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
021 /**
022 * Generate random vectors isotropically located on the surface of a sphere.
023 *
024 * @since 2.1
025 * @version $Revision: 925812 $ $Date: 2010-03-21 11:49:31 -0400 (Sun, 21 Mar 2010) $
026 */
027
028 public class UnitSphereRandomVectorGenerator
029 implements RandomVectorGenerator {
030 /**
031 * RNG used for generating the individual components of the vectors.
032 */
033 private final RandomGenerator rand;
034 /**
035 * Space dimension.
036 */
037 private final int dimension;
038
039 /**
040 * @param dimension Space dimension.
041 * @param rand RNG for the individual components of the vectors.
042 */
043 public UnitSphereRandomVectorGenerator(final int dimension,
044 final RandomGenerator rand) {
045 this.dimension = dimension;
046 this.rand = rand;
047 }
048 /**
049 * Create an object that will use a default RNG ({@link MersenneTwister}),
050 * in order to generate the individual components.
051 *
052 * @param dimension Space dimension.
053 */
054 public UnitSphereRandomVectorGenerator(final int dimension) {
055 this(dimension, new MersenneTwister());
056 }
057
058 /** {@inheritDoc} */
059 public double[] nextVector() {
060
061 final double[] v = new double[dimension];
062
063 double normSq;
064 do {
065 normSq = 0;
066 for (int i = 0; i < dimension; i++) {
067 final double comp = 2 * rand.nextDouble() - 1;
068 v[i] = comp;
069 normSq += comp * comp;
070 }
071 } while (normSq > 1);
072
073 final double f = 1 / Math.sqrt(normSq);
074 for (int i = 0; i < dimension; i++) {
075 v[i] *= f;
076 }
077
078 return v;
079
080 }
081
082 }