| /* |
| * 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; |
| |
| import org.apache.commons.math.ode.DerivativeException; |
| |
| /** This class converts second order differential equations to first |
| * order ones. |
| * |
| * <p>This class is a wrapper around a {@link |
| * SecondOrderDifferentialEquations} which allow to use a {@link |
| * FirstOrderIntegrator} to integrate it.</p> |
| * |
| * <p>The transformation is done by changing the n dimension state |
| * vector to a 2n dimension vector, where the first n components are |
| * the initial state variables and the n last components are their |
| * first time derivative. The first time derivative of this state |
| * vector then really contains both the first and second time |
| * derivative of the initial state vector, which can be handled by the |
| * underlying second order equations set.</p> |
| * |
| * <p>One should be aware that the data is duplicated during the |
| * transformation process and that for each call to {@link |
| * #computeDerivatives computeDerivatives}, this wrapper does copy 4n |
| * scalars : 2n before the call to {@link |
| * SecondOrderDifferentialEquations#computeSecondDerivatives |
| * computeSecondDerivatives} in order to dispatch the y state vector |
| * into z and zDot, and 2n after the call to gather zDot and zDDot |
| * into yDot. Since the underlying problem by itself perhaps also |
| * needs to copy data and dispatch the arrays into domain objects, |
| * this has an impact on both memory and CPU usage. The only way to |
| * avoid this duplication is to perform the transformation at the |
| * problem level, i.e. to implement the problem as a first order one |
| * and then avoid using this class.</p> |
| * |
| * @see FirstOrderIntegrator |
| * @see FirstOrderDifferentialEquations |
| * @see SecondOrderDifferentialEquations |
| * @version $Revision: 1073158 $ $Date: 2011-02-21 22:46:52 +0100 (lun. 21 févr. 2011) $ |
| * @since 1.2 |
| */ |
| |
| public class FirstOrderConverter implements FirstOrderDifferentialEquations { |
| |
| /** Underlying second order equations set. */ |
| private final SecondOrderDifferentialEquations equations; |
| |
| /** second order problem dimension. */ |
| private final int dimension; |
| |
| /** state vector. */ |
| private final double[] z; |
| |
| /** first time derivative of the state vector. */ |
| private final double[] zDot; |
| |
| /** second time derivative of the state vector. */ |
| private final double[] zDDot; |
| |
| /** Simple constructor. |
| * Build a converter around a second order equations set. |
| * @param equations second order equations set to convert |
| */ |
| public FirstOrderConverter (final SecondOrderDifferentialEquations equations) { |
| this.equations = equations; |
| dimension = equations.getDimension(); |
| z = new double[dimension]; |
| zDot = new double[dimension]; |
| zDDot = new double[dimension]; |
| } |
| |
| /** Get the dimension of the problem. |
| * <p>The dimension of the first order problem is twice the |
| * dimension of the underlying second order problem.</p> |
| * @return dimension of the problem |
| */ |
| public int getDimension() { |
| return 2 * dimension; |
| } |
| |
| /** Get the current time derivative of the state vector. |
| * @param t current value of the independent <I>time</I> variable |
| * @param y array containing the current value of the state vector |
| * @param yDot placeholder array where to put the time derivative of the state vector |
| * @throws DerivativeException this exception is propagated to the caller if the |
| * underlying user function triggers one |
| */ |
| public void computeDerivatives(final double t, final double[] y, final double[] yDot) |
| throws DerivativeException { |
| |
| // split the state vector in two |
| System.arraycopy(y, 0, z, 0, dimension); |
| System.arraycopy(y, dimension, zDot, 0, dimension); |
| |
| // apply the underlying equations set |
| equations.computeSecondDerivatives(t, z, zDot, zDDot); |
| |
| // build the result state derivative |
| System.arraycopy(zDot, 0, yDot, 0, dimension); |
| System.arraycopy(zDDot, 0, yDot, dimension, dimension); |
| |
| } |
| |
| } |