    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    package org.apache.commons.math.random;
    018
    019    import org.apache.commons.math.exception.NotStrictlyPositiveException;
    020    import org.apache.commons.math.util.FastMath;
    021
    022    /**
    023     * Abstract class implementing the {@link  RandomGenerator} interface.
    024     * Default implementations for all methods other than {@link #nextDouble()} and
    025     * {@link #setSeed(long)} are provided.
    026     * <p>
    027     * All data generation methods are based on {@code code nextDouble()}.
    028     * Concrete implementations <strong>must</strong> override
    029     * this method and <strong>should</strong> provide better / more
    030     * performant implementations of the other methods if the underlying PRNG
    031     * supplies them.</p>
    032     *
    033     * @since 1.1
    034     * @version $Revision: 990655 $ $Date: 2010-08-29 23:49:40 +0200 (dim. 29 ao??t 2010) $
    035     */
    036    public abstract class AbstractRandomGenerator implements RandomGenerator {
    037
    038        /**
    039         * Cached random normal value.  The default implementation for
    040         * {@link #nextGaussian} generates pairs of values and this field caches the
    041         * second value so that the full algorithm is not executed for every
    042         * activation.  The value {@code Double.NaN} signals that there is
    043         * no cached value.  Use {@link #clear} to clear the cached value.
    044         */
    045        private double cachedNormalDeviate = Double.NaN;
    046
    047        /**
    048         * Construct a RandomGenerator.
    049         */
    050        public AbstractRandomGenerator() {
    051            super();
    052
    053        }
    054
    055        /**
    056         * Clears the cache used by the default implementation of
    057         * {@link #nextGaussian}. Implemementations that do not override the
    058         * default implementation of {@code nextGaussian} should call this
    059         * method in the implementation of {@link #setSeed(long)}
    060         */
    061        public void clear() {
    062            cachedNormalDeviate = Double.NaN;
    063        }
    064
    065        /** {@inheritDoc} */
    066        public void setSeed(int seed) {
    067            setSeed((long) seed);
    068        }
    069
    070        /** {@inheritDoc} */
    071        public void setSeed(int[] seed) {
    072            // the following number is the largest prime that fits in 32 bits (it is 2^32 - 5)
    073            final long prime = 4294967291l;
    074
    075            long combined = 0l;
    076            for (int s : seed) {
    077                combined = combined * prime + s;
    078            }
    079            setSeed(combined);
    080        }
    081
    082        /**
    083         * Sets the seed of the underyling random number generator using a
    084         * {@code long} seed.  Sequences of values generated starting with the
    085         * same seeds should be identical.
    086         * <p>
    087         * Implementations that do not override the default implementation of
    088         * {@code nextGaussian} should include a call to {@link #clear} in the
    089         * implementation of this method.</p>
    090         *
    091         * @param seed the seed value
    092         */
    093        public abstract void setSeed(long seed);
    094
    095        /**
    096         * Generates random bytes and places them into a user-supplied
    097         * byte array.  The number of random bytes produced is equal to
    098         * the length of the byte array.
    099         * <p>
    100         * The default implementation fills the array with bytes extracted from
    101         * random integers generated using {@link #nextInt}.</p>
    102         *
    103         * @param bytes the non-null byte array in which to put the
    104         * random bytes
    105         */
    106        public void nextBytes(byte[] bytes) {
    107            int bytesOut = 0;
    108            while (bytesOut < bytes.length) {
    109              int randInt = nextInt();
    110              for (int i = 0; i < 3; i++) {
    111                  if ( i > 0) {
    112                      randInt = randInt >> 8;
    113                  }
    114                  bytes[bytesOut++] = (byte) randInt;
    115                  if (bytesOut == bytes.length) {
    116                      return;
    117                  }
    118              }
    119            }
    120        }
    121
    122         /**
    123         * Returns the next pseudorandom, uniformly distributed {@code int}
    124         * value from this random number generator's sequence.
    125         * All 2<font size="-1"><sup>32</sup></font> possible {@code int} values
    126         * should be produced with  (approximately) equal probability.
    127         * <p>
    128         * The default implementation provided here returns
    129         * <pre>
    130         * <code>(int) (nextDouble() * Integer.MAX_VALUE)</code>
    131         * </pre></p>
    132         *
    133         * @return the next pseudorandom, uniformly distributed {@code int}
    134         *  value from this random number generator's sequence
    135         */
    136        public int nextInt() {
    137            return (int) (nextDouble() * Integer.MAX_VALUE);
    138        }
    139
    140        /**
    141         * Returns a pseudorandom, uniformly distributed {@code int} value
    142         * between 0 (inclusive) and the specified value (exclusive), drawn from
    143         * this random number generator's sequence.
    144         * <p>
    145         * The default implementation returns
    146         * <pre>
    147         * <code>(int) (nextDouble() * n</code>
    148         * </pre></p>
    149         *
    150         * @param n the bound on the random number to be returned.  Must be
    151         * positive.
    152         * @return  a pseudorandom, uniformly distributed {@code int}
    153         * value between 0 (inclusive) and n (exclusive).
    154         * @throws NotStrictlyPositiveException if {@code n <= 0}.
    155         */
    156        public int nextInt(int n) {
    157            if (n <= 0 ) {
    158                throw new NotStrictlyPositiveException(n);
    159            }
    160            int result = (int) (nextDouble() * n);
    161            return result < n ? result : n - 1;
    162        }
    163
    164         /**
    165         * Returns the next pseudorandom, uniformly distributed {@code long}
    166         * value from this random number generator's sequence.  All
    167         * 2<font size="-1"><sup>64</sup></font> possible {@code long} values
    168         * should be produced with (approximately) equal probability.
    169         * <p>
    170         * The default implementation returns
    171         * <pre>
    172         * <code>(long) (nextDouble() * Long.MAX_VALUE)</code>
    173         * </pre></p>
    174         *
    175         * @return  the next pseudorandom, uniformly distributed {@code long}
    176         *value from this random number generator's sequence
    177         */
    178        public long nextLong() {
    179            return (long) (nextDouble() * Long.MAX_VALUE);
    180        }
    181
    182        /**
    183         * Returns the next pseudorandom, uniformly distributed
    184         * {@code boolean} value from this random number generator's
    185         * sequence.
    186         * <p>
    187         * The default implementation returns
    188         * <pre>
    189         * <code>nextDouble() <= 0.5</code>
    190         * </pre></p>
    191         *
    192         * @return  the next pseudorandom, uniformly distributed
    193         * {@code boolean} value from this random number generator's
    194         * sequence
    195         */
    196        public boolean nextBoolean() {
    197            return nextDouble() <= 0.5;
    198        }
    199
    200         /**
    201         * Returns the next pseudorandom, uniformly distributed {@code float}
    202         * value between {@code 0.0} and {@code 1.0} from this random
    203         * number generator's sequence.
    204         * <p>
    205         * The default implementation returns
    206         * <pre>
    207         * <code>(float) nextDouble() </code>
    208         * </pre></p>
    209         *
    210         * @return  the next pseudorandom, uniformly distributed {@code float}
    211         * value between {@code 0.0} and {@code 1.0} from this
    212         * random number generator's sequence
    213         */
    214        public float nextFloat() {
    215            return (float) nextDouble();
    216        }
    217
    218        /**
    219         * Returns the next pseudorandom, uniformly distributed
    220         * {@code double} value between {@code 0.0} and
    221         * {@code 1.0} from this random number generator's sequence.
    222         * <p>
    223         * This method provides the underlying source of random data used by the
    224         * other methods.</p>
    225         *
    226         * @return  the next pseudorandom, uniformly distributed
    227         *  {@code double} value between {@code 0.0} and
    228         *  {@code 1.0} from this random number generator's sequence
    229         */
    230        public abstract double nextDouble();
    231
    232        /**
    233         * Returns the next pseudorandom, Gaussian ("normally") distributed
    234         * {@code double} value with mean {@code 0.0} and standard
    235         * deviation {@code 1.0} from this random number generator's sequence.
    236         * <p>
    237         * The default implementation uses the <em>Polar Method</em>
    238         * due to G.E.P. Box, M.E. Muller and G. Marsaglia, as described in
    239         * D. Knuth, <u>The Art of Computer Programming</u>, 3.4.1C.</p>
    240         * <p>
    241         * The algorithm generates a pair of independent random values.  One of
    242         * these is cached for reuse, so the full algorithm is not executed on each
    243         * activation.  Implementations that do not override this method should
    244         * make sure to call {@link #clear} to clear the cached value in the
    245         * implementation of {@link #setSeed(long)}.</p>
    246         *
    247         * @return  the next pseudorandom, Gaussian ("normally") distributed
    248         * {@code double} value with mean {@code 0.0} and
    249         * standard deviation {@code 1.0} from this random number
    250         *  generator's sequence
    251         */
    252        public double nextGaussian() {
    253            if (!Double.isNaN(cachedNormalDeviate)) {
    254                double dev = cachedNormalDeviate;
    255                cachedNormalDeviate = Double.NaN;
    256                return dev;
    257            }
    258            double v1 = 0;
    259            double v2 = 0;
    260            double s = 1;
    261            while (s >=1 ) {
    262                v1 = 2 * nextDouble() - 1;
    263                v2 = 2 * nextDouble() - 1;
    264                s = v1 * v1 + v2 * v2;
    265            }
    266            if (s != 0) {
    267                s = FastMath.sqrt(-2 * FastMath.log(s) / s);
    268            }
    269            cachedNormalDeviate = v2 * s;
    270            return v1 * s;
    271        }
    272    }