1 /* 2 * Licensed to the Apache Software Foundation (ASF) under one or more 3 * contributor license agreements. See the NOTICE file distributed with 4 * this work for additional information regarding copyright ownership. 5 * The ASF licenses this file to You under the Apache License, Version 2.0 6 * (the "License"); you may not use this file except in compliance with 7 * the License. You may obtain a copy of the License at 8 * 9 * http://www.apache.org/licenses/LICENSE-2.0 10 * 11 * Unless required by applicable law or agreed to in writing, software 12 * distributed under the License is distributed on an "AS IS" BASIS, 13 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. 14 * See the License for the specific language governing permissions and 15 * limitations under the License. 16 */ 17 18 package org.apache.commons.math.ode; 19 20 /** This class converts second order differential equations to first 21 * order ones. 22 * 23 * <p>This class is a wrapper around a {@link 24 * SecondOrderDifferentialEquations} which allow to use a {@link 25 * FirstOrderIntegrator} to integrate it.</p> 26 * 27 * <p>The transformation is done by changing the n dimension state 28 * vector to a 2n dimension vector, where the first n components are 29 * the initial state variables and the n last components are their 30 * first time derivative. The first time derivative of this state 31 * vector then really contains both the first and second time 32 * derivative of the initial state vector, which can be handled by the 33 * underlying second order equations set.</p> 34 * 35 * <p>One should be aware that the data is duplicated during the 36 * transformation process and that for each call to {@link 37 * #computeDerivatives computeDerivatives}, this wrapper does copy 4n 38 * scalars : 2n before the call to {@link 39 * SecondOrderDifferentialEquations#computeSecondDerivatives 40 * computeSecondDerivatives} in order to dispatch the y state vector 41 * into z and zDot, and 2n after the call to gather zDot and zDDot 42 * into yDot. Since the underlying problem by itself perhaps also 43 * needs to copy data and dispatch the arrays into domain objects, 44 * this has an impact on both memory and CPU usage. The only way to 45 * avoid this duplication is to perform the transformation at the 46 * problem level, i.e. to implement the problem as a first order one 47 * and then avoid using this class.</p> 48 * 49 * @see FirstOrderIntegrator 50 * @see FirstOrderDifferentialEquations 51 * @see SecondOrderDifferentialEquations 52 * @version $Revision: 620312 $ $Date: 2008-02-10 12:28:59 -0700 (Sun, 10 Feb 2008) $ 53 * @since 1.2 54 */ 55 56 public class FirstOrderConverter 57 implements FirstOrderDifferentialEquations { 58 59 /** Simple constructor. 60 * Build a converter around a second order equations set. 61 * @param equations second order equations set to convert 62 */ 63 public FirstOrderConverter (SecondOrderDifferentialEquations equations) { 64 this.equations = equations; 65 dimension = equations.getDimension(); 66 z = new double[dimension]; 67 zDot = new double[dimension]; 68 zDDot = new double[dimension]; 69 } 70 71 /** Get the dimension of the problem. 72 * <p>The dimension of the first order problem is twice the 73 * dimension of the underlying second order problem.</p> 74 * @return dimension of the problem 75 */ 76 public int getDimension() { 77 return 2 * dimension; 78 } 79 80 /** Get the current time derivative of the state vector. 81 * @param t current value of the independent <I>time</I> variable 82 * @param y array containing the current value of the state vector 83 * @param yDot placeholder array where to put the time derivative of the state vector 84 * @throws DerivativeException this exception is propagated to the caller if the 85 * underlying user function triggers one 86 */ 87 public void computeDerivatives(double t, double[] y, double[] yDot) 88 throws DerivativeException { 89 90 // split the state vector in two 91 System.arraycopy(y, 0, z, 0, dimension); 92 System.arraycopy(y, dimension, zDot, 0, dimension); 93 94 // apply the underlying equations set 95 equations.computeSecondDerivatives(t, z, zDot, zDDot); 96 97 // build the result state derivative 98 System.arraycopy(zDot, 0, yDot, 0, dimension); 99 System.arraycopy(zDDot, 0, yDot, dimension, dimension); 100 101 } 102 103 /** Underlying second order equations set. */ 104 private SecondOrderDifferentialEquations equations; 105 106 /** second order problem dimension. */ 107 private int dimension; 108 109 /** state vector. */ 110 private double[] z; 111 112 /** first time derivative of the state vector. */ 113 private double[] zDot; 114 115 /** second time derivative of the state vector. */ 116 private double[] zDDot; 117 118 }