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/*
* File: LineMinimizerBacktracking.java
* Authors: Kevin R. Dixon
* Company: Sandia National Laboratories
* Project: Cognitive Foundry
*
* Copyright Mar 1, 2008, Sandia Corporation.
* Under the terms of Contract DE-AC04-94AL85000, there is a non-exclusive
* license for use of this work by or on behalf of the U.S. Government.
* Export of this program may require a license from the United States
* Government. See CopyrightHistory.txt for complete details.
*
*/
package gov.sandia.cognition.learning.algorithm.minimization.line;
import gov.sandia.cognition.annotation.PublicationReference;
import gov.sandia.cognition.annotation.PublicationType;
import gov.sandia.cognition.evaluator.Evaluator;
import gov.sandia.cognition.math.DifferentiableUnivariateScalarFunction;
import gov.sandia.cognition.math.matrix.NumericalDifferentiator;
/**
* Implementation of the backtracking line-minimization algorithm. This
* algorithm takes a full Newton step in the direction of a minimum and,
* if this does not find a sufficiently decreasing function evaluation, it
* then successively reduces the step length and tries again until a
* sufficiently decreasing value is found.
*
* This is a very inexact line minimizer, but it appears to help the
* performance of both Quasi-Newton and Conjugate Gradient algorithms.
* In my tests, it appears to reduce the computation by about 30%-60%.
*
* @author Kevin R. Dixon
* @since 2.1
*/
@PublicationReference(
author={
"William H. Press",
"Saul A. Teukolsky",
"William T. Vetterling",
"Brian P. Flannery"
},
title="Numerical Recipes in C, Second Edition",
type=PublicationType.Book,
year=1992,
pages={384,386},
url="http://www.nrbook.com/a/bookcpdf.php"
)
public class LineMinimizerBacktracking
extends AbstractAnytimeLineMinimizer>
{
/**
* Maximum step size allowed by a parabolic fit, {@value}
*/
public static final double STEP_MAX = 100.0;
/**
* Default sufficient decrease value, {@value}
*/
public static final double DEFAULT_SUFFICIENT_DECREASE = 0.5;
/**
* Default amount to decrease the step amount each iteration, {@value}.
*/
public static final double DEFAULT_GEOMETRIC_DECREASE = 0.5;
/**
* Default flag to use numerical differentiation, {@value}.
*/
public static final boolean DEFAULT_NUMERICAL_DERIVATIVE = true;
/**
* Sufficient decrease condition, must be (0,1). Smaller values (0.1)
* result in more accurate searches, larger values (0.9) tend to be
* easier to satisfy.
*/
private double sufficientDecrease;
/**
* Amount to decrease the step amount each iteration.
*/
private double geometricDecrease;
/**
* Flag whether or not to use the numerical differentiation.
*/
private boolean numericalDerivative;
/**
* Creates a new instance of LineMinimizerBacktracking
*/
public LineMinimizerBacktracking()
{
this( DEFAULT_GEOMETRIC_DECREASE );
}
/**
* Creates a new instance of LineMinimizerBacktracking
* @param geometricDecrease
* Amount to decrease the step amount each iteration.
*/
public LineMinimizerBacktracking(
double geometricDecrease )
{
this( geometricDecrease, DEFAULT_NUMERICAL_DERIVATIVE );
}
/**
* Creates a new instance of LineMinimizerBacktracking
* @param geometricDecrease
* Amount to decrease the step amount each iteration.
* @param numericalDerivative
* Flag whether or not to use the numerical differentiation.
*/
public LineMinimizerBacktracking(
double geometricDecrease,
boolean numericalDerivative )
{
this( geometricDecrease, numericalDerivative, DEFAULT_SUFFICIENT_DECREASE );
}
/**
* Creates a new instance of LineMinimizerBacktracking
* @param geometricDecrease
* Amount to decrease the step amount each iteration.
* @param numericalDerivative
* Flag whether or not to use the numerical differentiation.
* @param sufficientDecrease
* Sufficient decrease condition, must be (0,1). Smaller values (0.1)
* result in more accurate searches, larger values (0.9) tend to be
* easier to satisfy.
*/
public LineMinimizerBacktracking(
double geometricDecrease,
boolean numericalDerivative,
double sufficientDecrease )
{
super( null );
this.setGeometricDecrease(geometricDecrease);
this.setNumericalDerivative(numericalDerivative);
this.setSufficientDecrease( sufficientDecrease );
}
/**
* Current value of the step size.
*/
private double stepValue;
@Override
protected boolean initializeAlgorithm()
{
// Ignore the return from the super method, as all that matters is if
// we have an initial guess.
super.initializeAlgorithm();
return (this.getInitialGuess() != null);
}
@Override
public boolean bracketingStep()
{
Double initialGuessFunctionValue;
if( this.getInitialGuessFunctionValue() != null )
{
initialGuessFunctionValue = this.getInitialGuessFunctionValue();
}
else
{
initialGuessFunctionValue = this.data.evaluate( this.getInitialGuess() );
}
Double initialGuessSlope;
if( this.getInitialGuessSlope() != null )
{
initialGuessSlope = this.getInitialGuessSlope();
}
else if( !this.getNumericalDerivative() &&
(this.data instanceof DifferentiableUnivariateScalarFunction) )
{
initialGuessSlope =
((DifferentiableUnivariateScalarFunction) this.data).differentiate( this.getInitialGuess() );
}
else
{
initialGuessSlope = NumericalDifferentiator.DoubleJacobian.differentiate(
this.getInitialGuess(), this.data );
}
// The initial point will be considered 0.0
InputOutputSlopeTriplet initialTriplet = new InputOutputSlopeTriplet(
this.getInitialGuess(),
initialGuessFunctionValue,
initialGuessSlope );
double slope = initialTriplet.getSlope();
// The initialTriplet hasn't yet been converted, so we can just return
this.result = initialTriplet;
// Look for a nearly flat function and bound the search, because it's
// likely to be hopeless
if (Math.abs(slope) <= this.getTolerance())
{
this.stop();
return true;
}
// Compute the Newton step. If that's too far
double newtonStep = -Math.abs(initialGuessFunctionValue) / slope;
if( Math.abs(newtonStep) > STEP_MAX )
{
newtonStep = -Math.signum(slope) * STEP_MAX;
}
this.stepValue = newtonStep;
// We only know the lower bound right now
this.getBracket().setLowerBound( initialTriplet );
return true;
}
@Override
public boolean sectioningStep()
{
LineBracket bracket = this.getBracket();
InputOutputSlopeTriplet initialTriplet = bracket.getLowerBound();
double x = this.stepValue + initialTriplet.getInput();
double fx = this.data.evaluate( x );
InputOutputSlopeTriplet trialPoint =
new InputOutputSlopeTriplet( x, fx, null );
bracket.setOtherPoint( bracket.getUpperBound() );
bracket.setUpperBound( trialPoint );
// Store this point off
if( this.result.getOutput() > trialPoint.getOutput() )
{
this.result = trialPoint;
}
if( WolfeConditions.evaluateGoldsteinCondition(
initialTriplet, trialPoint, this.getSufficientDecrease() ) )
{
this.result = trialPoint;
return false;
}
this.stepValue *= this.getGeometricDecrease();
return (Math.abs(this.stepValue) > this.getTolerance());
}
/**
* Getter for sufficientDecrease
* @return
* Sufficient decrease condition, must be (0,1). Smaller values (0.1)
* result in more accurate searches, larger values (0.9) tend to be
* easier to satisfy.
*/
public double getSufficientDecrease()
{
return this.sufficientDecrease;
}
/**
* Setter for sufficientDecrease
* @param sufficientDecrease
* Sufficient decrease condition, must be (0,1). Smaller values (0.1)
* result in more accurate searches, larger values (0.9) tend to be
* easier to satisfy.
*/
public void setSufficientDecrease(
double sufficientDecrease )
{
if( (sufficientDecrease <= 0.0) ||
(sufficientDecrease >= 1.0) )
{
throw new IllegalArgumentException(
"sufficientDecrease must be (0,1)" );
}
this.sufficientDecrease = sufficientDecrease;
}
/**
* Getter for geometricDecrease.
* @return
* Amount to decrease the step amount each iteration.
*/
public double getGeometricDecrease()
{
return this.geometricDecrease;
}
/**
* Setter for geometricDecrease
* @param geometricDecrease
* Amount to decrease the step amount each iteration.
*/
public void setGeometricDecrease(
double geometricDecrease)
{
if( (geometricDecrease <= 0.0) ||
(geometricDecrease >= 1.0) )
{
throw new IllegalArgumentException( "geometricDecrease must be (0,1)" );
}
this.geometricDecrease = geometricDecrease;
}
/**
* Getter for numericalDerivative
* @return
* Flag whether or not to use the numerical differentiation.
*/
public boolean getNumericalDerivative()
{
return this.numericalDerivative;
}
/**
* Setter for numericalDerivative
* @param numericalDerivative
* Flag whether or not to use the numerical differentiation.
*/
public void setNumericalDerivative(
boolean numericalDerivative)
{
this.numericalDerivative = numericalDerivative;
}
}
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