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Data structure which allows accurate estimation of quantiles and related rank statistics
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/*
 * Licensed to Ted Dunning under one or more
 * contributor license agreements.  See the NOTICE file distributed with
 * this work for additional information regarding copyright ownership.
 * The ASF 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
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License 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.tdunning.math.stats;

import java.io.IOException;
import java.io.ObjectInputStream;
import java.io.Serializable;
import java.util.ArrayList;
import java.util.List;
import java.util.concurrent.atomic.AtomicInteger;

/**
 * A single centroid which represents a number of data points.
 */
public class Centroid implements Comparable, Serializable {
    private static final AtomicInteger uniqueCount = new AtomicInteger(1);

    private double centroid = 0;
    private int count = 0;

    // The ID is transient because it must be unique within a given JVM. A new
    // ID should be generated from uniqueCount when a Centroid is deserialized.
    private transient int id;

    private List actualData = null;

    private Centroid(boolean record) {
        id = uniqueCount.getAndIncrement();
        if (record) {
            actualData = new ArrayList<>();
        }
    }

    public Centroid(double x) {
        this(false);
        start(x, 1, uniqueCount.getAndIncrement());
    }

    public Centroid(double x, int w) {
        this(false);
        start(x, w, uniqueCount.getAndIncrement());
    }

    public Centroid(double x, int w, int id) {
        this(false);
        start(x, w, id);
    }

    public Centroid(double x, int id, boolean record) {
        this(record);
        start(x, 1, id);
    }

    Centroid(double x, int w, List data) {
        this(x, w);
        actualData = data;
    }

    private void start(double x, int w, int id) {
        this.id = id;
        add(x, w);
    }

    public void add(double x, int w) {
        if (actualData != null) {
            actualData.add(x);
        }
        count += w;
        centroid += w * (x - centroid) / count;
    }

    public double mean() {
        return centroid;
    }

    public int count() {
        return count;
    }

    public int id() {
        return id;
    }

    @Override
    public String toString() {
        return "Centroid{" +
                "centroid=" + centroid +
                ", count=" + count +
                '}';
    }

    @Override
    public int hashCode() {
        return id;
    }

    @Override
    public int compareTo(@SuppressWarnings("NullableProblems") Centroid o) {
        int r = Double.compare(centroid, o.centroid);
        if (r == 0) {
            r = id - o.id;
        }
        return r;
    }

    public List data() {
        return actualData;
    }

    @SuppressWarnings("WeakerAccess")
    public void insertData(double x) {
        if (actualData == null) {
            actualData = new ArrayList<>();
        }
        actualData.add(x);
    }

    public static Centroid createWeighted(double x, int w, Iterable data) {
        Centroid r = new Centroid(data != null);
        r.add(x, w, data);
        return r;
    }

    public void add(double x, int w, Iterable data) {
        if (actualData != null) {
            if (data != null) {
                for (Double old : data) {
                    actualData.add(old);
                }
            } else {
                actualData.add(x);
            }
        }
        centroid = AbstractTDigest.weightedAverage(centroid, count, x, w);
        count += w;
    }

    private void readObject(ObjectInputStream in) throws ClassNotFoundException, IOException {
        in.defaultReadObject();
        id = uniqueCount.getAndIncrement();
    }
}