    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    }