com.brettonw.math.DensityBasedScan Maven / Gradle / Ivy

Go to download
package com.brettonw.math;

import org.apache.logging.log4j.LogManager;
import org.apache.logging.log4j.Logger;

import java.util.ArrayList;
import java.util.Arrays;
import java.util.List;

// https://en.wikipedia.org/wiki/DBSCAN
public class DensityBasedScan extends ClusterAlgorithm {
    private static final Logger log = LogManager.getLogger (DensityBasedScan.class);

    // the assign array contains either UNTOUCHED, NOISE, or a cluster index
    private static final int FIRST_CLUSTER = 0;
    private static final int UNTOUCHED = -1;
    private static final int NOISE = -2;

    private int clusterCount;
    private int[] assign;

    public DensityBasedScan (DataSet dataSet, double range, int minPts) {
        super (dataSet);

        // initialize the clustering engine
        assign = new int[dataSet.getN ()];
        Arrays.fill (assign, UNTOUCHED);
        clusterCount = FIRST_CLUSTER;

        // scan over all the points...
        for (int i = 0, n = dataSet.getN (); i < n; ++i) {
            if (assign[i] == UNTOUCHED) {
                // get the neighbors
                Tuple tuple = dataSet.get (i);
                int[] neighbors = dataSet.rangeSearch (tuple, range);
                if (neighbors.length < minPts) {
                    assign[i] = NOISE;
                } else {
                    assign[i] = clusterCount;
                    expandCluster (neighbors, range, minPts);
                    ++clusterCount;
                }
            }
        }
    }

    /*
    private boolean neighborhoodPredicate (Tuple a, Tuple b) {
        double epsilon = 1.0e-1;
        return Tuple.deltaNorm (a, b) < epsilon;
    }

    private double densityPredicate (Tuple[] neighborhood) {
        return neighborhood.length;
    }
    */

    private void expandCluster (int[] neighbors, double range, int minPts) {
        for (int i : neighbors) {
            if (assign[i] == UNTOUCHED) {
                assign[i] = clusterCount;

                // get the neighbors
                Tuple tuple = dataSet.get (i);
                int[] neighborNeighbors = dataSet.rangeSearch (tuple, range);
                if (neighborNeighbors.length > minPts) {
                    expandCluster (neighborNeighbors, range, minPts);
                }
            }
        }
    }

    public int getClusterCount () {
        return clusterCount;
    }

    @Override
    public Tuple[] getCluster (int i) {
        // naive scan of the full list
        List list = new ArrayList<> ();

        // an exhaustive search over all the tuples to find tuples in the cluster
        for (int j = 0, n = dataSet.getN (); j < n; ++j) {
            if (assign[j] == i) {
                list.add (dataSet.get (j));
            }
        }

        return list.toArray (new Tuple[list.size ()]);
    }
}


/* Wikipedia (https://en.wikipedia.org/wiki/DBSCAN)

DBSCAN(D, eps, MinPts) {
   C = 0
   for each point P in dataset D {
      if P is visited
         continue next point
      mark P as visited
      NeighborPts = regionQuery(P, eps)
      if sizeof(NeighborPts) < MinPts
         mark P as NOISE
      else {
         C = next cluster
         expandCluster(P, NeighborPts, C, eps, MinPts)
      }
   }
}

expandCluster(P, NeighborPts, C, eps, MinPts) {
   add P to cluster C
   for each point P' in NeighborPts {
      if P' is not visited {
         mark P' as visited
         NeighborPts' = regionQuery(P', eps)
         if sizeof(NeighborPts') >= MinPts
            NeighborPts = NeighborPts joined with NeighborPts'
      }
      if P' is not yet member of any cluster
         add P' to cluster C
   }
}
*/