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With inspiration from other libraries

There is a newer version: 2.12.15

package io.virtdata.stathelpers;

import java.nio.ByteBuffer;
import java.util.Iterator;

public class DiscreteProbabilityBuffer implements Iterable {

    private static int REFERENT_ID =0;
    private static int PROBABILITY = REFERENT_ID +Integer.BYTES;
    public static int RECORD_LEN = PROBABILITY +Double.BYTES;
    private double cumulativeProbability = 0.0D;
    private boolean isNormalized=false;

    private final ByteBuffer buffer;

    public DiscreteProbabilityBuffer(int entries) {
        this.buffer = ByteBuffer.allocate(entries*RECORD_LEN);
    }

    public DiscreteProbabilityBuffer add(int i, double probability) {
        cumulativeProbability+=probability;
        buffer.putInt(i);
        buffer.putDouble(probability);
        return this;
    }

    /**
     * Normalize the dataset, but only if the cumulative probability is not close to
     * the unit probability of 1.0D, within some phi threshold. In either case,
     * mark the dataset as normalized.
     * @param phi A double value, preferably very small, like 0.000000001D
     */
    public void normalize(double phi) {
        if (Math.abs(cumulativeProbability-1.0D) iterator() {
        return new Iter(buffer.duplicate());
    }

    public double getCumulativeProbability() {
        return cumulativeProbability;
    }

    private static class Iter implements Iterator {

        private ByteBuffer iterbuf;

        public Iter(ByteBuffer iterbuf) {
            this.iterbuf = iterbuf;
        }

        @Override
        public boolean hasNext() {
            return iterbuf.remaining()>0;
        }

        @Override
        public Entry next() {
            int eid = iterbuf.getInt();
            double prob = iterbuf.getDouble();
            return new Entry(eid,prob);
        }
    }

    public static class Entry {
        private int eventId;
        private double probability;

        public Entry(int eventId, double probability) {
            this.eventId = eventId;
            this.probability = probability;
        }

        public int getEventId() {
            return eventId;
        }

        public double getProbability() {
            return probability;
        }
    }

}