es.uam.eps.ir.relison.diffusion.expiration.ExponentialDecayExpirationMechanism Maven / Gradle / Ivy
/*
* Copyright (C) 2020 Information Retrieval Group at Universidad Autónoma
* de Madrid, http://ir.ii.uam.es
*
* This Source Code Form is subject to the terms of the Mozilla Public
* License, v. 2.0. If a copy of the MPL was not distributed with this
* file, You can obtain one at http://mozilla.org/MPL/2.0/.
*/
package es.uam.eps.ir.relison.diffusion.expiration;
import es.uam.eps.ir.relison.diffusion.data.Data;
import es.uam.eps.ir.relison.diffusion.data.Information;
import es.uam.eps.ir.relison.diffusion.simulation.UserState;
import java.io.Serializable;
import java.util.Random;
import java.util.stream.Stream;
/**
* Expiration mechanism that increases exponentially the probability of discarding
* an information piece as the number of iterations since its creation increases.
*
* @author Javier Sanz-Cruzado ([email protected])
* @author Pablo Castells ([email protected])
*
* @param type of the users.
* @param type of the information.
* @param type of the parameters.
*/
public class ExponentialDecayExpirationMechanism implements ExpirationMechanism
{
/**
* Time before expiration.
*/
private final double decay;
/**
* Random number generator.
*/
private final Random rng;
/**
* Constructor.
* @param halfLife the time required for the piece to have p = 0.5 of being removed
*/
public ExponentialDecayExpirationMechanism(double halfLife)
{
this.rng = new Random();
this.decay = halfLife >= 0.0 ? Math.log(2.0)/halfLife : Double.POSITIVE_INFINITY;
}
@Override
public Stream expire(UserState user, Data data, int numIter, Long timestamp)
{
return user.getReceivedInformation().filter(piece ->
{
long time = numIter - piece.getTimestamp();
double rnd = rng.nextDouble();
return this.expdecay(time) < rnd;
}).map(Information::getInfoId);
}
/**
* Computes the probability that the information piece stays in the received list.
* @param time the difference between creation time and current time.
* @return the probability that the information piece stays in the received list.
*/
private double expdecay(long time)
{
return Math.exp(-this.decay*time);
}
}