Simple implementation of Merge Sort Algorithm in C++

MergeSort.cpp

// Fill a random vector
template <typename Comparable>
void fillRandom(vector<Comparable> &iArray, Comparable range)
{
 srand((unsigned)time(0));
 for(Comparable i = 0; i < iArray.size(); i++)
 {
  iArray[i] = rand() % range;
 }
}

/**********************************************************************
MERGE SORT:
- Recurse over first half
- Recurse over second half
- Merge by getting biggest elements from two data sets
**********************************************************************/
template <typename Comparable>
void mergeSort(vector<Comparable> &a, int first, int last)
{
 if(first < last)
 {
  int mid = (first + last) / 2;
  mergeSort(a, first, mid);
  mergeSort(a, mid + 1, last);
 
  merge(a, first, mid, last);
 }
}
 
template <typename Comparable>
void merge(vector<Comparable> &a, int first, int mid, int last)
{
 vector<Comparable>tmp(last - first + 1);
 int index1 = first;
 int index2 = mid + 1;
 int index = 0;
 
 // As long as both lists still have elements, add the next
 while(((index1 <= mid) || (index2 <= last)) && (index < (last - first + 1)))
 {
  if((index1 <= mid) && (index2 <= last))
  {
   tmp[index] = (a[index1] < a[index2])? a[index1] : a[index2];
   (a[index1] < a[index2])? index1++ : index2++;
  }
  else if(index1 > mid)
  {
   tmp[index] = a[index2];
   index2++;
  }
  else // index2 > last 
  {
   tmp[index] = a[index1];
   index1++;
  }
 
  index++;
 }
 
 for(int i = 0; i < last - first + 1; i++)
 {
  a[first + i] = tmp[i];
 }
}