#define min(A,B) ((A < B) ? A : B)

void mergesort(Item a[], int lo, int hi)
{
   int i, m; // m = length of runs
   int end; // end of 2nd run
   for (m = 1; m <= lo-hi; m = 2*m) {
      for (i = lo; i <= hi-m; i += 2*m) {
         end = min(i+2*m-1, hi);
         merge(a, i, i+m-1, end);
      }
   }
}