Class KdTree

java.lang.Object
org.locationtech.jts.index.kdtree.KdTree

public class KdTree extends Object
An implementation of a KD-Tree over two dimensions (X and Y). KD-trees provide fast range searching and fast lookup for point data. The tree is built dynamically by inserting points. The tree supports queries by range and for point equality. For querying an internal stack is used instead of recursion to avoid overflow.

This implementation supports detecting and snapping points which are closer than a given distance tolerance. If the same point (up to tolerance) is inserted more than once, it is snapped to the existing node. In other words, if a point is inserted which lies within the tolerance of a node already in the index, it is snapped to that node. When an inserted point is snapped to a node then a new node is not created but the count of the existing node is incremented. If more than one node in the tree is within tolerance of an inserted point, the closest and then lowest node is snapped to.

The structure of a KD-Tree depends on the order of insertion of the points. A tree may become unbalanced if the inserted points are coherent (e.g. monotonic in one or both dimensions). A perfectly balanced tree has depth of only log2(N), but an unbalanced tree may be much deeper. This has a serious impact on query efficiency. One solution to this is to randomize the order of points before insertion (e.g. by using Fisher-Yates shuffling).

Author:
David Skea, Martin Davis
  • Constructor Details

    • KdTree

      public KdTree()
      Creates a new instance of a KdTree with a snapping tolerance of 0.0. (I.e. distinct points will not be snapped)
    • KdTree

      public KdTree(double tolerance)
      Creates a new instance of a KdTree, specifying a snapping distance tolerance. Points which lie closer than the tolerance to a point already in the tree will be treated as identical to the existing point.
      Parameters:
      tolerance - the tolerance distance for considering two points equal
  • Method Details

    • toCoordinates

      public static Coordinate[] toCoordinates(Collection kdnodes)
      Converts a collection of KdNodes to an array of Coordinates.
      Parameters:
      kdnodes - a collection of nodes
      Returns:
      an array of the coordinates represented by the nodes
    • toCoordinates

      public static Coordinate[] toCoordinates(Collection kdnodes, boolean includeRepeated)
      Converts a collection of KdNodes to an array of Coordinates, specifying whether repeated nodes should be represented by multiple coordinates.
      Parameters:
      kdnodes - a collection of nodes
      includeRepeated - true if repeated nodes should be included multiple times
      Returns:
      an array of the coordinates represented by the nodes
    • getRoot

      public KdNode getRoot()
      Gets the root node of this tree.
      Returns:
      the root node of the tree
    • isEmpty

      public boolean isEmpty()
      Tests whether the index contains any items.
      Returns:
      true if the index does not contain any items
    • insert

      public KdNode insert(Coordinate p)
      Inserts a new point in the kd-tree, with no data.
      Parameters:
      p - the point to insert
      Returns:
      the kdnode containing the point
    • insert

      public KdNode insert(Coordinate p, Object data)
      Inserts a new point into the kd-tree.
      Parameters:
      p - the point to insert
      data - a data item for the point
      Returns:
      returns a new KdNode if a new point is inserted, else an existing node is returned with its counter incremented. This can be checked by testing returnedNode.getCount() > 1.
    • query

      public void query(Envelope queryEnv, KdNodeVisitor visitor)
      Performs a range search of the points in the index and visits all nodes found.
      Parameters:
      queryEnv - the range rectangle to query
      visitor - a visitor to visit all nodes found by the search
    • query

      public List query(Envelope queryEnv)
      Performs a range search of the points in the index.
      Parameters:
      queryEnv - the range rectangle to query
      Returns:
      a list of the KdNodes found
    • query

      public void query(Envelope queryEnv, List result)
      Performs a range search of the points in the index.
      Parameters:
      queryEnv - the range rectangle to query
      result - a list to accumulate the result nodes into
    • query

      public KdNode query(Coordinate queryPt)
      Searches for a given point in the index and returns its node if found.
      Parameters:
      queryPt - the point to query
      Returns:
      the point node, if it is found in the index, or null if not
    • depth

      public int depth()
      Computes the depth of the tree.
      Returns:
      the depth of the tree
    • size

      public int size()
      Computes the size (number of items) in the tree.
      Returns:
      the size of the tree