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* Licensed to the Apache Software Foundation (ASF) under one or more
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* contributor license agreements. See the NOTICE file distributed with
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* this work for additional information regarding copyright ownership.
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* The ASF licenses this file to You under the Apache License, Version 2.0
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* (the "License"); you may not use this file except in compliance with
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* the License. You may obtain a copy of the License at
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* http://www.apache.org/licenses/LICENSE-2.0
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* Unless required by applicable law or agreed to in writing, software
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* distributed under the License is distributed on an "AS IS" BASIS,
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* WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
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* See the License for the specific language governing permissions and
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* limitations under the License.
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package org.apache.commons.math.ode;
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* This class implements a step interpolator for second order
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* Runge-Kutta integrator.
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* <p>This interpolator allow to compute dense output inside the last
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* step computed. The interpolation equation is consistent with the
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* integration scheme :
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* y(t_n + theta h) = y (t_n + h) + (1-theta) h [theta y'_1 - (1+theta) y'_2]
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* where theta belongs to [0 ; 1] and where y'_1 and y'_2 are the two
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* evaluations of the derivatives already computed during the
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* @see MidpointIntegrator
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* @version $Revision: 620312 $ $Date: 2008-02-10 12:28:59 -0700 (Sun, 10 Feb 2008) $
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class MidpointStepInterpolator
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extends RungeKuttaStepInterpolator {
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/** Simple constructor.
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* This constructor builds an instance that is not usable yet, the
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* {@link AbstractStepInterpolator#reinitialize} method should be called
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* before using the instance in order to initialize the internal arrays. This
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* constructor is used only in order to delay the initialization in
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* some cases. The {@link RungeKuttaIntegrator} class uses the
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* prototyping design pattern to create the step interpolators by
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* cloning an uninitialized model and latter initializing the copy.
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public MidpointStepInterpolator() {
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* @param interpolator interpolator to copy from. The copy is a deep
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* copy: its arrays are separated from the original arrays of the
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public MidpointStepInterpolator(MidpointStepInterpolator interpolator) {
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/** Really copy the finalized instance.
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* @return a copy of the finalized instance
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protected StepInterpolator doCopy() {
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return new MidpointStepInterpolator(this);
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/** Compute the state at the interpolated time.
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* This is the main processing method that should be implemented by
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* the derived classes to perform the interpolation.
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* @param theta normalized interpolation abscissa within the step
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* (theta is zero at the previous time step and one at the current time step)
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* @param oneMinusThetaH time gap between the interpolated time and
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* @throws DerivativeException this exception is propagated to the caller if the
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* underlying user function triggers one
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protected void computeInterpolatedState(double theta,
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double oneMinusThetaH)
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throws DerivativeException {
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double coeff1 = oneMinusThetaH * theta;
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double coeff2 = oneMinusThetaH * (1.0 + theta);
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for (int i = 0; i < interpolatedState.length; ++i) {
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interpolatedState[i] = currentState[i] +
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coeff1 * yDotK[0][i] - coeff2 * yDotK[1][i];
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/** Serializable version identifier */
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private static final long serialVersionUID = -865524111506042509L;