private void Build(int treeArrayIndex, int segmentStartIndex, int segmentEndIndex)
{
if (segmentStartIndex == segmentEndIndex)
{
_treeArray[treeArrayIndex] = new MaximumSumQueryValue(_sourceArray[segmentStartIndex]);
return;
}
int leftChildTreeArrayIndex = 2 * treeArrayIndex + 1;
int rightChildTreeArrayIndex = leftChildTreeArrayIndex + 1;
Build(leftChildTreeArrayIndex, segmentStartIndex, (segmentStartIndex + segmentEndIndex) / 2);
Build(rightChildTreeArrayIndex, (segmentStartIndex + segmentEndIndex) / 2 + 1, segmentEndIndex);
_treeArray[treeArrayIndex] = _treeArray[leftChildTreeArrayIndex].Combine(_treeArray[rightChildTreeArrayIndex]);
}
public MaximumSumQueryValue Combine(MaximumSumQueryValue rightAdjacentValue)
=> new MaximumSumQueryValue
{
// The sum is just the sum of both.
Sum = Sum + rightAdjacentValue.Sum,
// The maximum sum either intersects both segments, or is entirely in one.
MaximumSum = Math.Max(
MaximumRightStartingSum + rightAdjacentValue.MaximumLeftStartingSum,
Math.Max(MaximumSum, rightAdjacentValue.MaximumSum)),
// The maximum left starting sum starts at the left, and may or may not cross into the right.
MaximumLeftStartingSum = Math.Max(
Sum + rightAdjacentValue.MaximumLeftStartingSum,
MaximumLeftStartingSum),
// The maximum right starting sum starts at the right, and may or may not cross into the left.
MaximumRightStartingSum = Math.Max(
rightAdjacentValue.Sum + MaximumRightStartingSum,
rightAdjacentValue.MaximumRightStartingSum)
};