// Merge Sort
#include <cstdlib>
#include <iostream>
using namespace std;

// Prototypes:
void merge(int sorted[], int leftHalf[], int leftCount, int rightHalf[], int rightCount);
void mergeSort(int myArray[], int elementCount);

int main()
{
  int num,i;
  int * a;
  
  cout<<"**********************************************************"<<endl;
  cout<<"                  MERGE SORT PROGRAM"<<endl;
  
  cout<<"*********************************************************"<<endl;
  cout<<endl<<endl;
  cout<<"Please Enter THE NUMBER OF ELEMENTS to sort: ";
  cin>>num;
  cout<<endl;

  // dynamically allocate an array of the correct size
  a = new int[num];

  for(i=0;i<num;i++) {
      a[i] = rand() % num;
  } // end for

  cout<<endl;
  cout<<"The unsorted list is: "<<endl;
  for(i=0;i<num;i++) {
    cout << a[i] << " ";
  } // end for
  cout<<endl<<endl;

  mergeSort(a,num);

  cout<<endl;
  cout<<"So, the sorted list (using MERGE SORT) will be: "<<endl;
  for(i=0;i<num;i++) {
    cout << a[i] << " ";
  } // end for
  cout<<endl<<endl;

  return 0;
} // end main


void mergeSort(int myArray[], int elementCount) {
  if (elementCount > 1) {
	int half = elementCount/2;
	int * leftHalf = new int[half];
	int * rightHalf = new int[elementCount - half];

	for (int i = 0; i < half; i++) {
	  leftHalf[i] = myArray[i];
	} // end for

	int right = 0;
	for (int i = half; i < elementCount; i++) {
	  rightHalf[right] = myArray[i];
	  right++;
	} // end for

	mergeSort(leftHalf, half);

	mergeSort(rightHalf, elementCount - half);
	merge(myArray, leftHalf, half, rightHalf, elementCount - half);

	// free up the memory allocated for the array created above
	delete [] leftHalf;
	delete [] rightHalf;
  } // end if
} // end mergeSort


void merge(int sorted[], int leftHalf[], int leftCount, int rightHalf[], int rightCount) {
	int left=0, right=0, nextSorted=0;
	
	while (left < leftCount && right < rightCount) {
		if (leftHalf[left] <= rightHalf[right]) {
			sorted[nextSorted] = leftHalf[left];
			left++;
		} else {
			sorted[nextSorted] = rightHalf[right];
			right++;
		} // end if
		nextSorted++;
	} // end while

	while (left < leftCount) {
		sorted[nextSorted] = leftHalf[left];
		left++;
		nextSorted++;
	} // end while

	while (right < rightCount) {
		sorted[nextSorted] = rightHalf[right];
		right++;
		nextSorted++;
	} // end while


} // end merge