/*
* (c) Copyright Christian P. Fries, Germany. All rights reserved. Contact: email@christian-fries.de.
*
* Created on 09.02.2004
*/
package net.finmath.montecarlo;
import java.io.Serializable;
import net.finmath.compatibility.java.util.function.DoubleUnaryOperator;
import net.finmath.compatibility.java.util.function.IntFunction;
import net.finmath.randomnumbers.MersenneTwister;
import net.finmath.stochastic.RandomVariableInterface;
import net.finmath.time.TimeDiscretizationInterface;
/**
* Implementation of a time-discrete n-dimensional sequence of independent increments
* <i>W = (W<sub>1</sub>,...,W<sub>n</sub>)</i> form a given set of inverse
* cumulative distribution functions.
*
* Independent increments is a sequence of independent random variables index
* by the time index associated with the time discretization. At each time step
* the increment is a d-dimensional random variable \( Z(t_{i}) \), where d is <code>numberOfFactors</code>.
* where each component of \( Z_{j}(t_{i}) \) is given by
* \[
* Z_{j}(t_{i}) = ICDF_{i,j}(U_{i,j})
* \]
* for a sequence of independent uniform distributes random variables U_{i,j}.
*
* The inverse cumulative distribution functions \( ICDF_{i,j} \) are given by
* <code>inverseCumulativeDistributionFunctions</code> as the
* map \( i \mapsto ( j \mapsto ICDF_{i,j} ) \) (here i is the time index and j is the factor (component).
*
* Each \( U_{i,j} \) is samples using <code>numberOfPaths</code>.
*
* The class is immutable and thread safe. It uses lazy initialization.
*
* @author Christian Fries
* @version 1.6
*/
public class IndependentIncrements implements IndependentIncrementsInterface, Serializable {
/**
*
*/
private static final long serialVersionUID = 6270884840989559532L;
private final TimeDiscretizationInterface timeDiscretization;
private final int numberOfFactors;
private final int numberOfPaths;
private final int seed;
private final AbstractRandomVariableFactory randomVariableFactory;
private transient RandomVariableInterface[][] increments;
private final Object incrementsLazyInitLock = new Object();
private final IntFunction<IntFunction<DoubleUnaryOperator>> inverseCumulativeDistributionFunctions;
/**
* Construct the simulation of independent increments.
*
* Independent increments is a sequence of independent random variables index
* by the time index associated with the time discretization. At each time step
* the increment is a d-dimensional random variable \( Z(t_{i}) \), where d is <code>numberOfFactors</code>.
* where each component of \( Z_{j}(t_{i}) \) is given by
* \[
* Z_{j}(t_{i}) = ICDF_{i,j}(U_{i,j})
* \]
* for a sequence of independent uniform distributes random variables U_{i,j}.
*
* The inverse cumulative distribution functions \( ICDF_{i,j} \) are given by
* <code>inverseCumulativeDistributionFunctions</code> as the
* map \( i \mapsto ( j \mapsto ICDF_{i,j} ) \) (here i is the time index and j is the factor (component).
*
* Each \( U_{i,j} \) is samples using <code>numberOfPaths</code>.
*
* The constructor allows to set the factory to be used for the construction of
* random variables. This allows to generate increments represented
* by different implementations of the RandomVariableInterface
* (e.g. the RandomVariableLowMemory internally using float representations).
*
* @param timeDiscretization The time discretization used for the increments.
* @param numberOfFactors Number of factors.
* @param numberOfPaths Number of paths to simulate.
* @param seed The seed of the random number generator.
* @param inverseCumulativeDistributionFunctions A map from the timeIndices to a map from the from the factors to the corresponding inverse cumulative distribution function.
* @param randomVariableFactory Factory to be used to create random variable.
*/
public IndependentIncrements(
TimeDiscretizationInterface timeDiscretization,
int numberOfFactors,
int numberOfPaths,
int seed,
IntFunction<IntFunction<DoubleUnaryOperator>> inverseCumulativeDistributionFunctions,
AbstractRandomVariableFactory randomVariableFactory) {
super();
this.timeDiscretization = timeDiscretization;
this.numberOfFactors = numberOfFactors;
this.numberOfPaths = numberOfPaths;
this.seed = seed;
this.inverseCumulativeDistributionFunctions = inverseCumulativeDistributionFunctions;
this.randomVariableFactory = randomVariableFactory;
this.increments = null; // Lazy initialization
}
/**
* Construct the simulation of independet increments.
*
* The independent increments is a sequence of independent random variables index
* by the time index associated with the time discretization. At each time step
* the increment is a d-dimensional random variable \( Z(t_{i}) \), where d is <code>numberOfFactors</code>.
* where each component of \( Z_{j}(t_{i}) \) is given by
* \[
* Z_{j}(t_{i}) = ICDF_{i,j}(U_{i,j})
* \]
* for a sequence of independent uniform distributes random variables U_{i,j}.
*
* The inverse cumulative distribution functions \( ICDF_{i,j} \) are given by
* <code>inverseCumulativeDistributionFunctions</code> as the
* map \( i \mapsto ( j \mapsto ICDF_{i,j} ) \) (here i is the time index and j is the factor (component).
*
* Each \( U_{i,j} \) is samples using <code>numberOfPaths</code>.
*
* @param timeDiscretization The time discretization used for the increments.
* @param numberOfFactors Number of factors.
* @param numberOfPaths Number of paths to simulate.
* @param seed The seed of the random number generator.
* @param inverseCumulativeDistributionFunctions A map from the timeIndices to a map from the from the factors to the corresponding inverse cumulative distribution function.
*/
public IndependentIncrements(
TimeDiscretizationInterface timeDiscretization,
int numberOfFactors,
int numberOfPaths,
int seed,
IntFunction<IntFunction<DoubleUnaryOperator>> inverseCumulativeDistributionFunctions) {
this(timeDiscretization, numberOfFactors, numberOfPaths, seed, inverseCumulativeDistributionFunctions, new RandomVariableFactory());
}
@Override
public IndependentIncrementsInterface getCloneWithModifiedSeed(int seed) {
return new IndependentIncrements(getTimeDiscretization(), getNumberOfFactors(), getNumberOfPaths(), seed, inverseCumulativeDistributionFunctions, randomVariableFactory);
}
@Override
public IndependentIncrementsInterface getCloneWithModifiedTimeDiscretization(TimeDiscretizationInterface newTimeDiscretization) {
return new IndependentIncrements(newTimeDiscretization, getNumberOfFactors(), getNumberOfPaths(), getSeed(), inverseCumulativeDistributionFunctions, randomVariableFactory);
}
@Override
public RandomVariableInterface getIncrement(int timeIndex, int factor) {
// Thread safe lazy initialization
synchronized(incrementsLazyInitLock) {
if(increments == null) doGenerateIncrements();
}
/*
* We return an immutable object which ensures that the receiver does not alter the data.
*/
return increments[timeIndex][factor];
}
/**
* Lazy initialization of brownianIncrement. Synchronized to ensure thread safety of lazy init.
*/
private void doGenerateIncrements() {
if(increments != null) return; // Nothing to do
// Create random number sequence generator
MersenneTwister mersenneTwister = new MersenneTwister(seed);
// Allocate memory
double[][][] incrementsArray = new double[timeDiscretization.getNumberOfTimeSteps()][numberOfFactors][numberOfPaths];
// Pre-fetch icdfs
DoubleUnaryOperator[][] inverseCumulativeDistributionFunctions = new DoubleUnaryOperator[timeDiscretization.getNumberOfTimeSteps()][numberOfFactors];
for(int timeIndex=0; timeIndex<timeDiscretization.getNumberOfTimeSteps(); timeIndex++) {
for(int factor=0; factor<numberOfFactors; factor++) {
inverseCumulativeDistributionFunctions[timeIndex][factor] = this.inverseCumulativeDistributionFunctions.apply(timeIndex).apply(factor);
}
}
/*
* Generate normal distributed independent increments.
*
* The inner loop goes over time and factors.
* MersenneTwister is known to generate "independent" increments in 623 dimensions.
* Since we want to generate independent streams (paths), the loop over path is the outer loop.
*/
for(int path=0; path<numberOfPaths; path++) {
for(int timeIndex=0; timeIndex<timeDiscretization.getNumberOfTimeSteps(); timeIndex++) {
// Generate uncorrelated Brownian increment
for(int factor=0; factor<numberOfFactors; factor++) {
double uniformIncrement = mersenneTwister.nextDouble();
incrementsArray[timeIndex][factor][path] = inverseCumulativeDistributionFunctions[timeIndex][factor].applyAsDouble(uniformIncrement);
}
}
}
// Allocate memory for RandomVariable wrapper objects.
increments = new RandomVariableInterface[timeDiscretization.getNumberOfTimeSteps()][numberOfFactors];
// Wrap the values in RandomVariable objects
for(int timeIndex=0; timeIndex<timeDiscretization.getNumberOfTimeSteps(); timeIndex++) {
double time = timeDiscretization.getTime(timeIndex+1);
for(int factor=0; factor<numberOfFactors; factor++) {
increments[timeIndex][factor] =
randomVariableFactory.createRandomVariable(time, incrementsArray[timeIndex][factor]);
}
}
}
@Override
public TimeDiscretizationInterface getTimeDiscretization() {
return timeDiscretization;
}
@Override
public int getNumberOfFactors() {
return numberOfFactors;
}
@Override
public int getNumberOfPaths() {
return numberOfPaths;
}
@Override
public RandomVariableInterface getRandomVariableForConstant(double value) {
return randomVariableFactory.createRandomVariable(value);
}
/**
* @return Returns the seed.
*/
public int getSeed() {
return seed;
}
public String toString() {
return super.toString()
+ "\n" + "timeDiscretization: " + timeDiscretization.toString()
+ "\n" + "numberOfPaths: " + numberOfPaths
+ "\n" + "numberOfFactors: " + numberOfFactors
+ "\n" + "seed: " + seed;
}
@Override
public boolean equals(Object o) {
if (this == o) return true;
if (o == null || getClass() != o.getClass()) return false;
IndependentIncrements that = (IndependentIncrements) o;
if (numberOfFactors != that.numberOfFactors) return false;
if (numberOfPaths != that.numberOfPaths) return false;
if (seed != that.seed) return false;
if (!timeDiscretization.equals(that.timeDiscretization)) return false;
return true;
}
@Override
public int hashCode() {
int result = timeDiscretization.hashCode();
result = 31 * result + numberOfFactors;
result = 31 * result + numberOfPaths;
result = 31 * result + seed;
return result;
}
}