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: 786881 $ $Date: 2009-06-20 14:53:08 -0400 (Sat, 20 Jun 2009) $ 53 * @since 1.2 54 */ 55 56 public class FirstOrderConverter implements FirstOrderDifferentialEquations { 57 58 /** Simple constructor. 59 * Build a converter around a second order equations set. 60 * @param equations second order equations set to convert 61 */ 62 public FirstOrderConverter (final SecondOrderDifferentialEquations equations) { 63 this.equations = equations; 64 dimension = equations.getDimension(); 65 z = new double[dimension]; 66 zDot = new double[dimension]; 67 zDDot = new double[dimension]; 68 } 69 70 /** Get the dimension of the problem. 71 * <p>The dimension of the first order problem is twice the 72 * dimension of the underlying second order problem.</p> 73 * @return dimension of the problem 74 */ 75 public int getDimension() { 76 return 2 * dimension; 77 } 78 79 /** Get the current time derivative of the state vector. 80 * @param t current value of the independent <I>time</I> variable 81 * @param y array containing the current value of the state vector 82 * @param yDot placeholder array where to put the time derivative of the state vector 83 * @throws DerivativeException this exception is propagated to the caller if the 84 * underlying user function triggers one 85 */ 86 public void computeDerivatives(final double t, final double[] y, final double[] yDot) 87 throws DerivativeException { 88 89 // split the state vector in two 90 System.arraycopy(y, 0, z, 0, dimension); 91 System.arraycopy(y, dimension, zDot, 0, dimension); 92 93 // apply the underlying equations set 94 equations.computeSecondDerivatives(t, z, zDot, zDDot); 95 96 // build the result state derivative 97 System.arraycopy(zDot, 0, yDot, 0, dimension); 98 System.arraycopy(zDDot, 0, yDot, dimension, dimension); 99 100 } 101 102 /** Underlying second order equations set. */ 103 private SecondOrderDifferentialEquations equations; 104 105 /** second order problem dimension. */ 106 private int dimension; 107 108 /** state vector. */ 109 private double[] z; 110 111 /** first time derivative of the state vector. */ 112 private double[] zDot; 113 114 /** second time derivative of the state vector. */ 115 private double[] zDDot; 116 117 }