As discussed earlier, RPT is based on a model where the world is divided into grid squares or cells. This is done recursively to get almost any amount of accuracy required. Each cell is indexed in Lucene with a byte string that has the parent cell's byte string as a prefix. Therefore, it is named PrefixTree. The PrefixTreeStrategy class for indexing and search uses a SpatialPrefixTree abstraction that decides how to divide the world into grid squares and what the byte encoding looks like to represent each grid square. It has two implementations, namely geohash and quadtrees. Let us look at both implementations in detail.
A quadtree is a simple and effective spatial indexing technique wherein each of the nodes represents a bounding box covering the parts of the space that has been indexed. Each node is either a leaf-node that contains one or more indexed points with no child, or an internal node with four children, one for each quadrant. The index structure of a quadtree...