Ejemplo n.º 1
0
        /// <summary>
        /// A top-down recursive method to find the nearest neighbors of a given point.
        /// </summary>
        /// <param name="nodeIndex">The index of the node for the current recursion branch.</param>
        /// <param name="target">The point whose neighbors we are trying to find.</param>
        /// <param name="rect">The <see cref="HyperRect{T}"/> containing the possible nearest neighbors.</param>
        /// <param name="dimension">The current splitting dimension for this recursion branch.</param>
        /// <param name="nearestNeighbors">The <see cref="BoundedPriorityList{TElement,TPriority}"/> containing the nearest neighbors already discovered.</param>
        /// <param name="maxSearchRadiusSquared">The squared radius of the current largest distance to search from the <paramref name="target"/></param>
        private void SearchForNearestNeighbors(
            int nodeIndex,
            TDimension[] target,
            HyperRect <TDimension> rect,
            int dimension,
            BoundedPriorityList <int, double> nearestNeighbors,
            double maxSearchRadiusSquared)
        {
            if (this.InternalPointArray.Length <= nodeIndex || nodeIndex < 0 ||
                this.InternalPointArray[nodeIndex] == null)
            {
                return;
            }

            // Work out the current dimension
            var dim = dimension % this.Dimensions;

            var leftRect = rect.Clone();

            leftRect.MaxPoint[dim] = this.InternalPointArray[nodeIndex][dim];

            var rightRect = rect.Clone();

            rightRect.MinPoint[dim] = this.InternalPointArray[nodeIndex][dim];

            // Determine which side the target resides in
            var compare = target[dim].CompareTo(this.InternalPointArray[nodeIndex][dim]);

            var nearerRect  = compare <= 0 ? leftRect : rightRect;
            var furtherRect = compare <= 0 ? rightRect : leftRect;

            var nearerNode  = compare <= 0 ? BinaryTreeNavigation.LeftChildIndex(nodeIndex) : BinaryTreeNavigation.RightChildIndex(nodeIndex);
            var furtherNode = compare <= 0 ? BinaryTreeNavigation.RightChildIndex(nodeIndex) : BinaryTreeNavigation.LeftChildIndex(nodeIndex);

            // Move down into the nearer branch
            this.SearchForNearestNeighbors(
                nearerNode,
                target,
                nearerRect,
                dimension + 1,
                nearestNeighbors,
                maxSearchRadiusSquared);

            // Walk down into the further branch but only if our capacity hasn't been reached
            // OR if there's a region in the further rectangle that's closer to the target than our
            // current furtherest nearest neighbor
            var closestPointInFurtherRect = furtherRect.GetClosestPoint(target);
            var distanceSquaredToTarget   = this.Metric(closestPointInFurtherRect, target);

            if (distanceSquaredToTarget.CompareTo(maxSearchRadiusSquared) <= 0)
            {
                if (nearestNeighbors.IsFull)
                {
                    if (distanceSquaredToTarget.CompareTo(nearestNeighbors.MaxPriority) < 0)
                    {
                        this.SearchForNearestNeighbors(
                            furtherNode,
                            target,
                            furtherRect,
                            dimension + 1,
                            nearestNeighbors,
                            maxSearchRadiusSquared);
                    }
                }
                else
                {
                    this.SearchForNearestNeighbors(
                        furtherNode,
                        target,
                        furtherRect,
                        dimension + 1,
                        nearestNeighbors,
                        maxSearchRadiusSquared);
                }
            }

            // Try to add the current node to our nearest neighbors list
            distanceSquaredToTarget = this.Metric(this.InternalPointArray[nodeIndex], target);
            if (distanceSquaredToTarget.CompareTo(maxSearchRadiusSquared) <= 0 && this.InternalNodeArray[nodeIndex].Used == false)
            {
                nearestNeighbors.Add(nodeIndex, distanceSquaredToTarget);
            }
        }
Ejemplo n.º 2
0
        /// <summary>
        /// Grows a KD tree recursively via median splitting. We find the median by doing a full sort.
        /// </summary>
        /// <param name="index">The array index for the current node.</param>
        /// <param name="dim">The current splitting dimension.</param>
        /// <param name="points">The set of points remaining to be added to the kd-tree</param>
        /// <param name="nodes">The set of nodes RE</param>
        private void GenerateTree(
            int index,
            int dim,
            IReadOnlyCollection <TDimension[]> points,
            IEnumerable <TNode> nodes)
        {
            // See wikipedia for a good explanation kd-tree construction.
            // https://en.wikipedia.org/wiki/K-d_tree

            // zip both lists so we can sort nodes according to points
            var zippedList = points.Zip(nodes, (p, n) => new { Point = p, Node = n });

            // sort the points along the current dimension
            var sortedPoints = zippedList.OrderBy(z => z.Point[dim]).ToArray();

            // get the point which has the median value of the current dimension.
            var medianPoint    = sortedPoints[points.Count / 2];
            var medianPointIdx = sortedPoints.Length / 2;

            // The point with the median value all the current dimension now becomes the value of the current tree node
            // The previous node becomes the parents of the current node.
            this.InternalPointArray[index] = medianPoint.Point;
            this.InternalNodeArray[index]  = medianPoint.Node;

            // We now split the sorted points into 2 groups
            // 1st group: points before the median
            var leftPoints = new TDimension[medianPointIdx][];
            var leftNodes  = new TNode[medianPointIdx];

            Array.Copy(sortedPoints.Select(z => z.Point).ToArray(), leftPoints, leftPoints.Length);
            Array.Copy(sortedPoints.Select(z => z.Node).ToArray(), leftNodes, leftNodes.Length);

            // 2nd group: Points after the median
            var rightPoints = new TDimension[sortedPoints.Length - (medianPointIdx + 1)][];
            var rightNodes  = new TNode[sortedPoints.Length - (medianPointIdx + 1)];

            Array.Copy(
                sortedPoints.Select(z => z.Point).ToArray(),
                medianPointIdx + 1,
                rightPoints,
                0,
                rightPoints.Length);
            Array.Copy(sortedPoints.Select(z => z.Node).ToArray(), medianPointIdx + 1, rightNodes, 0, rightNodes.Length);

            // We new recurse, passing the left and right arrays for arguments.
            // The current node's left and right values become the "roots" for
            // each recursion call. We also forward cycle to the next dimension.
            var nextDim = (dim + 1) % this.Dimensions; // select next dimension

            // We only need to recurse if the point array contains more than one point
            // If the array has no points then the node stay a null value
            if (leftPoints.Length <= 1)
            {
                if (leftPoints.Length == 1)
                {
                    this.InternalPointArray[BinaryTreeNavigation.LeftChildIndex(index)] = leftPoints[0];
                    this.InternalNodeArray[BinaryTreeNavigation.LeftChildIndex(index)]  = leftNodes[0];
                }
            }
            else
            {
                this.GenerateTree(BinaryTreeNavigation.LeftChildIndex(index), nextDim, leftPoints, leftNodes);
            }

            // Do the same for the right points
            if (rightPoints.Length <= 1)
            {
                if (rightPoints.Length == 1)
                {
                    this.InternalPointArray[BinaryTreeNavigation.RightChildIndex(index)] = rightPoints[0];
                    this.InternalNodeArray[BinaryTreeNavigation.RightChildIndex(index)]  = rightNodes[0];
                }
            }
            else
            {
                this.GenerateTree(BinaryTreeNavigation.RightChildIndex(index), nextDim, rightPoints, rightNodes);
            }
        }