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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.math3.ode.nonstiff;
import org.apache.commons.math3.Field;
import org.apache.commons.math3.RealFieldElement;
import org.apache.commons.math3.ode.FieldEquationsMapper;
import org.apache.commons.math3.ode.FieldODEStateAndDerivative;
/**
* This class implements a step interpolator for second order
* Runge-Kutta integrator.
*
* <p>This interpolator computes dense output inside the last
* step computed. The interpolation equation is consistent with the
* integration scheme :
* <ul>
* <li>Using reference point at step start:<br>
* y(t<sub>n</sub> + θ h) = y (t<sub>n</sub>) + θ h [(1 - θ) y'<sub>1</sub> + θ y'<sub>2</sub>]
* </li>
* <li>Using reference point at step end:<br>
* y(t<sub>n</sub> + θ h) = y (t<sub>n</sub> + h) + (1-θ) h [θ y'<sub>1</sub> - (1+θ) y'<sub>2</sub>]
* </li>
* </ul>
* </p>
*
* where θ belongs to [0 ; 1] and where y'<sub>1</sub> and y'<sub>2</sub> are the two
* evaluations of the derivatives already computed during the
* step.</p>
*
* @see MidpointFieldIntegrator
* @param <T> the type of the field elements
* @since 3.6
*/
class MidpointFieldStepInterpolator<T extends RealFieldElement<T>>
extends RungeKuttaFieldStepInterpolator<T> {
/** Simple constructor.
* @param field field to which the time and state vector elements belong
* @param forward integration direction indicator
* @param yDotK slopes at the intermediate points
* @param globalPreviousState start of the global step
* @param globalCurrentState end of the global step
* @param softPreviousState start of the restricted step
* @param softCurrentState end of the restricted step
* @param mapper equations mapper for the all equations
*/
MidpointFieldStepInterpolator(final Field<T> field, final boolean forward,
final T[][] yDotK,
final FieldODEStateAndDerivative<T> globalPreviousState,
final FieldODEStateAndDerivative<T> globalCurrentState,
final FieldODEStateAndDerivative<T> softPreviousState,
final FieldODEStateAndDerivative<T> softCurrentState,
final FieldEquationsMapper<T> mapper) {
super(field, forward, yDotK,
globalPreviousState, globalCurrentState, softPreviousState, softCurrentState,
mapper);
}
/** {@inheritDoc} */
@Override
protected MidpointFieldStepInterpolator<T> create(final Field<T> newField, final boolean newForward, final T[][] newYDotK,
final FieldODEStateAndDerivative<T> newGlobalPreviousState,
final FieldODEStateAndDerivative<T> newGlobalCurrentState,
final FieldODEStateAndDerivative<T> newSoftPreviousState,
final FieldODEStateAndDerivative<T> newSoftCurrentState,
final FieldEquationsMapper<T> newMapper) {
return new MidpointFieldStepInterpolator<T>(newField, newForward, newYDotK,
newGlobalPreviousState, newGlobalCurrentState,
newSoftPreviousState, newSoftCurrentState,
newMapper);
}
/** {@inheritDoc} */
@SuppressWarnings("unchecked")
@Override
protected FieldODEStateAndDerivative<T> computeInterpolatedStateAndDerivatives(final FieldEquationsMapper<T> mapper,
final T time, final T theta,
final T thetaH, final T oneMinusThetaH) {
final T coeffDot2 = theta.multiply(2);
final T coeffDot1 = time.getField().getOne().subtract(coeffDot2);
final T[] interpolatedState;
final T[] interpolatedDerivatives;
if (getGlobalPreviousState() != null && theta.getReal() <= 0.5) {
final T coeff1 = theta.multiply(oneMinusThetaH);
final T coeff2 = theta.multiply(thetaH);
interpolatedState = previousStateLinearCombination(coeff1, coeff2);
interpolatedDerivatives = derivativeLinearCombination(coeffDot1, coeffDot2);
} else {
final T coeff1 = oneMinusThetaH.multiply(theta);
final T coeff2 = oneMinusThetaH.multiply(theta.add(1)).negate();
interpolatedState = currentStateLinearCombination(coeff1, coeff2);
interpolatedDerivatives = derivativeLinearCombination(coeffDot1, coeffDot2);
}
return new FieldODEStateAndDerivative<T>(time, interpolatedState, interpolatedDerivatives);
}
}
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