// Program to demonstrate the implementation and usage
//  of the mergesort algorithm.
import java.util.*;

public class Sorter {
    public static void main(String[] args) {
        Queue<Integer> list = buildRandom(20);
        System.out.println(list);
        mergeSort(list);
        System.out.println(list);
    }
    
    // creates a queue populated with size random integers
    //   between -99 and 99
    public static Queue<Integer> buildRandom(int size) {
        Random r = new Random();
        Queue<Integer> queue = new LinkedList<Integer>();
        for (int i = 0; i < size; i++) {
            queue.add(r.nextInt() % 100);
        }
        return queue;
    }
    
    // sorts the queue using the mergesort algorithm
    // pre: queue != null
    public static void mergeSort(Queue<Integer> queue) {
        if (queue.size() > 1) {
            // split in half
            Queue<Integer> half1 = new LinkedList<Integer>();
            Queue<Integer> half2 = new LinkedList<Integer>();
            
            int size = queue.size();
            for (int i = 0; i < size / 2; i++) {
                half1.add(queue.remove());
            }
            while (!queue.isEmpty()) {
                half2.add(queue.remove());
            }
            
            // sort the halves
            mergeSort(half1);
            mergeSort(half2);
            
            // merge back together
            mergeInto(queue, half1, half2);
        }
    }
    
    // merges the values in queue1 and queue2 into result
    // pre: queue1 != null, queue2 != null, result != null
    //      result is empty, queue1 and queue2 are sorted
    // post: result is sorted, queue1 and queue2 are empty
    public static void mergeInto(Queue<Integer> result, 
                                 Queue<Integer> queue1, Queue<Integer> queue2) {
        while (!queue1.isEmpty() && !queue2.isEmpty()) {
            if (queue1.peek() <= queue2.peek()) {
                result.add(queue1.remove());
            } else {
                result.add(queue2.remove());
            }
        }
        while (!queue1.isEmpty()) {
            result.add(queue1.remove());
        }
        while (!queue2.isEmpty()) {
            result.add(queue2.remove());
        }
    }
}