com.cloudera.oryx.kmeans.common.WeightedSampler.java Source code

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Here is the source code for com.cloudera.oryx.kmeans.common.WeightedSampler.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.oryx.kmeans.common;

import org.apache.commons.math3.random.RandomGenerator;

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

import com.cloudera.oryx.common.random.RandomManager;

public final class WeightedSampler<T, W extends Weighted<T>> {

    private final double[] cumulativeSum;
    private final List<W> things;
    private final RandomGenerator random;

    public WeightedSampler(List<W> things, RandomGenerator random) {
        this.things = things;
        this.cumulativeSum = new double[things.size() + 1];
        for (int i = 0; i < things.size(); i++) {
            cumulativeSum[i + 1] = cumulativeSum[i] + things.get(i).weight();
        }
        this.random = (random == null) ? RandomManager.getRandom() : random;
    }

    public T sample() {
        double offset = random.nextDouble() * cumulativeSum[cumulativeSum.length - 1];
        int next = Arrays.binarySearch(cumulativeSum, offset);
        Weighted<T> item = (next >= 0) ? things.get(next - 1) : things.get(-2 - next);
        return item.thing();
    }
}