SOURCE FILE: mergeSort.cpp

const int MAX_SIZE = maximum-number-of-items-in-array;

void Merge(dataType A[], int F, int Mid, int L)
// ---------------------------------------------------------
// Merges two sorted array segments A[F..Mid] and 
// A[Mid+1..L] into one sorted array.
// Precondition: F <= Mid <= L. The subarrays A[F..Mid] and
// A[Mid+1..L] are each sorted in increasing order.
// Postcondition: A[F..L] is sorted.
// Implementation note: This function merges the two
// subarrays into a temporary array and copies the result
// into the original array A.
// ---------------------------------------------------------
{
   dataType TempArray[MAX_SIZE];    // temporary array

   // initialize the local indexes to indicate the subarrays
   int First1 = F;        // beginning of first subarray
   int Last1  = Mid;      // end of first subarray
   int First2 = Mid + 1;  // beginning of second subarray
   int Last2  = L;        // end of second subarray

   // while both subarrays are not empty, copy the
   // smaller item into the temporary array
   int Index = First1;    // next available location in 
                          // TempArray
   for (; (First1 <= Last1) && (First2 <= Last2); ++Index)
   {  // Invariant: TempArray[First1..Index-1] is in order
      if (A[First1] < A[First2])
      {  TempArray[Index] = A[First1];
         ++First1;
      }
      else
      {  TempArray[Index] = A[First2];
         ++First2;
      }  // end if
   }  // end for

   // finish off the nonempty subarray

   // finish off the first subarray, if necessary
   for (; First1 <= Last1; ++First1, ++Index)
      // Invariant: TempArray[First1..Index-1] is in order
      TempArray[Index] = A[First1];

   // finish off the second subarray, if necessary
   for (; First2 <= Last2; ++First2, ++Index)
      // Invariant: TempArray[First1..Index-1] is in order
      TempArray[Index] = A[First2];

   // copy the result back into the original array
   for (Index = F; Index <= L; ++Index)
      A[Index] = TempArray[Index];
}  // end Merge

void Mergesort(dataType A[], int F, int L)
// ---------------------------------------------------------
// Sorts the items in an array into ascending order. 
// Precondition: A[F..L] is an array.
// Postcondition: A[F..L] is sorted in ascending order.
// Calls: Merge.
// ---------------------------------------------------------
{
   if (F < L)
   {  // sort each half
      int Mid = (F + L)/2;    // index of midpoint
      Mergesort(A, F, Mid);   // sort left half A[F..Mid]
      Mergesort(A, Mid+1, L); // sort right half A[Mid+1..L]

      // merge the two halves
      Merge(A, F, Mid, L);
   }  // end if
}  // end Mergesort