import java.util.Arrays;

/* Java class that includes methods for determining the maximum segment sum
** of an array (or segment thereof) containing int values.
**
** Author: R. McCloskey
** Date: January 2020
*/
public class MaxSegSumViaPrefixSums implements MaxSegSum {

   // instance variables
   // ------------------

   private int maxDiffSoFar; // result of most recent call to maxSegSum()
   private long workMeasure; // # calls and loop iterations since reset()


   // constructor
   // -----------

   public MaxSegSumViaPrefixSums() {
      workMeasure = 0;
      maxDiffSoFar = -1;
   }


   // observer
   // --------

   public int maxSegSumVal() { return maxDiffSoFar; }

   public long measureOfWork() { return workMeasure; }


   // mutators
   // --------

   public void reset() { workMeasure = 0L; }


   /* Returns the maximum segment sum of the given array, computed using
   ** the prefix sums approach.
   */
   public int maxSegSum(int[] a) { 
      return maxSegSum(a, 0, a.length);
   }

   /* Returns the maximum segment sum of the given array segment
   ** (i.e., a[low..high)), computed using the prefix sums approach.
   ** pre: 0 <= low <= high <= a.length
   */
   public int maxSegSum(int[] a, int low, int high) { 
      int prefixSum = 0;
      int minPrefixSumSoFar = 0;
      maxDiffSoFar = 0;
      int i = low;
      // loop invariant: 
      //    prefixSum = sum of elements in a[low..i) and
      //    minPrefixSumSoFar = minimum among sums of all prefixes of a[low..i)
      //    maxDiffSoFar = difference between maximum of sums of prefixes of
      //                   of a[low..i) and minimum thereof
      while (i != high) {
         prefixSum = prefixSum + a[i];
         int thisDiff = prefixSum - minPrefixSumSoFar;
         if (thisDiff >= maxDiffSoFar) {
            maxDiffSoFar = thisDiff;
            //System.out.println("new max difference at " + i);
         }
         if (prefixSum < minPrefixSumSoFar) {
            minPrefixSumSoFar = prefixSum;
            //System.out.println("new min prefix sum at " + i);
         }
         i = i+1;
      }
      workMeasure = workMeasure + high - low + 1;
      return maxDiffSoFar;
   }

}