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.random;
019    
020    import org.apache.commons.math3.util.FastMath;
021    
022    
023    /**
024     * Generate random vectors isotropically located on the surface of a sphere.
025     *
026     * @since 2.1
027     * @version $Id: UnitSphereRandomVectorGenerator.java 1416643 2012-12-03 19:37:14Z tn $
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    }