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Java example source code file (FirstOrderConverter.java)

This example Java source code file (FirstOrderConverter.java) is included in the alvinalexander.com "Java Source Code Warehouse" project. The intent of this project is to help you "Learn Java by Example" TM.

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Java - Java tags/keywords

firstorderconverter, firstorderdifferentialequations, secondorderdifferentialequations

The FirstOrderConverter.java Java example source code

/*
 * 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;


/** 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
 * @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 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
   */
  public void computeDerivatives(final double t, final double[] y, final double[] yDot) {

    // 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);

  }

}

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