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.math3.ode; 019 020 021 /** This class converts second order differential equations to first 022 * order ones. 023 * 024 * <p>This class is a wrapper around a {@link 025 * SecondOrderDifferentialEquations} which allow to use a {@link 026 * FirstOrderIntegrator} to integrate it.</p> 027 * 028 * <p>The transformation is done by changing the n dimension state 029 * vector to a 2n dimension vector, where the first n components are 030 * the initial state variables and the n last components are their 031 * first time derivative. The first time derivative of this state 032 * vector then really contains both the first and second time 033 * derivative of the initial state vector, which can be handled by the 034 * underlying second order equations set.</p> 035 * 036 * <p>One should be aware that the data is duplicated during the 037 * transformation process and that for each call to {@link 038 * #computeDerivatives computeDerivatives}, this wrapper does copy 4n 039 * scalars : 2n before the call to {@link 040 * SecondOrderDifferentialEquations#computeSecondDerivatives 041 * computeSecondDerivatives} in order to dispatch the y state vector 042 * into z and zDot, and 2n after the call to gather zDot and zDDot 043 * into yDot. Since the underlying problem by itself perhaps also 044 * needs to copy data and dispatch the arrays into domain objects, 045 * this has an impact on both memory and CPU usage. The only way to 046 * avoid this duplication is to perform the transformation at the 047 * problem level, i.e. to implement the problem as a first order one 048 * and then avoid using this class.</p> 049 * 050 * @see FirstOrderIntegrator 051 * @see FirstOrderDifferentialEquations 052 * @see SecondOrderDifferentialEquations 053 * @version $Id: FirstOrderConverter.java 1416643 2012-12-03 19:37:14Z tn $ 054 * @since 1.2 055 */ 056 057 public class FirstOrderConverter implements FirstOrderDifferentialEquations { 058 059 /** Underlying second order equations set. */ 060 private final SecondOrderDifferentialEquations equations; 061 062 /** second order problem dimension. */ 063 private final int dimension; 064 065 /** state vector. */ 066 private final double[] z; 067 068 /** first time derivative of the state vector. */ 069 private final double[] zDot; 070 071 /** second time derivative of the state vector. */ 072 private final double[] zDDot; 073 074 /** Simple constructor. 075 * Build a converter around a second order equations set. 076 * @param equations second order equations set to convert 077 */ 078 public FirstOrderConverter (final SecondOrderDifferentialEquations equations) { 079 this.equations = equations; 080 dimension = equations.getDimension(); 081 z = new double[dimension]; 082 zDot = new double[dimension]; 083 zDDot = new double[dimension]; 084 } 085 086 /** Get the dimension of the problem. 087 * <p>The dimension of the first order problem is twice the 088 * dimension of the underlying second order problem.</p> 089 * @return dimension of the problem 090 */ 091 public int getDimension() { 092 return 2 * dimension; 093 } 094 095 /** Get the current time derivative of the state vector. 096 * @param t current value of the independent <I>time</I> variable 097 * @param y array containing the current value of the state vector 098 * @param yDot placeholder array where to put the time derivative of the state vector 099 */ 100 public void computeDerivatives(final double t, final double[] y, final double[] yDot) { 101 102 // split the state vector in two 103 System.arraycopy(y, 0, z, 0, dimension); 104 System.arraycopy(y, dimension, zDot, 0, dimension); 105 106 // apply the underlying equations set 107 equations.computeSecondDerivatives(t, z, zDot, zDDot); 108 109 // build the result state derivative 110 System.arraycopy(zDot, 0, yDot, 0, dimension); 111 System.arraycopy(zDDot, 0, yDot, dimension, dimension); 112 113 } 114 115 }