com.analog.lyric.dimple.factorfunctions.Gamma.java Source code

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/*******************************************************************************
*   Copyright 2012 Analog Devices, Inc.
*
*   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
*
*       http://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 com.analog.lyric.dimple.factorfunctions;

import java.util.Map;

import org.eclipse.jdt.annotation.Nullable;

import com.analog.lyric.dimple.exceptions.DimpleException;
import com.analog.lyric.dimple.factorfunctions.core.FactorFunction;
import com.analog.lyric.dimple.factorfunctions.core.FactorFunctionUtilities;
import com.analog.lyric.dimple.factorfunctions.core.IParametricFactorFunction;
import com.analog.lyric.dimple.model.values.Value;

/**
 * Gamma distribution. The variables in the argument list are ordered as follows:
 * 
 * 1) Alpha: Alpha parameter of the Gamma distribution (non-negative)
 * 2) Beta: Beta parameter of the Gamma distribution (non-negative)
 * 3...) An arbitrary number of real variables
 * 
 * Alpha and Beta parameters may optionally be specified as constants in the constructor.
 * In this case, they are not included in the list of arguments.
 * 
 */
public class Gamma extends FactorFunction implements IParametricFactorFunction {
    protected double _alpha;
    protected double _beta;
    protected double _alphaMinusOne;
    protected double _logBeta;
    protected double _logGammaAlphaMinusAlphaLogBeta;
    protected boolean _parametersConstant = false;
    protected int _firstDirectedToIndex = 2;

    /*--------------
     * Construction
     */

    public Gamma() {
        super();
    }

    public Gamma(double alpha, double beta) {
        this();
        _alpha = alpha;
        _beta = beta;
        _alphaMinusOne = _alpha - 1;
        _logBeta = Math.log(_beta);
        _logGammaAlphaMinusAlphaLogBeta = org.apache.commons.math3.special.Gamma.logGamma(_alpha)
                - _alpha * _logBeta;
        _parametersConstant = true;
        _firstDirectedToIndex = 0;
        if (_alpha <= 0)
            throw new DimpleException("Non-positive alpha parameter. This must be a positive value.");
        if (_beta <= 0)
            throw new DimpleException("Non-positive beta parameter. This must be a positive value.");
    }

    /**
     * Construct from specified parameters
     * @param parameters the following values are supported
     * <ul>
     * <li>alpha (default 1.0)
     * <li>beta (default 1.0)
     * </ul>
     * @since 0.07
     */
    public Gamma(Map<String, Object> parameters) {
        this((double) getOrDefault(parameters, "alpha", 1.0), (double) getOrDefault(parameters, "beta", 1.0));
    }

    /*------------------------
     * FactorFunction methods
     */

    @Override
    public final double evalEnergy(Value[] arguments) {
        int index = 0;
        if (!_parametersConstant) {
            _alpha = arguments[index++].getDouble(); // First input is alpha parameter (must be non-negative)
            if (_alpha <= 0)
                return Double.POSITIVE_INFINITY;
            _beta = arguments[index++].getDouble(); // Second input is beta parameter (must be non-negative)
            if (_beta <= 0)
                return Double.POSITIVE_INFINITY;
            _logBeta = Math.log(_beta);
        }
        final int length = arguments.length;
        final int N = length - index; // Number of non-parameter variables
        double sum = 0;
        if (_alpha == 1) {
            for (; index < length; index++) {
                double x = arguments[index].getDouble(); // Remaining inputs are Gamma variables
                if (x < 0)
                    return Double.POSITIVE_INFINITY;
                else
                    sum += x;
            }
            return sum * _beta - N * _logBeta;
        } else {
            if (!_parametersConstant) {
                _alphaMinusOne = _alpha - 1;
                _logGammaAlphaMinusAlphaLogBeta = org.apache.commons.math3.special.Gamma.logGamma(_alpha)
                        - _alpha * _logBeta;
            }
            for (; index < length; index++) {
                final double x = arguments[index].getDouble(); // Remaining inputs are Gamma variables
                if (x < 0)
                    return Double.POSITIVE_INFINITY;
                else
                    sum += x * _beta - Math.log(x) * _alphaMinusOne;
            }
            return sum + N * _logGammaAlphaMinusAlphaLogBeta;
        }
    }

    @Override
    public final boolean isDirected() {
        return true;
    }

    @Override
    public final int[] getDirectedToIndices(int numEdges) {
        // All edges except the parameter edges (if present) are directed-to edges
        return FactorFunctionUtilities.getListOfIndices(_firstDirectedToIndex, numEdges - 1);
    }

    /*-----------------------------------
     * IParametricFactorFunction methods
     */

    @Override
    public int copyParametersInto(Map<String, Object> parameters) {
        if (_parametersConstant) {
            parameters.put("alpha", _alpha);
            parameters.put("beta", _beta);
            return 2;
        }
        return 0;
    }

    @Override
    public @Nullable Object getParameter(String parameterName) {
        if (_parametersConstant) {
            switch (parameterName) {
            case "alpha":
                return _alpha;
            case "beta":
                return _beta;
            }
        }
        return null;
    }

    @Override
    public final boolean hasConstantParameters() {
        return _parametersConstant;
    }

    /*-------------------------
     * Factor-specific methods
     */

    public final double getAlphaMinusOne() {
        return _alphaMinusOne;
    }

    public final double getBeta() {
        return _beta;
    }
}