    001    /*
    002     * Licensed to the Apache Software Foundation (ASF) under one or more
    003     * contributor license agreements.  See the NOTICE file distributed with
    004     * this work for additional information regarding copyright ownership.
    005     * The ASF licenses this file to You under the Apache License, Version 2.0
    006     * (the "License"); you may not use this file except in compliance with
    007     * the License.  You may obtain a copy of the License at
    008     *
    009     *      http://www.apache.org/licenses/LICENSE-2.0
    010     *
    011     * Unless required by applicable law or agreed to in writing, software
    012     * distributed under the License is distributed on an "AS IS" BASIS,
    013     * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
    014     * See the License for the specific language governing permissions and
    015     * limitations under the License.
    016     */
    017
    018    package org.apache.commons.math.ode;
    019
    020    import org.apache.commons.math.ode.DerivativeException;
    021
    022    /** This class converts second order differential equations to first
    023     * order ones.
    024     *
    025     * <p>This class is a wrapper around a {@link
    026     * SecondOrderDifferentialEquations} which allow to use a {@link
    027     * FirstOrderIntegrator} to integrate it.</p>
    028     *
    029     * <p>The transformation is done by changing the n dimension state
    030     * vector to a 2n dimension vector, where the first n components are
    031     * the initial state variables and the n last components are their
    032     * first time derivative. The first time derivative of this state
    033     * vector then really contains both the first and second time
    034     * derivative of the initial state vector, which can be handled by the
    035     * underlying second order equations set.</p>
    036     *
    037     * <p>One should be aware that the data is duplicated during the
    038     * transformation process and that for each call to {@link
    039     * #computeDerivatives computeDerivatives}, this wrapper does copy 4n
    040     * scalars : 2n before the call to {@link
    041     * SecondOrderDifferentialEquations#computeSecondDerivatives
    042     * computeSecondDerivatives} in order to dispatch the y state vector
    043     * into z and zDot, and 2n after the call to gather zDot and zDDot
    044     * into yDot. Since the underlying problem by itself perhaps also
    045     * needs to copy data and dispatch the arrays into domain objects,
    046     * this has an impact on both memory and CPU usage. The only way to
    047     * avoid this duplication is to perform the transformation at the
    048     * problem level, i.e. to implement the problem as a first order one
    049     * and then avoid using this class.</p>
    050     *
    051     * @see FirstOrderIntegrator
    052     * @see FirstOrderDifferentialEquations
    053     * @see SecondOrderDifferentialEquations
    054     * @version $Revision: 1073158 $ $Date: 2011-02-21 22:46:52 +0100 (lun. 21 f??vr. 2011) $
    055     * @since 1.2
    056     */
    057
    058    public class FirstOrderConverter implements FirstOrderDifferentialEquations {
    059
    060        /** Underlying second order equations set. */
    061        private final SecondOrderDifferentialEquations equations;
    062
    063        /** second order problem dimension. */
    064        private final int dimension;
    065
    066        /** state vector. */
    067        private final double[] z;
    068
    069        /** first time derivative of the state vector. */
    070        private final double[] zDot;
    071
    072        /** second time derivative of the state vector. */
    073        private final double[] zDDot;
    074
    075      /** Simple constructor.
    076       * Build a converter around a second order equations set.
    077       * @param equations second order equations set to convert
    078       */
    079      public FirstOrderConverter (final SecondOrderDifferentialEquations equations) {
    080          this.equations = equations;
    081          dimension      = equations.getDimension();
    082          z              = new double[dimension];
    083          zDot           = new double[dimension];
    084          zDDot          = new double[dimension];
    085      }
    086
    087      /** Get the dimension of the problem.
    088       * <p>The dimension of the first order problem is twice the
    089       * dimension of the underlying second order problem.</p>
    090       * @return dimension of the problem
    091       */
    092      public int getDimension() {
    093        return 2 * dimension;
    094      }
    095
    096      /** Get the current time derivative of the state vector.
    097       * @param t current value of the independent <I>time</I> variable
    098       * @param y array containing the current value of the state vector
    099       * @param yDot placeholder array where to put the time derivative of the state vector
    100       * @throws DerivativeException this exception is propagated to the caller if the
    101       * underlying user function triggers one
    102       */
    103      public void computeDerivatives(final double t, final double[] y, final double[] yDot)
    104          throws DerivativeException {
    105
    106        // split the state vector in two
    107        System.arraycopy(y, 0,         z,    0, dimension);
    108        System.arraycopy(y, dimension, zDot, 0, dimension);
    109
    110        // apply the underlying equations set
    111        equations.computeSecondDerivatives(t, z, zDot, zDDot);
    112
    113        // build the result state derivative
    114        System.arraycopy(zDot,  0, yDot, 0,         dimension);
    115        System.arraycopy(zDDot, 0, yDot, dimension, dimension);
    116
    117      }
    118
    119    }