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/*
* File: OptimizedKMeansClusterer.java
* Authors: Justin Basilico and Kevin R. Dixon
* Company: Sandia National Laboratories
* Project: Cognitive Foundry
*
* Copyright February 21, 2006, Sandia Corporation. Under the terms of Contract
* DE-AC04-94AL85000, there is a non-exclusive license for use of this work by
* or on behalf of the U.S. Government. Export of this program may require a
* license from the United States Government. See CopyrightHistory.txt for
* complete details.
*
*/
package gov.sandia.cognition.learning.algorithm.clustering;
import gov.sandia.cognition.annotation.CodeReview;
import gov.sandia.cognition.annotation.PublicationReference;
import gov.sandia.cognition.annotation.PublicationType;
import gov.sandia.cognition.learning.algorithm.clustering.cluster.CentroidCluster;
import gov.sandia.cognition.learning.algorithm.clustering.cluster.ClusterCreator;
import gov.sandia.cognition.learning.algorithm.clustering.divergence.CentroidClusterDivergenceFunction;
import gov.sandia.cognition.learning.algorithm.clustering.initializer.FixedClusterInitializer;
import gov.sandia.cognition.math.Metric;
import java.util.ArrayList;
import java.util.Iterator;
/**
* This class implements an optimized version of the k-means algorithm that
* makes use of the triangle inequality to compute the same answer as k-means
* while using less distance calculations. The only restriction that the
* algorithm places is that the divergence function it is given must be a
* metric because it must obey the triangle inequality.
*
* @param The type of the data to cluster. This is typically
* defined by the divergence function used.
* @author Justin Basilico
* @author Kevin R. Dixon
* @since 1.0
*/
@CodeReview(
reviewer="Kevin R. Dixon",
date="2008-07-22",
changesNeeded=false,
comments={
"Added PublicationReference",
"Code generally looks fine."
}
)
@PublicationReference(
author="C. Elkan",
title="Using the Triangle Inequality to Accelerate k-Means",
type=PublicationType.Conference,
year=2003,
publication="Proceedings of the Twentieth International Conference on Machine Learning",
pages={147,153},
url="www-cse.ucsd.edu/~elkan/kmeansicml03.pdf"
)
public class OptimizedKMeansClusterer
extends KMeansClusterer>
{
/** The metric being used. */
private Metric metric;
/** The lower bounds on the distances to the clusters. */
protected double[][] lowerBounds;
/** The upper bounds on the distance to the current assigned cluster. */
protected double[] upperBounds;
/** The distances between clusters. */
protected double[][] clusterDistances;
/**
* Creates a new instance of OptimizedKMeansClusterer.
*
* @param numClusters The number of clusters to create.
* @param maxIterations Number of iterations before stopping
* @param initializer The cluster initializer.
* @param metric The metric to use.
* @param creator The cluster creator to use.
*/
public OptimizedKMeansClusterer(
int numClusters,
int maxIterations,
FixedClusterInitializer, DataType> initializer,
Metric metric,
ClusterCreator, DataType> creator)
{
super(numClusters, maxIterations, initializer,
new CentroidClusterDivergenceFunction(metric), creator);
this.setMetric(metric);
this.setLowerBounds(null);
this.setUpperBounds(null);
this.setClusterDistances(null);
}
@Override
@SuppressWarnings("unchecked")
public OptimizedKMeansClusterer clone()
{
final OptimizedKMeansClusterer result =
(OptimizedKMeansClusterer) super.clone();
result.metric = (Metric) result.divergenceFunction;
result.lowerBounds = null;
result.upperBounds = null;
result.clusterDistances = null;
return result;
}
@Override
protected boolean initializeAlgorithm()
{
boolean superReturn = super.initializeAlgorithm();
// Initialize the bounds.
int N = this.getNumElements();
int k = this.getNumClusters();
this.setLowerBounds(new double[N][k]);
this.setUpperBounds(new double[N]);
this.setClusterDistances(new double[k][k]);
return superReturn;
}
/**
* Computes the distances between the clusters.
*/
protected void computeClusterDistances()
{
// Go through all the clusters and compute their distances.
for (int i = 0; i < this.getNumClusters(); i++)
{
// Get the i-th cluster.
DataType clusterI = this.getClusterCentroid(i);
// We only compute the distance once since it must be symmetric.
// We hit the diagonal first so we do not assign to it twice.
clusterDistances[i][i] =
this.metric.evaluate(clusterI, clusterI);
// Now do the off-diagonal component of the distances.
for (int j = i + 1; j < this.getNumClusters(); j++)
{
// Get the j-th cluster.
DataType clusterJ = this.getClusterCentroid(j);
// Compute the distance between the two clusters.
double distance = this.metric.evaluate(clusterI, clusterJ);
// Save the cluster distance.
clusterDistances[i][j] = distance;
clusterDistances[j][i] = distance;
}
}
}
@Override
protected boolean step()
{
// Note: The comments in this function refer to the paper
// C. Elkan. "Using the Triangle Inequality to Accelerate k-Means". In
// Proceedings of the Twentieth International Conference on Machine
// Learning, 2003, pp. 147-153.
// Please use that paper as a guide if you need to follow the code
// in more detail.
// Keep track of the number of assignments that changed.
this.setNumChanged(0);
if (this.getNumClusters() <= 0)
{
// No clusters.
return false;
}
// Step 1: Computer distances between all centers.
computeClusterDistances();
// Compute the s values, which are based on the minimum distance to
// between clusters.
double[] s = new double[this.getNumClusters()];
for (int i = 0; i < this.getNumClusters(); i++)
{
double minDistance = Double.MAX_VALUE;
for (int j = 0; j < this.getNumClusters(); j++)
{
double distance = clusterDistances[i][j];
if (i != j && distance < minDistance)
{
minDistance = distance;
}
}
s[i] = 0.5 * minDistance;
}
// Step 2 & 3: Identify points... and Compute...
// This is the big loop that does all the real work by looping over
// all the points.
Iterator iterator = this.data.iterator();
for (int i = 0; i < this.getNumElements(); i++)
{
// Evaluate the i-th point.
DataType element = iterator.next();
int assignment = this.assignments[i];
if (assignment < 0)
{
// Assignments not initialized.
double minDistance = Double.MAX_VALUE;
for (int j = 0; j < this.getNumClusters(); j++)
{
// TO DO: Use Lemma 1 to avoid redundant distance calculations.
double distance = this.metric.evaluate(element,
this.getClusterCentroid(j));
this.lowerBounds[i][j] = distance;
if (assignment < 0 || distance < minDistance)
{
assignment = j;
minDistance = distance;
}
}
this.setAssignment(i, assignment);
this.upperBounds[i] = minDistance;
this.setNumChanged(this.getNumChanged() + 1);
continue;
}
// See if we need to update this element at all.
if (this.upperBounds[i] <= s[assignment])
{
// Step 2: u(x) <= s(c(x))
continue;
}
// We may need to update the cluster so keep this information.
int oldAssignment = assignment;
double distanceToCluster = 0.0;
boolean distanceToClusterComputed = false;
for (int j = 0; j < this.getNumClusters(); j++)
{
if (j == this.assignments[i])
{
// Condition (i): c != c(x)
continue;
}
else if (this.upperBounds[i] <= this.lowerBounds[i][j])
{
// Condition (ii): u(x) > l(x, c)
continue;
}
else if (this.upperBounds[i]
<= 0.5 * this.clusterDistances[assignment][j])
{
// Condition (iii): u(x) > 0.5 * d(c(x), c))
continue;
}
// See if we need to recompute the distance to the current
// assigned cluster.
if (!distanceToClusterComputed)
{
distanceToCluster = this.metric.evaluate(element,
this.getClusterCentroid(assignment));
this.lowerBounds[i][assignment] = distanceToCluster;
distanceToClusterComputed = true;
}
// See if we need to compute the distance to the j-th cluster.
if (distanceToCluster > this.lowerBounds[i][j]
|| distanceToCluster > 0.5 * this.clusterDistances[assignment][j])
{
double distance = this.metric.evaluate(element,
this.getClusterCentroid(j));
this.lowerBounds[i][j] = distance;
if (distance < distanceToCluster)
{
distanceToCluster = distance;
assignment = j;
this.setAssignment(i, j);
this.upperBounds[i] = distance;
}
}
}
// If ew changed the assignment keep track of that in the counter.
if (oldAssignment != assignment)
{
this.setNumChanged(this.getNumChanged() + 1);
}
}
// Step 4: For each center, find the means.
ArrayList oldCentroids =
new ArrayList(this.getNumClusters());
for (int i = 0; i < this.getNumClusters(); i++)
{
oldCentroids.add(this.getClusterCentroid(i));
}
this.createClustersFromAssignments();
// Step 5: Evaluate the amount each cluster has changed and update
// the lower bounds.
double[] clusterDeltas = new double[this.getNumClusters()];
for (int j = 0; j < this.getNumClusters(); j++)
{
// Get the old and new centroids for this cluster.
DataType oldCentroid = oldCentroids.get(j);
DataType newCentroid = this.getClusterCentroid(j);
// Compute the change in the centroid.
double meanChange = this.metric.evaluate(oldCentroid, newCentroid);
clusterDeltas[j] = meanChange;
// Update the lower bounds for each element.
for (int i = 0; i < this.getNumElements(); i++)
{
this.lowerBounds[i][j] =
Math.max(0.0, this.lowerBounds[i][j] - meanChange);
}
}
// Step 6: Update the upper bounds on the distance.
for (int i = 0; i < this.getNumElements(); i++)
{
this.upperBounds[i] += clusterDeltas[this.assignments[i]];
}
return (this.getNumChanged() > 0);
}
/**
* Gets the centroid for the given cluster index.
*
* @param clusterIndex The index of the cluster to get the centroid for.
* @return The centroid for the given cluster.
*/
public DataType getClusterCentroid(
int clusterIndex)
{
// Attempt to get the cluster.
CentroidCluster cluster = this.clusters.get(clusterIndex);
if (cluster == null)
{
// Error: The cluster was null so the centroid must also be null.
return null;
}
else
{
// Return the centroid.
return cluster.getCentroid();
}
}
/**
* Gets the metric being used by the algorithm.
*
* @return The metric being used.
*/
public Metric getMetric()
{
return this.metric;
}
/**
* Sets the metric to use in cluster.
*
* @param metric The metric being used.
*/
private void setMetric(
Metric metric)
{
if (metric == null)
{
// Error: Bad metric.
throw new NullPointerException("The metric cannot be null.");
}
this.metric = metric;
}
/**
* Sets the lower bounds of the distances to the cluster.
*
* @param lowerBounds The new lower bounds.
*/
private void setLowerBounds(
double[][] lowerBounds)
{
this.lowerBounds = lowerBounds;
}
/**
* Sets the upper bounds of the distances to the cluster.
*
* @param upperBounds The new upper bounds.
*/
private void setUpperBounds(
double[] upperBounds)
{
this.upperBounds = upperBounds;
}
/**
* Sets the distances between clusters
*
* @param clusterDistances The new distances between clusters.
*/
private void setClusterDistances(
double[][] clusterDistances)
{
this.clusterDistances = clusterDistances;
}
}