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/*
 * Copyright (c) 2020 Oracle and/or its affiliates.
 *
 * Licensed under the Universal Permissive License v 1.0 as shown at
 * http://oss.oracle.com/licenses/upl.
 */

package com.tangosol.internal.net.metrics;

import java.io.OutputStream;
import java.io.OutputStreamWriter;
import java.io.PrintWriter;

import java.nio.charset.Charset;

import java.util.Arrays;
import java.util.Collection;
import java.util.Comparator;

/*
 * This class is heavily inspired by:
 * WeightedSnapshot
 *
 * From Helidon v 2.0.2
 * Distributed under Apache License, Version 2.0
 */

/**
 * A statistical snapshot of a {@link WeightedSnapshot}.
 */
class WeightedSnapshot
        extends Snapshot
    {
    /**
     * Create a new {@link Snapshot} with the given values.
     *
     * @param values an unordered set of values in the reservoir
     */
    WeightedSnapshot(Collection values)
        {
        final WeightedSample[] copy = values.toArray(new WeightedSample[]{});

        Arrays.sort(copy, Comparator.comparingLong(o -> o.value));

        this.values = new long[copy.length];
        this.normWeights = new double[copy.length];
        this.quantiles = new double[copy.length];

        double sumWeight = 0;
        for (WeightedSample sample : copy)
            {
            sumWeight += sample.weight;
            }

        for (int i = 0; i < copy.length; i++)
            {
            this.values[i] = copy[i].value;
            this.normWeights[i] = copy[i].weight / sumWeight;
            }

        for (int i = 1; i < copy.length; i++)
            {
            this.quantiles[i] = this.quantiles[i - 1] + this.normWeights[i - 1];
            }
        }

    /**
     * Returns the value at the given quantile.
     *
     * @param quantile a given quantile, in {@code [0..1]}
     * @return the value in the distribution at {@code quantile}
     */
    @Override
    public double getValue(double quantile)
        {
        if ((quantile < 0.0) || (quantile > 1.0) || Double.isNaN(quantile))
            {
            throw new IllegalArgumentException(quantile + " is not in [0..1]");
            }

        if (values.length == 0)
            {
            return 0.0;
            }

        int posx = Arrays.binarySearch(quantiles, quantile);
        if (posx < 0)
            {
            posx = ((-posx) - 1) - 1;
            }

        if (posx < 1)
            {
            return values[0];
            }

        if (posx >= values.length)
            {
            return values[values.length - 1];
            }

        return values[(int) posx];
        }

    /**
     * Returns the number of values in the snapshot.
     *
     * @return the number of values
     */
    @Override
    public int size()
        {
        return values.length;
        }

    /**
     * Returns the entire set of values in the snapshot.
     *
     * @return the entire set of values
     */
    @Override
    public long[] getValues()
        {
        return Arrays.copyOf(values, values.length);
        }

    /**
     * Returns the highest value in the snapshot.
     *
     * @return the highest value
     */
    @Override
    public long getMax()
        {
        if (values.length == 0)
            {
            return 0;
            }
        return values[values.length - 1];
        }

    /**
     * Returns the lowest value in the snapshot.
     *
     * @return the lowest value
     */
    @Override
    public long getMin()
        {
        if (values.length == 0)
            {
            return 0;
            }
        return values[0];
        }

    /**
     * Returns the weighted arithmetic mean of the values in the snapshot.
     *
     * @return the weighted arithmetic mean
     */
    @Override
    public double getMean()
        {
        if (values.length == 0)
            {
            return 0;
            }

        double sum = 0;
        for (int i = 0; i < values.length; i++)
            {
            sum += values[i] * normWeights[i];
            }
        return sum;
        }

    /**
     * Returns the weighted standard deviation of the values in the snapshot.
     *
     * @return the weighted standard deviation value
     */
    @Override
    public double getStdDev()
        {
        // two-pass algorithm for variance, avoids numeric overflow

        if (values.length <= 1)
            {
            return 0;
            }

        final double mean = getMean();
        double variance = 0;

        for (int i = 0; i < values.length; i++)
            {
            final double diff = values[i] - mean;
            variance += normWeights[i] * diff * diff;
            }

        return Math.sqrt(variance);
        }

    /**
     * Writes the values of the snapshot to the given stream.
     *
     * @param output an output stream
     */
    @Override
    public void dump(OutputStream output)
        {
        final PrintWriter out = new PrintWriter(new OutputStreamWriter(output, UTF_8));
        try
            {
            for (long value : values)
                {
                out.printf("%d%n", value);
                }
            }
        finally
            {
            out.close();
            }
        }
    private final long[] values;
    private final double[] normWeights;
    private final double[] quantiles;
    private static final Charset UTF_8 = Charset.forName("UTF-8");

    /**
     * A single sample item with value and its weights for {@link WeightedSnapshot}.
     */
    static class WeightedSample
        {
        WeightedSample(long value, double weight)
            {
            this.value = value;
            this.weight = weight;
            }

        long getValue()
            {
            return value;
            }

        double getWeight()
            {
            return weight;
            }
        private final long value;
        private final double weight;
        }
    }