Circular array queue demonstration

circular_queue.cpp

#include <iostream>
#include <iomanip>

using namespace std;

/*-
 *
 *   0  1  2  3  4  5
 *      a  b  c  d
 *      ^        ^
 *    first    last
 */

template<typename T>
class circ_queue {
private:
  int size;
  T* array;
  int first, last;
  int count;

  // Create a new array of specified size, put the queue's contents
  // into the new array.
  void resize(int newSize) {
    if (count > newSize) throw "size too small";

    delete[] array;
    T* newArray = new T[newSize];

    // fill new array
    int startIndex = 0;
    for (int i = 0; i < count; i ++) {
      newArray[(startIndex + i) % newSize] = array[(first + i) % size];
    }

    // update variables
    last = (startIndex + count) % newSize;
    first = startIndex;
    size = newSize;
    array = newArray;
  }

public:
  circ_queue(int sz) :
    size(sz), array(new T[size]), first(0), last(0), count(0) { }

  ~circ_queue() { delete[] array; }

  void enqueue(T val) {
    // if queue is full, double array size
    if (count == size) {
      resize(size * 2);
    }

    // put new item to last
    count ++;
    array[last] = val;
    last = (last + 1) % size;
  }

  T dequeue() {
    if (count == 0) throw "queue is empty";

    // move first forward
    count --;
    T val = array[first];
    first = (first + 1) % size;

    // shrink if count is less than 1/3 of total size
    // (prevents shrinking immediately after expanding)
    if (size > 10 && count * 3 < size)
      resize(size / 2);

    return val;
  }

  bool is_empty() { return count == 0; }
  bool is_full() { return count == size; }

  int get_size() { return size; }
  int get_count() { return count; }

  // Print out the contents and variables of this queue
  void print_info() {
    const int WIDTH = 8;

    // array contents
    cout << endl << "array:" << endl;
    cout << setw(WIDTH + 2) << "index |";
    for (int i = 0; i < size; i ++) {
      cout << setw(WIDTH) << i;
    }
    cout << endl;

    cout << setw(WIDTH + 2) << "value |";
    for (int i = 0; i < size; i ++) {
      cout << setw(WIDTH) << array[i];
    }
    cout << endl;

    // first and last positions
    cout << setw(WIDTH + 2) << " |";
    for (int i = 0; i < size; i ++) {
      const char* lbl = (i == first ? "first^" : (i == last ? "last^" : " "));
      cout << setw(WIDTH) << lbl;
    }
    cout << endl;

    if (first == last) {
      cout << setw(WIDTH + 2) << " |";
      for (int i = 0; i < size; i ++) {
        cout << setw(WIDTH) << (i == last ? "last^" : " ");
      }
      cout << endl;
    }

    // contents
    cout << endl << "contents: ";
    for (int i = 0; i < count; i ++) {
      cout << " " << array[(first + i) % size] << " ";
    }
    cout << endl;

    // variables
    cout << "size:  " << size << endl;
    cout << "count: " << count << endl;

    cout << endl;
  }

};

int main(int argc, char** argv) {
  circ_queue<int>* q = new circ_queue<int>(4);

  cout << "Enter a number to enqueue." << endl;
  cout << "Enter 'd' to dequeue." << endl;
  cout << "Enter 'x' to exit." << endl;

  // interactive
  while (true) {
    try {
      q->print_info();

      cout << "> ";

      string line;
      getline(cin, line);

      if (line.length() == 0) continue;

      char act = line[0];
      if (act == 'x') {
        // exit
        return 0;
      } else if (act == 'd') {
        // dequeue
        short c = q->dequeue();
        cout << "Got: " << c << endl;
      } else {
        try {
        // enqueue
        q->enqueue(stoi(line));
        } catch (invalid_argument) {
          cerr << "Not a number." << endl;
        } catch (out_of_range) {
          cerr << "Number out of range." << endl;
        }
      }

    } catch (const char* msg) {
      cerr << "Error: " << msg << endl;
    }
  }
  return 0;
}