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root/jsr166/jsr166/src/main/java/util/SplittableRandom.java

Comparing jsr166/src/main/java/util/SplittableRandom.java (file contents):
Revision 1.11 by dl, Tue Jul 16 12:32:05 2013 UTC vs.
Revision 1.13 by dl, Thu Jul 25 13:19:09 2013 UTC

#	Line 25 | Line 25
25
26		package java.util;
27
28	+	import java.security.SecureRandom;
29		import java.util.concurrent.atomic.AtomicLong;
30		import java.util.Spliterator;
31		import java.util.function.IntConsumer;
#	Line 114 | Line 115 | public class SplittableRandom {
115		* are encountered; see method addGammaModGeorge. For this to
116		* work, initial gamma values must be at least 13.
117		*
117	–	* The value of gamma differs for each instance across a series of
118	–	* splits, and is generated using a slightly stripped-down variant
119	–	* of the same algorithm, but operating across calls to split(),
120	–	* not calls to nextSeed(): Each instance carries the state of
121	–	* this generator as nextSplit, and uses mix64(nextSplit) as its
122	–	* own gamma value. Computations of gammas themselves use a fixed
123	–	* constant as the second argument to the addGammaModGeorge
124	–	* function, GAMMA_GAMMA, a "genuinely random" number from a
125	–	* radioactive decay reading (obtained from
126	–	* http://www.fourmilab.ch/hotbits/) meeting the above range
127	–	* constraint. Using a fixed constant maintains the invariant that
128	–	* the value of gamma is the same for every instance that is at
129	–	* the same split-distance from their common root. (Note: there is
130	–	* nothing especially magic about obtaining this constant from a
131	–	* "truly random" physical source rather than just choosing one
132	–	* arbitrarily; using "hotbits" was merely an aesthetically pleasing
133	–	* choice.  In either case, good statistical behavior of the
134	–	* algorithm should be, and was, verified by using the DieHarder
135	–	* test suite.)
136	–	*
118		* The mix64 bit-mixing function called by nextLong and other
119		* methods computes the same value as the "64-bit finalizer"
120		* function in Austin Appleby's MurmurHash3 algorithm.  See
121		* http://code.google.com/p/smhasher/wiki/MurmurHash3 , which
122		* comments: "The constants for the finalizers were generated by a
123		* simple simulated-annealing algorithm, and both avalanche all
124	<	* bits of 'h' to within 0.25% bias." It also appears to work to
125	<	* use instead any of the variants proposed by David Stafford at
126	<	* http://zimbry.blogspot.com/2011/09/better-bit-mixing-improving-on.html
127	<	* but these variants have not yet been tested as thoroughly
128	<	* in the context of the implementation of SplittableRandom.
124	>	* bits of 'h' to within 0.25% bias."
125	>	*
126	>	* The value of gamma differs for each instance across a series of
127	>	* splits, and is generated using an independent variant of the
128	>	* same algorithm, but operating across calls to split(), not
129	>	* calls to nextSeed(): Each instance carries the state of this
130	>	* generator as nextSplit. Gammas are treated as 57bit values,
131	>	* advancing by adding GAMMA_GAMMA mod GAMMA_PRIME, and bit-mixed
132	>	* with a 57-bit version of mix, using the "Mix01" multiplicative
133	>	* constants for MurmurHash3 described by David Stafford
134	>	* (http://zimbry.blogspot.com/2011/09/better-bit-mixing-improving-on.html).
135	>	* The value of GAMMA_GAMMA is arbitrary (except must be at least
136	>	* 13 and less than GAMMA_PRIME), but because it serves as the
137	>	* base of split sequences, should be subject to validation of
138	>	* consequent random number quality metrics.
139		*
140		* The mix32 function used for nextInt just consists of two of the
141	<	* five lines of mix64; avalanche testing shows that the 64-bit result
142	<	* has its top 32 bits avalanched well, though not the bottom 32 bits.
143	<	* DieHarder tests show that it is adequate for generating one
144	<	* random int from the 64-bit result of nextSeed.
141	>	* five lines of mix64; avalanche testing shows that the 64-bit
142	>	* result has its top 32 bits avalanched well, though not the
143	>	* bottom 32 bits.  DieHarder tests show that it is adequate for
144	>	* generating one random int from the 64-bit result of nextSeed.
145		*
146		* Support for the default (no-argument) constructor relies on an
147		* AtomicLong (defaultSeedGenerator) to help perform the
#	Line 158 | Line 149 | public class SplittableRandom {
149		* SplittableRandom. Unlike other cases, this split must be
150		* performed in a thread-safe manner. We use
151		* AtomicLong.compareAndSet as the (typically) most efficient
152	<	* mechanism. To bootstrap, we start off using a function of the
153	<	* current System time as seed, and update using another
154	<	* "genuinely random" constant DEFAULT_SEED_GAMMA. The default
155	<	* constructor uses GAMMA_GAMMA, not 0, for its splitSeed argument
156	<	* (addGammaModGeorge(0, GAMMA_GAMMA) == GAMMA_GAMMA) to reflect
157	<	* that each is split from this root generator, even though the
158	<	* root is not explicitly represented as a SplittableRandom.  When
159	<	* establishing the initial seed, we use both
160	<	* System.currentTimeMillis and System.nanoTime(), to avoid
161	<	* regularities that may occur if using either alone.
152	>	* mechanism. To bootstrap, we start off using a SecureRandom
153	>	* initial default seed, and update using a fixed
154	>	* DEFAULT_SEED_GAMMA. The default constructor uses GAMMA_GAMMA,
155	>	* not 0, for its splitSeed argument (addGammaModGeorge(0,
156	>	* GAMMA_GAMMA) == GAMMA_GAMMA) to reflect that each is split from
157	>	* this root generator, even though the root is not explicitly
158	>	* represented as a SplittableRandom.
159	>	*/
160	>
161	>	/**
162	>	* The prime modulus for gamma values.
163		*/
164	+	private static final long GAMMA_PRIME = (1L << 57) - 13L;
165
166		/**
167	<	* The "genuinely random" value for producing new gamma values.
168	<	* The value is arbitrary, subject to the requirement that it be
169	<	* greater or equal to 13.
167	>	* The value for producing new gamma values. Must be greater or
168	>	* equal to 13 and less than GAMMA_PRIME. Otherwise, the value is
169	>	* arbitrary subject to validation of the resulting statistical
170	>	* quality of splits.
171		*/
172	<	private static final long GAMMA_GAMMA = 0xF2281E2DBA6606F3L;
172	>	private static final long GAMMA_GAMMA = 0x00aae38294f712aabL;
173
174		/**
175	<	* The "genuinely random" seed update value for default constructors.
176	<	* The value is arbitrary, subject to the requirement that it be
177	<	* greater or equal to 13.
175	>	* The seed update value for default constructors.  Must be
176	>	* greater or equal to 13. Otherwise, the value is arbitrary
177	>	* subject to quality checks.
178		*/
179	<	private static final long DEFAULT_SEED_GAMMA = 0xBD24B73A95FB84D9L;
179	>	private static final long DEFAULT_SEED_GAMMA = 0x9e3779b97f4a7c15L;
180
181		/**
182		* The value 13 with 64bit sign bit set. Used in the signed
#	Line 200 | Line 194 | public class SplittableRandom {
194		* The next seed for default constructors.
195		*/
196		private static final AtomicLong defaultSeedGenerator =
197	<	new AtomicLong(mix64(System.currentTimeMillis()) ^
204	<	mix64(System.nanoTime()));
197	>	new AtomicLong(getInitialDefaultSeed());
198
199		/**
200		* The seed, updated only via method nextSeed.
#	Line 265 | Line 258 | public class SplittableRandom {
258		}
259
260		/**
261	+	* Returns a 57-bit mixed transformation of its argument.  See
262	+	* above for explanation.
263	+	*/
264	+	private static long mix57(long z) {
265	+	z ^= (z >>> 33);
266	+	z *= 0x7fb5d329728ea185L;
267	+	z &= 0x01FFFFFFFFFFFFFFL;
268	+	z ^= (z >>> 33);
269	+	z *= 0x81dadef4bc2dd44dL;
270	+	z &= 0x01FFFFFFFFFFFFFFL;
271	+	z ^= (z >>> 33);
272	+	return z;
273	+	}
274	+
275	+	/**
276		* Internal constructor used by all other constructors and by
277		* method split. Establishes the initial seed for this instance,
278		* and uses the given splitSeed to establish gamma, as well as the
#	Line 275 | Line 283 | public class SplittableRandom {
283		this.seed = seed;
284		long s = splitSeed, g;
285		do { // ensure gamma >= 13, considered as an unsigned integer
286	<	s = addGammaModGeorge(s, GAMMA_GAMMA);
287	<	g = mix64(s);
288	<	} while (g >= 0L && g < 13L);
286	>	s += GAMMA_GAMMA;
287	>	if (s >= GAMMA_PRIME)
288	>	s -= GAMMA_PRIME;
289	>	g = mix57(s);
290	>	} while (g < 13L);
291		this.gamma = g;
292		this.nextSplit = s;
293		}
#	Line 302 | Line 312 | public class SplittableRandom {
312		return mix64(newSeed);
313		}
314
315	+	/**
316	+	* Returns an initial default seed.
317	+	*/
318	+	private static long getInitialDefaultSeed() {
319	+	byte[] seedBytes = java.security.SecureRandom.getSeed(8);
320	+	long s = (long)(seedBytes[0]) & 0xffL;
321	+	for (int i = 1; i < 8; ++i)
322	+	s = (s << 8) | ((long)(seedBytes[i]) & 0xffL);
323	+	return s;
324	+	}
325	+
326		/*
327		* Internal versions of nextX methods used by streams, as well as
328		* the public nextX(origin, bound) methods.  These exist mainly to
#	Line 377 | Line 398 | public class SplittableRandom {
398		int r = mix32(nextSeed());
399		if (origin < bound) {
400		int n = bound - origin, m = n - 1;
401	<	if ((n & m) == 0L)
401	>	if ((n & m) == 0)
402		r = (r & m) + origin;
403		else if (n > 0) {
404		for (int u = r >>> 1;
#	Line 421 | Line 442 | public class SplittableRandom {
442		* @param seed the initial seed
443		*/
444		public SplittableRandom(long seed) {
445	<	this(seed, 0);
445	>	this(seed, 0L);
446		}
447
448		/**
#	Line 477 | Line 498 | public class SplittableRandom {
498		// Specialize internalNextInt for origin 0
499		int r = mix32(nextSeed());
500		int m = bound - 1;
501	<	if ((bound & m) == 0L) // power of two
501	>	if ((bound & m) == 0) // power of two
502		r &= m;
503		else { // reject over-represented candidates
504		for (int u = r >>> 1;

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