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/*
 * File:                KernelScalarFunction.java
 * Authors:             Justin Basilico
 * Company:             Sandia National Laboratories
 * Project:             Cognitive Framework Lite
 *
 * Copyright August 27, 2007, 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.function.scalar;

import gov.sandia.cognition.learning.function.regression.Regressor;
import gov.sandia.cognition.learning.function.kernel.DefaultKernelContainer;
import gov.sandia.cognition.learning.function.kernel.Kernel;
import gov.sandia.cognition.util.WeightedValue;
import java.util.ArrayList;
import java.util.Collection;

/**
 * The {@code KernelScalarFunction} class implements a scalar function that
 * uses a kernel to compute its output value. It is parameterized by a kernel
 * function, a list of examples and their weights, and a bias term. This types
 * of scalar function represents what is learned by algorithms such as 
 * Support Vector Regression.
 *
 * @param   The type of the input to the function. 
 *      For example, Vector.
 * @author Justin Basilico
 * @since  2.0
 */
public class KernelScalarFunction
    extends DefaultKernelContainer
    implements Regressor
{
    /** The default value for the bias is 0.0. */
    public static final double DEFAULT_BIAS = 0.0;
    
    /** The list of weighted examples that are used for categorization. */
    protected Collection> examples;
    
    /** The bias term. */
    protected double bias;
    
    /**
     * Creates a new instance of KernelScalarFunction.
     */
    public KernelScalarFunction()
    {
        this((Kernel) null);
    }
    
    /**
     * Creates a new instance of KernelScalarFunction with the given kernel.
     *
     * @param  kernel The kernel to use.
     */
    public KernelScalarFunction(
        final Kernel kernel)
    {
        this(kernel, new ArrayList>(), DEFAULT_BIAS);
    }
        
    /**
     * Creates a new instance of KernelScalarFunction with the given kernel,
     * weighted examples, and bias.
     *
     * @param  kernel The kernel to use.
     * @param  examples The weighted examples.
     * @param  bias The bias.
     */
    public KernelScalarFunction(
        final Kernel kernel,
        final Collection> examples,
        final double bias)
    {
        super(kernel);
        
        this.setExamples(examples);
        this.setBias(bias);
    }
    
    /**
     * Creates a new copy of a KernelScalarFunction.
     *
     * @param  other The KernelScalarFunction to copy.
     */
    public KernelScalarFunction(
        final KernelScalarFunction other)
    {
        super(other);

        this.setExamples(
            other.getExamples() == null ? null : 
                new ArrayList>(
                    other.getExamples()));
        this.setBias(other.getBias());
    }
    
    /**
     * Evaluates the given input vector as a double by:
     * 
     *     sum w_i * k(input, x_i)
     *
     * @param  input The input to categorize.
     * @return The output categorization as a double where the sign is the
     *         categorization.
     */
    public Double evaluate(
        final InputType input)
    {
        return this.evaluateAsDouble(input);
    }
    
    /**
     * Evaluates the given input vector as a double by:
     *
     *     sum w_i * k(input, x_i)
     *
     * @param  input The input to categorize.
     * @return The output categorization as a double where the sign is the
     *         categorization.
     */
    @Override
    public double evaluateAsDouble(
        final InputType input)
    {
        // The sum starts out with the bias term.
        double sum = this.bias;

        // Loop over all the examples.
        for ( WeightedValue example : this.examples )
        {
            final double weight = example.getWeight();

            if ( weight == 0.0 )
            {
                continue;
            }

            // Evaluate the kernel between the example and the input.
            final double value = 
                this.kernel.evaluate(input, example.getValue());

            // Updatre the sum.
            sum += weight * value;
        }
        
        return sum;
    }
    
    /**
     * Gets the list of weighted examples that categorizer is using.
     *
     * @return The list of weighted examples.
     */
    public Collection> 
        getExamples()
    {
        return this.examples;
    }
    
    /**
     * Sets the list of weighted examples that categorizer is using.
     *
     * @param  examples The list of weighted examples.
     */
    public void setExamples(
        final Collection> examples)
    {
        this.examples = examples;
    }

    /**
     * Gets the bias term.
     *
     * @return bias The bias term.
     */
    public double getBias()
    {
        return this.bias;
    }

    /**
     * Sets the bias term.
     *
     * @param  bias The bias term.
     */
    public void setBias(
        final double bias)
    {
        this.bias = bias;
    }
}