ParallelCascadedPolygonUnion

ParallelCascadedPolygonUnion.java

import com.vividsolutions.jts.geom.Geometry;
import com.vividsolutions.jts.geom.util.GeometryCombiner;
import com.vividsolutions.jts.index.strtree.STRtree;

import java.util.Collection;
import java.util.List;
import java.util.stream.Collectors;

public class ParallelCascadedPolygonUnion {
    public static Geometry union(Collection<Geometry> geoms) {
        // Construct an STRtree from the input geometries.  If the number of geometries is
        // small, we should probably skip this step or at least adjust the nodeCapacity. 
        int nodeCapacity = 16;
        STRtree index = new STRtree(nodeCapacity);

        geoms.forEach(g-> index.insert(g.getEnvelopeInternal(), g));
        index.build();

        // Pull out the internal representation of the tree (a list of lists)
        // and perform the union operation.
        return performUnion(index.itemsTree());
    }

    private static Geometry unionTwoGeometries(Geometry g1, Geometry g2) {
        // This handles our base case when this function is used in a 
        // stream::reduce context.
        if (g1 == null) {
            return g2;
        }

        // Avoid doing the union if the geometries cannot intersect.
        if (!g1.getEnvelopeInternal().intersects(g2.getEnvelopeInternal())) {
            return GeometryCombiner.combine(g1, g2);
        }
        
        return g2.union(g1);
    }

    @SuppressWarnings("unchecked")
    private static Geometry performUnion(List items) {
        boolean isLeafNode = items.iterator().next() instanceof Geometry;

        // If we're not on a leaf node, first perform the union operation in parallel on the child nodes.  This will
        // transform the current node into a leaf node of a shallower tree. 
        if (!isLeafNode)
            items = ((List<List>) items).parallelStream().map(ParallelCascadedPolygonUnion::performUnion).collect(Collectors.toList());

        // Now, union the geometry stored in the child nodes 
        return ((List<Geometry>) items).parallelStream().reduce(null, ParallelCascadedPolygonUnion::unionTwoGeometries);
    }
}