import java.util.*;

public class MergeSort {
	public static final int SIZE = 10;

	public static void main(String[] args) {
		Random rand = new Random();
		int[] a = new int[SIZE];
		for (int i = 0; i < a.length; i++) {
			a[i] = rand.nextInt(SIZE);
		}	

		System.out.println("BEFORE: " + Arrays.toString(a));
		mergeSort(a);
		System.out.println("AFTER: " + Arrays.toString(a));
	}
	
	// Rearranges the elements of a into sorted order using
	// the merge sort algorithm (recursive).
	public static void mergeSort(int[] a) {
		if (a.length >= 2) {
			// split array into two halves
			int[] left  = Arrays.copyOfRange(a, 0, a.length/2);
			int[] right = Arrays.copyOfRange(a, a.length/2, a.length);

			// sort the two halves
			mergeSort(left);
			mergeSort(right);

			// merge the sorted halves into a sorted whole
			merge(a, left, right);
		}
	}

	// pre:  left and right are sorted
   //       result.length == left.length + right.length   
   // post: copies the values from left and right 
   //       into result so that the values in result
   //       are in sorted order
	private static void merge(int[] result, int[] left, int[] right) {
		int i1 = 0;   // index into left array
		int i2 = 0;   // index into right array

		for (int i = 0; i < result.length; i++) {
			if (i2 >= right.length ||
				(i1 < left.length && left[i1] <= right[i2])) {
				result[i] = left[i1];    // take from left
				i1++;
			} else {
				result[i] = right[i2];   // take from right
				i2++;
			}
		}
	}
}