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The Math project is a library of lightweight, self-contained mathematics and statistics components addressing the most common practical problems not immediately available in the Java programming language or commons-lang.
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/*
 * Licensed to the Apache Software Foundation (ASF) under one or more
 * contributor license agreements.  See the NOTICE file distributed with
 * this work for additional information regarding copyright ownership.
 * The ASF licenses this file to You 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 org.apache.commons.math.distribution;

import java.io.Serializable;

import org.apache.commons.math.MathException;
import org.apache.commons.math.MathRuntimeException;
import org.apache.commons.math.special.Beta;

/**
 * Default implementation of
 * {@link org.apache.commons.math.distribution.TDistribution}.
 *
 * @version $Revision: 772119 $ $Date: 2009-05-06 05:43:28 -0400 (Wed, 06 May 2009) $
 */
public class TDistributionImpl
    extends AbstractContinuousDistribution
    implements TDistribution, Serializable  {

    /** Serializable version identifier */
    private static final long serialVersionUID = -5852615386664158222L;
    
    /** The degrees of freedom*/
    private double degreesOfFreedom;

    /**
     * Create a t distribution using the given degrees of freedom.
     * @param degreesOfFreedom the degrees of freedom.
     */
    public TDistributionImpl(double degreesOfFreedom) {
        super();
        setDegreesOfFreedom(degreesOfFreedom);
    }

    /**
     * Modify the degrees of freedom.
     * @param degreesOfFreedom the new degrees of freedom.
     */
    public void setDegreesOfFreedom(double degreesOfFreedom) {
        if (degreesOfFreedom <= 0.0) {
            throw MathRuntimeException.createIllegalArgumentException(
                  "degrees of freedom must be positive ({0})",
                  degreesOfFreedom);
        }
        this.degreesOfFreedom = degreesOfFreedom;
    }

    /**
     * Access the degrees of freedom.
     * @return the degrees of freedom.
     */
    public double getDegreesOfFreedom() {
        return degreesOfFreedom;
    }

    /**
     * For this distribution, X, this method returns P(X < x).
     * @param x the value at which the CDF is evaluated.
     * @return CDF evaluted at x. 
     * @throws MathException if the cumulative probability can not be
     *            computed due to convergence or other numerical errors.
     */
    public double cumulativeProbability(double x) throws MathException{
        double ret;
        if (x == 0.0) {
            ret = 0.5;
        } else {
            double t =
                Beta.regularizedBeta(
                    getDegreesOfFreedom() / (getDegreesOfFreedom() + (x * x)),
                    0.5 * getDegreesOfFreedom(),
                    0.5);
            if (x < 0.0) {
                ret = 0.5 * t;
            } else {
                ret = 1.0 - 0.5 * t;
            }
        }

        return ret;
    }
    
    /**
     * For this distribution, X, this method returns the critical point x, such
     * that P(X < x) = p.
     * 
     * Returns Double.NEGATIVE_INFINITY for p=0 and 
     * Double.POSITIVE_INFINITY for p=1.
     *
     * @param p the desired probability
     * @return x, such that P(X < x) = p
     * @throws MathException if the inverse cumulative probability can not be
     *         computed due to convergence or other numerical errors.
     * @throws IllegalArgumentException if p is not a valid
     *         probability.
     */
    @Override
    public double inverseCumulativeProbability(final double p) 
    throws MathException {
        if (p == 0) {
            return Double.NEGATIVE_INFINITY;
        }
        if (p == 1) {
            return Double.POSITIVE_INFINITY;
        }
        return super.inverseCumulativeProbability(p);
    }

    /**
     * Access the domain value lower bound, based on p, used to
     * bracket a CDF root.  This method is used by
     * {@link #inverseCumulativeProbability(double)} to find critical values.
     * 
     * @param p the desired probability for the critical value
     * @return domain value lower bound, i.e.
     *         P(X < lower bound) < p 
     */
    @Override
    protected double getDomainLowerBound(double p) {
        return -Double.MAX_VALUE;
    }

    /**
     * Access the domain value upper bound, based on p, used to
     * bracket a CDF root.  This method is used by
     * {@link #inverseCumulativeProbability(double)} to find critical values.
     * 
     * @param p the desired probability for the critical value
     * @return domain value upper bound, i.e.
     *         P(X < upper bound) > p 
     */
    @Override
    protected double getDomainUpperBound(double p) {
        return Double.MAX_VALUE;
    }

    /**
     * Access the initial domain value, based on p, used to
     * bracket a CDF root.  This method is used by
     * {@link #inverseCumulativeProbability(double)} to find critical values.
     * 
     * @param p the desired probability for the critical value
     * @return initial domain value
     */
    @Override
    protected double getInitialDomain(double p) {
        return 0.0;
    }
}