)) { // if we can't read the input integer, then just fail.
      cout << "Illegal input.  Good-bye.\n";
      return 1;
    } else if (option == 2) { 
      break; // quit by breaking out of the loop.
    } else if (option != 0 && option != 1) {
      cout << "Invalid option.  Try again.\n";
    } else { // option is 0 or 1
      string id;
      cout << "Enter student id: ";
      if (!(cin >> id)) {
        cout << "Illegal input.  Good-bye.\n";
        return 1;
      } else if (option == 0) {
        enroll_student(id, max_students, enrolled, waiting);
      } else {
        remove_student(id, max_students, enrolled, waiting); 
      }
    }
  }
  while (true);
  
  // some nice output
  sort(enrolled.begin(), enrolled.end());
  cout << "\nAt the end of the enrollment period, the following students are in the class:\n\n";
  for (unsigned int i=0; i<enrolled.size(); ++i) { cout << enrolled[i] << endl;  }
  if (!waiting.empty()) {
    cout << "\nStudents are on the waiting list in the following order:\n";
    for (unsigned int j=0; j<waiting.size(); ++j) { cout << waiting[j] << endl; }
  }
  return 0;
}