com.cloudera.science.ml.kmeans.core.LloydsUpdateStrategy.java Source code

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Here is the source code for com.cloudera.science.ml.kmeans.core.LloydsUpdateStrategy.java

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/**
 * Copyright (c) 2013, Cloudera, Inc. All Rights Reserved.
 *
 * Cloudera, Inc. licenses this file to you under the Apache License,
 * Version 2.0 (the "License"). You may not use this file except in
 * compliance with the License. You may obtain a copy of the License at
 *
 *     http://www.apache.org/licenses/LICENSE-2.0
 *
 * This software is distributed on an "AS IS" BASIS, WITHOUT WARRANTIES OR
 * CONDITIONS OF ANY KIND, either express or implied. See the License for
 * the specific language governing permissions and limitations under the
 * License.
 */
package com.cloudera.science.ml.kmeans.core;

import java.util.Collection;
import java.util.List;
import java.util.Map;

import org.apache.mahout.math.Vector;

import com.cloudera.science.ml.core.vectors.Centers;
import com.cloudera.science.ml.core.vectors.Weighted;
import com.google.common.collect.Lists;
import com.google.common.collect.Maps;

/**
 *
 */
public class LloydsUpdateStrategy implements KMeansUpdateStrategy {

    private final int numIterations;

    public LloydsUpdateStrategy(int numIterations) {
        this.numIterations = numIterations;
    }

    @Override
    public <V extends Vector> Centers update(List<Weighted<V>> points, Centers centers) {
        for (int iter = 0; iter < numIterations; iter++) {
            Map<Integer, List<Weighted<V>>> assignments = Maps.newHashMap();
            for (int i = 0; i < centers.size(); i++) {
                assignments.put(i, Lists.<Weighted<V>>newArrayList());
            }
            for (Weighted<V> weightedVec : points) {
                assignments.get(centers.indexOfClosest(weightedVec.thing())).add(weightedVec);
            }
            List<Vector> centroids = Lists.newArrayList();
            for (Map.Entry<Integer, List<Weighted<V>>> e : assignments.entrySet()) {
                if (e.getValue().isEmpty()) {
                    centroids.add(centers.get(e.getKey())); // fix the no-op center
                } else {
                    centroids.add(centroid(e.getValue()));
                }
            }
            centers = new Centers(centroids);
        }
        return centers;
    }

    /**
     * Compute the {@code Vector} that is the centroid of the given weighted points.
     * 
     * @param points The weighted points
     * @return The centroid of the weighted points
     */
    public <V extends Vector> Vector centroid(Collection<Weighted<V>> points) {
        Vector center = null;
        double sz = 0.0;
        for (Weighted<V> v : points) {
            Vector weighted = v.thing().times(v.weight());
            if (center == null) {
                center = weighted;
            } else {
                center = center.plus(weighted);
            }
            sz += v.weight();
        }
        return center.divide(sz);
    }
}