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root/jsr166/jsr166/src/main/java/util/Random.java
Revision: 1.7
Committed: Sun Apr 11 04:50:24 2004 UTC (20 years, 1 month ago) by jsr166
Branch: MAIN
Changes since 1.6: +1 -1 lines
Log Message:
Martin Buchholz: sync with Tiger

File Contents

# User Rev Content
1 dl 1.1 /*
2 jsr166 1.4 * %W% %E%
3 dl 1.1 *
4 jsr166 1.7 * Copyright 2004 Sun Microsystems, Inc. All rights reserved.
5 dl 1.1 * SUN PROPRIETARY/CONFIDENTIAL. Use is subject to license terms.
6     */
7    
8     package java.util;
9     import java.io.*;
10 dl 1.3 import java.util.concurrent.atomic.AtomicLong;
11 dl 1.1
12     /**
13     * An instance of this class is used to generate a stream of
14     * pseudorandom numbers. The class uses a 48-bit seed, which is
15     * modified using a linear congruential formula. (See Donald Knuth,
16     * <i>The Art of Computer Programming, Volume 2</i>, Section 3.2.1.)
17     * <p>
18     * If two instances of <code>Random</code> are created with the same
19     * seed, and the same sequence of method calls is made for each, they
20     * will generate and return identical sequences of numbers. In order to
21     * guarantee this property, particular algorithms are specified for the
22     * class <tt>Random</tt>. Java implementations must use all the algorithms
23     * shown here for the class <tt>Random</tt>, for the sake of absolute
24     * portability of Java code. However, subclasses of class <tt>Random</tt>
25     * are permitted to use other algorithms, so long as they adhere to the
26     * general contracts for all the methods.
27     * <p>
28     * The algorithms implemented by class <tt>Random</tt> use a
29     * <tt>protected</tt> utility method that on each invocation can supply
30     * up to 32 pseudorandomly generated bits.
31     * <p>
32     * Many applications will find the <code>random</code> method in
33     * class <code>Math</code> simpler to use.
34     *
35     * @author Frank Yellin
36 jsr166 1.4 * @version %I%, %G%
37 dl 1.1 * @see java.lang.Math#random()
38     * @since JDK1.0
39     */
40     public
41     class Random implements java.io.Serializable {
42     /** use serialVersionUID from JDK 1.1 for interoperability */
43     static final long serialVersionUID = 3905348978240129619L;
44    
45     /**
46     * The internal state associated with this pseudorandom number generator.
47     * (The specs for the methods in this class describe the ongoing
48     * computation of this value.)
49     *
50     * @serial
51     */
52 dl 1.3 private AtomicLong seed;
53 dl 1.1
54     private final static long multiplier = 0x5DEECE66DL;
55     private final static long addend = 0xBL;
56     private final static long mask = (1L << 48) - 1;
57    
58 jsr166 1.4 /**
59     * Creates a new random number generator. This constructor sets
60     * the seed of the random number generator to a value very likely
61     * to be distinct from any other invocation of this constructor.
62 dl 1.1 */
63 jsr166 1.4 public Random() { this(++seedUniquifier + System.nanoTime()); }
64     private static volatile long seedUniquifier = 8682522807148012L;
65 dl 1.1
66     /**
67     * Creates a new random number generator using a single
68     * <code>long</code> seed:
69     * <blockquote><pre>
70     * public Random(long seed) { setSeed(seed); }</pre></blockquote>
71     * Used by method <tt>next</tt> to hold
72     * the state of the pseudorandom number generator.
73     *
74     * @param seed the initial seed.
75     * @see java.util.Random#setSeed(long)
76     */
77     public Random(long seed) {
78 dl 1.3 this.seed = new AtomicLong(0L);
79 dl 1.1 setSeed(seed);
80     }
81    
82     /**
83     * Sets the seed of this random number generator using a single
84     * <code>long</code> seed. The general contract of <tt>setSeed</tt>
85     * is that it alters the state of this random number generator
86     * object so as to be in exactly the same state as if it had just
87     * been created with the argument <tt>seed</tt> as a seed. The method
88     * <tt>setSeed</tt> is implemented by class Random as follows:
89     * <blockquote><pre>
90     * synchronized public void setSeed(long seed) {
91     * this.seed = (seed ^ 0x5DEECE66DL) & ((1L << 48) - 1);
92     * haveNextNextGaussian = false;
93     * }</pre></blockquote>
94     * The implementation of <tt>setSeed</tt> by class <tt>Random</tt>
95     * happens to use only 48 bits of the given seed. In general, however,
96     * an overriding method may use all 64 bits of the long argument
97     * as a seed value.
98     *
99 dl 1.3 * Note: Although the seed value is an AtomicLong, this method
100 dl 1.1 * must still be synchronized to ensure correct semantics
101     * of haveNextNextGaussian.
102     *
103     * @param seed the initial seed.
104     */
105     synchronized public void setSeed(long seed) {
106 dl 1.3 seed = (seed ^ multiplier) & mask;
107     this.seed.set(seed);
108 dl 1.1 haveNextNextGaussian = false;
109     }
110    
111     /**
112     * Generates the next pseudorandom number. Subclass should
113     * override this, as this is used by all other methods.<p>
114     * The general contract of <tt>next</tt> is that it returns an
115     * <tt>int</tt> value and if the argument bits is between <tt>1</tt>
116     * and <tt>32</tt> (inclusive), then that many low-order bits of the
117     * returned value will be (approximately) independently chosen bit
118     * values, each of which is (approximately) equally likely to be
119     * <tt>0</tt> or <tt>1</tt>. The method <tt>next</tt> is implemented
120     * by class <tt>Random</tt> as follows:
121     * <blockquote><pre>
122     * synchronized protected int next(int bits) {
123     * seed = (seed * 0x5DEECE66DL + 0xBL) & ((1L << 48) - 1);
124     * return (int)(seed >>> (48 - bits));
125     * }</pre></blockquote>
126     * This is a linear congruential pseudorandom number generator, as
127     * defined by D. H. Lehmer and described by Donald E. Knuth in <i>The
128     * Art of Computer Programming,</i> Volume 2: <i>Seminumerical
129     * Algorithms</i>, section 3.2.1.
130     *
131     * @param bits random bits
132     * @return the next pseudorandom value from this random number generator's sequence.
133     * @since JDK1.1
134     */
135     protected int next(int bits) {
136     long oldseed, nextseed;
137 dl 1.6 AtomicLong seed = this.seed;
138 dl 1.1 do {
139 jozart 1.5 oldseed = seed.get();
140     nextseed = (oldseed * multiplier + addend) & mask;
141 dl 1.3 } while (!seed.compareAndSet(oldseed, nextseed));
142 dl 1.1 return (int)(nextseed >>> (48 - bits));
143     }
144    
145     private static final int BITS_PER_BYTE = 8;
146     private static final int BYTES_PER_INT = 4;
147    
148     /**
149     * Generates random bytes and places them into a user-supplied
150     * byte array. The number of random bytes produced is equal to
151     * the length of the byte array.
152     *
153     * @param bytes the non-null byte array in which to put the
154     * random bytes.
155     * @since JDK1.1
156     */
157     public void nextBytes(byte[] bytes) {
158     int numRequested = bytes.length;
159    
160     int numGot = 0, rnd = 0;
161    
162     while (true) {
163     for (int i = 0; i < BYTES_PER_INT; i++) {
164     if (numGot == numRequested)
165     return;
166    
167     rnd = (i==0 ? next(BITS_PER_BYTE * BYTES_PER_INT)
168     : rnd >> BITS_PER_BYTE);
169     bytes[numGot++] = (byte)rnd;
170     }
171     }
172     }
173    
174     /**
175     * Returns the next pseudorandom, uniformly distributed <code>int</code>
176     * value from this random number generator's sequence. The general
177     * contract of <tt>nextInt</tt> is that one <tt>int</tt> value is
178     * pseudorandomly generated and returned. All 2<font size="-1"><sup>32
179     * </sup></font> possible <tt>int</tt> values are produced with
180     * (approximately) equal probability. The method <tt>nextInt</tt> is
181     * implemented by class <tt>Random</tt> as follows:
182     * <blockquote><pre>
183     * public int nextInt() { return next(32); }</pre></blockquote>
184     *
185     * @return the next pseudorandom, uniformly distributed <code>int</code>
186     * value from this random number generator's sequence.
187     */
188     public int nextInt() { return next(32); }
189    
190     /**
191     * Returns a pseudorandom, uniformly distributed <tt>int</tt> value
192     * between 0 (inclusive) and the specified value (exclusive), drawn from
193     * this random number generator's sequence. The general contract of
194     * <tt>nextInt</tt> is that one <tt>int</tt> value in the specified range
195     * is pseudorandomly generated and returned. All <tt>n</tt> possible
196     * <tt>int</tt> values are produced with (approximately) equal
197     * probability. The method <tt>nextInt(int n)</tt> is implemented by
198     * class <tt>Random</tt> as follows:
199     * <blockquote><pre>
200     * public int nextInt(int n) {
201     * if (n<=0)
202     * throw new IllegalArgumentException("n must be positive");
203     *
204     * if ((n & -n) == n) // i.e., n is a power of 2
205     * return (int)((n * (long)next(31)) >> 31);
206     *
207     * int bits, val;
208     * do {
209     * bits = next(31);
210     * val = bits % n;
211     * } while(bits - val + (n-1) < 0);
212     * return val;
213     * }
214     * </pre></blockquote>
215     * <p>
216     * The hedge "approximately" is used in the foregoing description only
217     * because the next method is only approximately an unbiased source of
218     * independently chosen bits. If it were a perfect source of randomly
219     * chosen bits, then the algorithm shown would choose <tt>int</tt>
220     * values from the stated range with perfect uniformity.
221     * <p>
222     * The algorithm is slightly tricky. It rejects values that would result
223     * in an uneven distribution (due to the fact that 2^31 is not divisible
224     * by n). The probability of a value being rejected depends on n. The
225     * worst case is n=2^30+1, for which the probability of a reject is 1/2,
226     * and the expected number of iterations before the loop terminates is 2.
227     * <p>
228     * The algorithm treats the case where n is a power of two specially: it
229     * returns the correct number of high-order bits from the underlying
230     * pseudo-random number generator. In the absence of special treatment,
231     * the correct number of <i>low-order</i> bits would be returned. Linear
232     * congruential pseudo-random number generators such as the one
233     * implemented by this class are known to have short periods in the
234     * sequence of values of their low-order bits. Thus, this special case
235     * greatly increases the length of the sequence of values returned by
236     * successive calls to this method if n is a small power of two.
237     *
238     * @param n the bound on the random number to be returned. Must be
239     * positive.
240     * @return a pseudorandom, uniformly distributed <tt>int</tt>
241     * value between 0 (inclusive) and n (exclusive).
242     * @exception IllegalArgumentException n is not positive.
243     * @since 1.2
244     */
245    
246     public int nextInt(int n) {
247     if (n<=0)
248     throw new IllegalArgumentException("n must be positive");
249    
250     if ((n & -n) == n) // i.e., n is a power of 2
251     return (int)((n * (long)next(31)) >> 31);
252    
253     int bits, val;
254     do {
255     bits = next(31);
256     val = bits % n;
257     } while(bits - val + (n-1) < 0);
258     return val;
259     }
260    
261     /**
262     * Returns the next pseudorandom, uniformly distributed <code>long</code>
263     * value from this random number generator's sequence. The general
264     * contract of <tt>nextLong</tt> is that one long value is pseudorandomly
265     * generated and returned. All 2<font size="-1"><sup>64</sup></font>
266     * possible <tt>long</tt> values are produced with (approximately) equal
267     * probability. The method <tt>nextLong</tt> is implemented by class
268     * <tt>Random</tt> as follows:
269     * <blockquote><pre>
270     * public long nextLong() {
271     * return ((long)next(32) << 32) + next(32);
272     * }</pre></blockquote>
273     *
274     * @return the next pseudorandom, uniformly distributed <code>long</code>
275     * value from this random number generator's sequence.
276     */
277     public long nextLong() {
278     // it's okay that the bottom word remains signed.
279     return ((long)(next(32)) << 32) + next(32);
280     }
281    
282     /**
283     * Returns the next pseudorandom, uniformly distributed
284     * <code>boolean</code> value from this random number generator's
285     * sequence. The general contract of <tt>nextBoolean</tt> is that one
286     * <tt>boolean</tt> value is pseudorandomly generated and returned. The
287     * values <code>true</code> and <code>false</code> are produced with
288     * (approximately) equal probability. The method <tt>nextBoolean</tt> is
289     * implemented by class <tt>Random</tt> as follows:
290     * <blockquote><pre>
291     * public boolean nextBoolean() {return next(1) != 0;}
292     * </pre></blockquote>
293     * @return the next pseudorandom, uniformly distributed
294     * <code>boolean</code> value from this random number generator's
295     * sequence.
296     * @since 1.2
297     */
298     public boolean nextBoolean() {return next(1) != 0;}
299    
300     /**
301     * Returns the next pseudorandom, uniformly distributed <code>float</code>
302     * value between <code>0.0</code> and <code>1.0</code> from this random
303     * number generator's sequence. <p>
304     * The general contract of <tt>nextFloat</tt> is that one <tt>float</tt>
305     * value, chosen (approximately) uniformly from the range <tt>0.0f</tt>
306     * (inclusive) to <tt>1.0f</tt> (exclusive), is pseudorandomly
307     * generated and returned. All 2<font size="-1"><sup>24</sup></font>
308     * possible <tt>float</tt> values of the form
309     * <i>m&nbsp;x&nbsp</i>2<font size="-1"><sup>-24</sup></font>, where
310     * <i>m</i> is a positive integer less than 2<font size="-1"><sup>24</sup>
311     * </font>, are produced with (approximately) equal probability. The
312     * method <tt>nextFloat</tt> is implemented by class <tt>Random</tt> as
313     * follows:
314     * <blockquote><pre>
315     * public float nextFloat() {
316     * return next(24) / ((float)(1 << 24));
317     * }</pre></blockquote>
318     * The hedge "approximately" is used in the foregoing description only
319     * because the next method is only approximately an unbiased source of
320     * independently chosen bits. If it were a perfect source or randomly
321     * chosen bits, then the algorithm shown would choose <tt>float</tt>
322     * values from the stated range with perfect uniformity.<p>
323     * [In early versions of Java, the result was incorrectly calculated as:
324     * <blockquote><pre>
325     * return next(30) / ((float)(1 << 30));</pre></blockquote>
326     * This might seem to be equivalent, if not better, but in fact it
327     * introduced a slight nonuniformity because of the bias in the rounding
328     * of floating-point numbers: it was slightly more likely that the
329     * low-order bit of the significand would be 0 than that it would be 1.]
330     *
331     * @return the next pseudorandom, uniformly distributed <code>float</code>
332     * value between <code>0.0</code> and <code>1.0</code> from this
333     * random number generator's sequence.
334     */
335     public float nextFloat() {
336     int i = next(24);
337     return i / ((float)(1 << 24));
338     }
339    
340     /**
341     * Returns the next pseudorandom, uniformly distributed
342     * <code>double</code> value between <code>0.0</code> and
343     * <code>1.0</code> from this random number generator's sequence. <p>
344     * The general contract of <tt>nextDouble</tt> is that one
345     * <tt>double</tt> value, chosen (approximately) uniformly from the
346     * range <tt>0.0d</tt> (inclusive) to <tt>1.0d</tt> (exclusive), is
347     * pseudorandomly generated and returned. All
348     * 2<font size="-1"><sup>53</sup></font> possible <tt>float</tt>
349     * values of the form <i>m&nbsp;x&nbsp;</i>2<font size="-1"><sup>-53</sup>
350     * </font>, where <i>m</i> is a positive integer less than
351     * 2<font size="-1"><sup>53</sup></font>, are produced with
352     * (approximately) equal probability. The method <tt>nextDouble</tt> is
353     * implemented by class <tt>Random</tt> as follows:
354     * <blockquote><pre>
355     * public double nextDouble() {
356     * return (((long)next(26) << 27) + next(27))
357     * / (double)(1L << 53);
358     * }</pre></blockquote><p>
359     * The hedge "approximately" is used in the foregoing description only
360     * because the <tt>next</tt> method is only approximately an unbiased
361     * source of independently chosen bits. If it were a perfect source or
362     * randomly chosen bits, then the algorithm shown would choose
363     * <tt>double</tt> values from the stated range with perfect uniformity.
364     * <p>[In early versions of Java, the result was incorrectly calculated as:
365     * <blockquote><pre>
366     * return (((long)next(27) << 27) + next(27))
367     * / (double)(1L << 54);</pre></blockquote>
368     * This might seem to be equivalent, if not better, but in fact it
369     * introduced a large nonuniformity because of the bias in the rounding
370     * of floating-point numbers: it was three times as likely that the
371     * low-order bit of the significand would be 0 than that it would be
372     * 1! This nonuniformity probably doesn't matter much in practice, but
373     * we strive for perfection.]
374     *
375     * @return the next pseudorandom, uniformly distributed
376     * <code>double</code> value between <code>0.0</code> and
377     * <code>1.0</code> from this random number generator's sequence.
378     */
379     public double nextDouble() {
380     long l = ((long)(next(26)) << 27) + next(27);
381     return l / (double)(1L << 53);
382     }
383    
384     private double nextNextGaussian;
385     private boolean haveNextNextGaussian = false;
386    
387     /**
388     * Returns the next pseudorandom, Gaussian ("normally") distributed
389     * <code>double</code> value with mean <code>0.0</code> and standard
390     * deviation <code>1.0</code> from this random number generator's sequence.
391     * <p>
392     * The general contract of <tt>nextGaussian</tt> is that one
393     * <tt>double</tt> value, chosen from (approximately) the usual
394     * normal distribution with mean <tt>0.0</tt> and standard deviation
395     * <tt>1.0</tt>, is pseudorandomly generated and returned. The method
396     * <tt>nextGaussian</tt> is implemented by class <tt>Random</tt> as follows:
397     * <blockquote><pre>
398     * synchronized public double nextGaussian() {
399     * if (haveNextNextGaussian) {
400     * haveNextNextGaussian = false;
401     * return nextNextGaussian;
402     * } else {
403     * double v1, v2, s;
404     * do {
405     * v1 = 2 * nextDouble() - 1; // between -1.0 and 1.0
406     * v2 = 2 * nextDouble() - 1; // between -1.0 and 1.0
407     * s = v1 * v1 + v2 * v2;
408     * } while (s >= 1 || s == 0);
409     * double multiplier = Math.sqrt(-2 * Math.log(s)/s);
410     * nextNextGaussian = v2 * multiplier;
411     * haveNextNextGaussian = true;
412     * return v1 * multiplier;
413     * }
414     * }</pre></blockquote>
415     * This uses the <i>polar method</i> of G. E. P. Box, M. E. Muller, and
416     * G. Marsaglia, as described by Donald E. Knuth in <i>The Art of
417     * Computer Programming</i>, Volume 2: <i>Seminumerical Algorithms</i>,
418     * section 3.4.1, subsection C, algorithm P. Note that it generates two
419     * independent values at the cost of only one call to <tt>Math.log</tt>
420     * and one call to <tt>Math.sqrt</tt>.
421     *
422     * @return the next pseudorandom, Gaussian ("normally") distributed
423     * <code>double</code> value with mean <code>0.0</code> and
424     * standard deviation <code>1.0</code> from this random number
425     * generator's sequence.
426     */
427     synchronized public double nextGaussian() {
428     // See Knuth, ACP, Section 3.4.1 Algorithm C.
429     if (haveNextNextGaussian) {
430     haveNextNextGaussian = false;
431     return nextNextGaussian;
432     } else {
433     double v1, v2, s;
434     do {
435     v1 = 2 * nextDouble() - 1; // between -1 and 1
436     v2 = 2 * nextDouble() - 1; // between -1 and 1
437     s = v1 * v1 + v2 * v2;
438     } while (s >= 1 || s == 0);
439     double multiplier = Math.sqrt(-2 * Math.log(s)/s);
440     nextNextGaussian = v2 * multiplier;
441     haveNextNextGaussian = true;
442     return v1 * multiplier;
443     }
444     }
445    
446     /**
447     * Serializable fields for Random.
448     *
449     * @serialField seed long;
450     * seed for random computations
451     * @serialField nextNextGaussian double;
452     * next Gaussian to be returned
453     * @serialField haveNextNextGaussian boolean
454     * nextNextGaussian is valid
455     */
456     private static final ObjectStreamField[] serialPersistentFields = {
457     new ObjectStreamField("seed", Long.TYPE),
458     new ObjectStreamField("nextNextGaussian", Double.TYPE),
459     new ObjectStreamField("haveNextNextGaussian", Boolean.TYPE)
460     };
461    
462     /**
463     * Reconstitute the <tt>Random</tt> instance from a stream (that is,
464     * deserialize it). The seed is read in as long for
465     * historical reasons, but it is converted to an AtomicLong.
466     */
467     private void readObject(java.io.ObjectInputStream s)
468     throws java.io.IOException, ClassNotFoundException {
469    
470     ObjectInputStream.GetField fields = s.readFields();
471     long seedVal;
472    
473     seedVal = (long) fields.get("seed", -1L);
474     if (seedVal < 0)
475     throw new java.io.StreamCorruptedException(
476     "Random: invalid seed");
477 dl 1.3 seed = new AtomicLong(seedVal);
478 dl 1.1 nextNextGaussian = fields.get("nextNextGaussian", 0.0);
479     haveNextNextGaussian = fields.get("haveNextNextGaussian", false);
480     }
481    
482    
483     /**
484     * Save the <tt>Random</tt> instance to a stream.
485     * The seed of a Random is serialized as a long for
486     * historical reasons.
487     *
488     */
489     synchronized private void writeObject(ObjectOutputStream s) throws IOException {
490     // set the values of the Serializable fields
491     ObjectOutputStream.PutField fields = s.putFields();
492 dl 1.3 fields.put("seed", seed.get());
493 dl 1.1 fields.put("nextNextGaussian", nextNextGaussian);
494     fields.put("haveNextNextGaussian", haveNextNextGaussian);
495    
496     // save them
497     s.writeFields();
498    
499     }
500    
501     }