src/main/java/org/apache/commons/math/ode/nonstiff/ClassicalRungeKuttaStepInterpolator.java

/*
 * 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.ode.nonstiff;

import org.apache.commons.math.ode.DerivativeException;
import org.apache.commons.math.ode.sampling.StepInterpolator;

/**
 * This class implements a step interpolator for the classical fourth
 * order Runge-Kutta integrator.
 *
 * <p>This interpolator allows to compute dense output inside the last
 * step computed. The interpolation equation is consistent with the
 * integration scheme :

 * <pre>
 *   y(t_n + theta h) = y (t_n + h)
 *                    + (1 - theta) (h/6) [ (-4 theta^2 + 5 theta - 1) y'_1
 *                                          +(4 theta^2 - 2 theta - 2) (y'_2 + y'_3)
 *                                          -(4 theta^2 +   theta + 1) y'_4
 *                                        ]
 * </pre>
 *
 * where theta belongs to [0 ; 1] and where y'_1 to y'_4 are the four
 * evaluations of the derivatives already computed during the
 * step.</p>
 *
 * @see ClassicalRungeKuttaIntegrator
 * @version $Revision: 1073158 $ $Date: 2011-02-21 22:46:52 +0100 (lun. 21 févr. 2011) $
 * @since 1.2
 */

class ClassicalRungeKuttaStepInterpolator
    extends RungeKuttaStepInterpolator {

    /** Serializable version identifier */
    private static final long serialVersionUID = -6576285612589783992L;

    /** Simple constructor.
     * This constructor builds an instance that is not usable yet, the
     * {@link RungeKuttaStepInterpolator#reinitialize} method should be
     * called before using the instance in order to initialize the
     * internal arrays. This constructor is used only in order to delay
     * the initialization in some cases. The {@link RungeKuttaIntegrator}
     * class uses the prototyping design pattern to create the step
     * interpolators by cloning an uninitialized model and latter initializing
     * the copy.
     */
    public ClassicalRungeKuttaStepInterpolator() {
    }

    /** Copy constructor.
     * @param interpolator interpolator to copy from. The copy is a deep
     * copy: its arrays are separated from the original arrays of the
     * instance
     */
    public ClassicalRungeKuttaStepInterpolator(final ClassicalRungeKuttaStepInterpolator interpolator) {
        super(interpolator);
    }

    /** {@inheritDoc} */
    @Override
    protected StepInterpolator doCopy() {
        return new ClassicalRungeKuttaStepInterpolator(this);
    }

    /** {@inheritDoc} */
    @Override
    protected void computeInterpolatedStateAndDerivatives(final double theta,
                                            final double oneMinusThetaH)
        throws DerivativeException {

        final double fourTheta      = 4 * theta;
        final double oneMinusTheta  = 1 - theta;
        final double oneMinus2Theta = 1 - 2 * theta;
        final double s             = oneMinusThetaH / 6.0;
        final double coeff1        = s * ((-fourTheta + 5) * theta - 1);
        final double coeff23       = s * (( fourTheta - 2) * theta - 2);
        final double coeff4        = s * ((-fourTheta - 1) * theta - 1);
        final double coeffDot1     = oneMinusTheta * oneMinus2Theta;
        final double coeffDot23    = 2 * theta * oneMinusTheta;
        final double coeffDot4     = -theta * oneMinus2Theta;
        for (int i = 0; i < interpolatedState.length; ++i) {
            final double yDot1  = yDotK[0][i];
            final double yDot23 = yDotK[1][i] + yDotK[2][i];
            final double yDot4  = yDotK[3][i];
            interpolatedState[i] =
                currentState[i] + coeff1  * yDot1 + coeff23 * yDot23 + coeff4  * yDot4;
            interpolatedDerivatives[i] =
                coeffDot1 * yDot1 + coeffDot23 * yDot23 + coeffDot4 * yDot4;
        }

    }

}