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Efficient reliable UDP unicast, UDP multicast, and IPC transport protocol.
There is a newer version: 1.48.0
/*
 * Copyright 2014-2024 Real Logic Limited.
 *
 * Licensed 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
 *
 * https://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 io.aeron.driver;

import java.util.concurrent.ThreadLocalRandom;

/**
 * Feedback delay used for NAKs as well as for some retransmission use cases.
 * 
 * Generates delay based on Optimal Multicast Feedback

 * http://tools.ietf.org/html/rfc5401#page-13
 * 

 * {@code maxBackoffT} is max interval for delay
 * 

 * C version of the code:
 * 
{@code
 * double RandomBackoff(double maxBackoffT, double groupSize)
 * {
 *     double lambda = log(groupSize) + 1;
 *     double x = UniformRand(lambda / maxBackoffT) + lambda / (maxBackoffT * (exp(lambda) - 1));
 *     return ((maxBackoffT / lambda) * log(x * (exp(lambda) - 1) * (maxBackoffT / lambda)));
 * }
 * }
 * where {@code UniformRand(x)} is uniform distribution from {@code 0..max}
 * 
 * In this implementation's calculation:
 * 

 *     the {@code groupSize} is a constant (could be configurable as system property)
 *     {@code maxBackoffT} is a constant (could be configurable as system property)
 *     {@code GRTT} is a constant (could be configurable as a system property)
 * 
 * 
 * {@code N} (the expected number of feedback messages per RTT) is:

 * {@code N = exp(1.2 * L / (2 * maxBackoffT / GRTT))}
 * 

 * Assumptions:
 * 

 * {@code maxBackoffT = K * GRTT (K >= 1)}
 * 

 * Recommended {@code K}:
 * 

 *     K = 4 for situations where responses come from multiple places (i.e. for NAKs, multiple retransmitters)
 *     K = 6 for situations where responses come from single places (i.e. for NAKs, source only retransmit)
 * 
 */
public class OptimalMulticastDelayGenerator implements FeedbackDelayGenerator
{
    private final double randMax;
    private final double baseX;
    private final double constantT;
    private final double factorT;

    /**
     * Create new feedback delay generator based on estimates. Pre-calculating some parameters upfront.
     *
     * @param maxBackoffT of the delay interval
     * @param groupSize   estimate
     */
    public OptimalMulticastDelayGenerator(final double maxBackoffT, final double groupSize)
    {
        final double lambda = Math.log(groupSize) + 1;

        this.randMax = lambda / maxBackoffT;
        this.baseX = lambda / (maxBackoffT * (Math.exp(lambda) - 1));
        this.constantT = maxBackoffT / lambda;
        this.factorT = (Math.exp(lambda) - 1) * (maxBackoffT / lambda);
    }

    /**
     * {@inheritDoc}
     */
    public long generateDelayNs()
    {
        return (long)generateNewOptimalDelay();
    }

    /**
     * {@inheritDoc}
     */
    public boolean shouldFeedbackImmediately()
    {
        return false;
    }

    /**
     * Generate a new randomized delay value in the units of {@code maxBackoffT}}.
     *
     * @return delay in units of {@code maxBackoffT}.
     */
    public double generateNewOptimalDelay()
    {
        final double x = uniformRandom(randMax) + baseX;

        return constantT * Math.log(x * factorT);
    }

    /**
     * Return uniform random value in the range 0 to max.
     *
     * @param max of the random range
     * @return random value
     */
    public static double uniformRandom(final double max)
    {
        return ThreadLocalRandom.current().nextDouble() * max;
    }

    /**
     * {@inheritDoc}
     */
    public String toString()
    {
        return "OptimalMulticastDelayGenerator{" +
            "randMax=" + randMax +
            ", baseX=" + baseX +
            ", constantT=" + constantT +
            ", factorT=" + factorT +
            ", shouldFeedbackImmediately=" + shouldFeedbackImmediately() +
            '}';
    }
}