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.random; 019 020 import org.apache.commons.math.util.FastMath; 021 022 023 /** 024 * Generate random vectors isotropically located on the surface of a sphere. 025 * 026 * @since 2.1 027 * @version $Revision: 990655 $ $Date: 2010-08-29 23:49:40 +0200 (dim. 29 ao??t 2010) $ 028 */ 029 030 public class UnitSphereRandomVectorGenerator 031 implements RandomVectorGenerator { 032 /** 033 * RNG used for generating the individual components of the vectors. 034 */ 035 private final RandomGenerator rand; 036 /** 037 * Space dimension. 038 */ 039 private final int dimension; 040 041 /** 042 * @param dimension Space dimension. 043 * @param rand RNG for the individual components of the vectors. 044 */ 045 public UnitSphereRandomVectorGenerator(final int dimension, 046 final RandomGenerator rand) { 047 this.dimension = dimension; 048 this.rand = rand; 049 } 050 /** 051 * Create an object that will use a default RNG ({@link MersenneTwister}), 052 * in order to generate the individual components. 053 * 054 * @param dimension Space dimension. 055 */ 056 public UnitSphereRandomVectorGenerator(final int dimension) { 057 this(dimension, new MersenneTwister()); 058 } 059 060 /** {@inheritDoc} */ 061 public double[] nextVector() { 062 063 final double[] v = new double[dimension]; 064 065 double normSq; 066 do { 067 normSq = 0; 068 for (int i = 0; i < dimension; i++) { 069 final double comp = 2 * rand.nextDouble() - 1; 070 v[i] = comp; 071 normSq += comp * comp; 072 } 073 } while (normSq > 1); 074 075 final double f = 1 / FastMath.sqrt(normSq); 076 for (int i = 0; i < dimension; i++) { 077 v[i] *= f; 078 } 079 080 return v; 081 082 } 083 084 }