--- jsr166/src/main/java/util/SplittableRandom.java	2013/07/14 03:47:31	1.9
+++ jsr166/src/main/java/util/SplittableRandom.java	2013/08/05 13:58:02	1.14
@@ -25,6 +25,7 @@
 
 package java.util;
 
+import java.security.SecureRandom;
 import java.util.concurrent.atomic.AtomicLong;
 import java.util.Spliterator;
 import java.util.function.IntConsumer;
@@ -51,7 +52,9 @@ import java.util.stream.DoubleStream;
  * href="http://www.phy.duke.edu/~rgb/General/dieharder.php"> version
  * 3.31.1</a>.) These tests validate only the methods for certain
  * types and ranges, but similar properties are expected to hold, at
- * least approximately, for others as well.  </li>
+ * least approximately, for others as well. The <em>period</em>
+ * (length of any series of generated values before it repeats) is at
+ * least 2<sup>64</sup>. </li>
  *
  * <li> Method {@link #split} constructs and returns a new
  * SplittableRandom instance that shares no mutable state with the
@@ -112,43 +115,33 @@ public class SplittableRandom {
      * are encountered; see method addGammaModGeorge. For this to
      * work, initial gamma values must be at least 13.
      *
-     * The value of gamma differs for each instance across a series of
-     * splits, and is generated using a slightly stripped-down variant
-     * of the same algorithm, but operating across calls to split(),
-     * not calls to nextSeed(): Each instance carries the state of
-     * this generator as nextSplit, and uses mix64(nextSplit) as its
-     * own gamma value. Computations of gammas themselves use a fixed
-     * constant as the second argument to the addGammaModGeorge
-     * function, GAMMA_GAMMA, a "genuinely random" number from a
-     * radioactive decay reading (obtained from
-     * http://www.fourmilab.ch/hotbits/) meeting the above range
-     * constraint. Using a fixed constant maintains the invariant that
-     * the value of gamma is the same for every instance that is at
-     * the same split-distance from their common root. (Note: there is
-     * nothing especially magic about obtaining this constant from a
-     * "truly random" physical source rather than just choosing one
-     * arbitrarily; using "hotbits" was merely an aesthetically pleasing
-     * choice.  In either case, good statistical behavior of the
-     * algorithm should be, and was, verified by using the DieHarder
-     * test suite.)
-     *
      * The mix64 bit-mixing function called by nextLong and other
      * methods computes the same value as the "64-bit finalizer"
      * function in Austin Appleby's MurmurHash3 algorithm.  See
      * http://code.google.com/p/smhasher/wiki/MurmurHash3 , which
      * comments: "The constants for the finalizers were generated by a
      * simple simulated-annealing algorithm, and both avalanche all
-     * bits of 'h' to within 0.25% bias." It also appears to work to
-     * use instead any of the variants proposed by David Stafford at
-     * http://zimbry.blogspot.com/2011/09/better-bit-mixing-improving-on.html
-     * but these variants have not yet been tested as thoroughly
-     * in the context of the implementation of SplittableRandom.
+     * bits of 'h' to within 0.25% bias."
+     *
+     * The value of gamma differs for each instance across a series of
+     * splits, and is generated using an independent variant of the
+     * same algorithm, but operating across calls to split(), not
+     * calls to nextSeed(): Each instance carries the state of this
+     * generator as nextSplit. Gammas are treated as 57bit values,
+     * advancing by adding GAMMA_GAMMA mod GAMMA_PRIME, and bit-mixed
+     * with a 57-bit version of mix, using the "Mix13" multiplicative
+     * constants for MurmurHash3 described by David Stafford
+     * (http://zimbry.blogspot.com/2011/09/better-bit-mixing-improving-on.html).
+     * The value of GAMMA_GAMMA is arbitrary (except must be at least
+     * 13 and less than GAMMA_PRIME), but because it serves as the
+     * base of split sequences, should be subject to validation of
+     * consequent random number quality metrics.
      *
      * The mix32 function used for nextInt just consists of two of the
-     * five lines of mix64; avalanche testing shows that the 64-bit result
-     * has its top 32 bits avalanched well, though not the bottom 32 bits.
-     * DieHarder tests show that it is adequate for generating one
-     * random int from the 64-bit result of nextSeed.
+     * five lines of mix64; avalanche testing shows that the 64-bit
+     * result has its top 32 bits avalanched well, though not the
+     * bottom 32 bits.  DieHarder tests show that it is adequate for
+     * generating one random int from the 64-bit result of nextSeed.
      *
      * Support for the default (no-argument) constructor relies on an
      * AtomicLong (defaultSeedGenerator) to help perform the
@@ -156,8 +149,8 @@ public class SplittableRandom {
      * SplittableRandom. Unlike other cases, this split must be
      * performed in a thread-safe manner. We use
      * AtomicLong.compareAndSet as the (typically) most efficient
-     * mechanism. To bootstrap, we start off using System.nanotime(),
-     * and update using another "genuinely random" constant
+     * mechanism. To bootstrap, we start off using a SecureRandom
+     * initial default seed, and update using a fixed
      * DEFAULT_SEED_GAMMA. The default constructor uses GAMMA_GAMMA,
      * not 0, for its splitSeed argument (addGammaModGeorge(0,
      * GAMMA_GAMMA) == GAMMA_GAMMA) to reflect that each is split from
@@ -166,18 +159,30 @@ public class SplittableRandom {
      */
 
     /**
-     * The "genuinely random" value for producing new gamma values.
-     * The value is arbitrary, subject to the requirement that it be
-     * greater or equal to 13.
+     * The prime modulus for gamma values.
+     */
+    private static final long GAMMA_PRIME = (1L << 57) - 13L;
+
+    /**
+     * The value for producing new gamma values. Must be greater or
+     * equal to 13 and less than GAMMA_PRIME. Otherwise, the value is
+     * arbitrary subject to validation of the resulting statistical
+     * quality of splits.
      */
-    private static final long GAMMA_GAMMA = 0xF2281E2DBA6606F3L;
+    private static final long GAMMA_GAMMA = 0x00aae38294f712aabL;
 
     /**
-     * The "genuinely random" seed update value for default constructors.
-     * The value is arbitrary, subject to the requirement that it be
-     * greater or equal to 13.
+     * The seed update value for default constructors.  Must be
+     * greater or equal to 13. Otherwise, the value is arbitrary
+     * subject to quality checks.
      */
-    private static final long DEFAULT_SEED_GAMMA = 0xBD24B73A95FB84D9L;
+    private static final long DEFAULT_SEED_GAMMA = 0x9e3779b97f4a7c15L;
+
+    /**
+     * The value 13 with 64bit sign bit set. Used in the signed
+     * comparison in addGammaModGeorge.
+     */
+    private static final long BOTTOM13 = 0x800000000000000DL;
 
     /**
      * The least non-zero value returned by nextDouble(). This value
@@ -189,7 +194,7 @@ public class SplittableRandom {
      * The next seed for default constructors.
      */
     private static final AtomicLong defaultSeedGenerator =
-        new AtomicLong(System.nanoTime());
+        new AtomicLong(getInitialDefaultSeed());
 
     /**
      * The seed, updated only via method nextSeed.
@@ -217,15 +222,16 @@ public class SplittableRandom {
      * George < 2^64; thus we need only a conditional, not a loop,
      * to be sure of getting a representable value.
      *
-     * @param s a seed value
+     * Because Java comparison operators are signed, we implement this
+     * by conceptually offsetting seed values downwards by 2^63, so
+     * 0..13 is represented as Long.MIN_VALUE..BOTTOM13.
+     *
+     * @param s a seed value, viewed as a signed long
      * @param g a gamma value, 13 <= g (as unsigned)
      */
     private static long addGammaModGeorge(long s, long g) {
         long p = s + g;
-        if (Long.compareUnsigned(p, g) >= 0)
-            return p;
-        long q = p - 13L;
-        return (Long.compareUnsigned(p, 13L) >= 0) ? q : (q + g);
+        return (p >= s) ? p : ((p >= BOTTOM13) ? p  : p + g) - 13L;
     }
 
     /**
@@ -252,6 +258,19 @@ public class SplittableRandom {
     }
 
     /**
+     * Returns a 57-bit mixed transformation of its argument.  See
+     * above for explanation.
+     */
+    private static long mix57(long z) {
+        z = (z ^ (z >>> 30)) * 0xbf58476d1ce4e5b9L;
+        z &= 0x01FFFFFFFFFFFFFFL;
+        z = (z ^ (z >>> 27)) * 0x94d049bb133111ebL;
+        z &= 0x01FFFFFFFFFFFFFFL;
+        z ^= (z >>> 31);
+        return z;
+    }
+
+    /**
      * Internal constructor used by all other constructors and by
      * method split. Establishes the initial seed for this instance,
      * and uses the given splitSeed to establish gamma, as well as the
@@ -262,9 +281,11 @@ public class SplittableRandom {
         this.seed = seed;
         long s = splitSeed, g;
         do { // ensure gamma >= 13, considered as an unsigned integer
-            s = addGammaModGeorge(s, GAMMA_GAMMA);
-            g = mix64(s);
-        } while (Long.compareUnsigned(g, 13L) < 0);
+            s += GAMMA_GAMMA;
+            if (s >= GAMMA_PRIME)
+                s -= GAMMA_PRIME;
+            g = mix57(s);
+        } while (g < 13L);
         this.gamma = g;
         this.nextSplit = s;
     }
@@ -289,6 +310,17 @@ public class SplittableRandom {
         return mix64(newSeed);
     }
 
+    /**
+     * Returns an initial default seed.
+     */
+    private static long getInitialDefaultSeed() {
+        byte[] seedBytes = java.security.SecureRandom.getSeed(8);
+        long s = (long)(seedBytes[0]) & 0xffL;
+        for (int i = 1; i < 8; ++i)
+            s = (s << 8) | ((long)(seedBytes[i]) & 0xffL);
+        return s;
+    }
+
     /*
      * Internal versions of nextX methods used by streams, as well as
      * the public nextX(origin, bound) methods.  These exist mainly to
@@ -364,7 +396,7 @@ public class SplittableRandom {
         int r = mix32(nextSeed());
         if (origin < bound) {
             int n = bound - origin, m = n - 1;
-            if ((n & m) == 0L)
+            if ((n & m) == 0)
                 r = (r & m) + origin;
             else if (n > 0) {
                 for (int u = r >>> 1;
@@ -403,12 +435,12 @@ public class SplittableRandom {
     /**
      * Creates a new SplittableRandom instance using the specified
      * initial seed. SplittableRandom instances created with the same
-     * seed generate identical sequences of values.
+     * seed in the same program generate identical sequences of values.
      *
      * @param seed the initial seed
      */
     public SplittableRandom(long seed) {
-        this(seed, 0);
+        this(seed, 0L);
     }
 
     /**
@@ -455,7 +487,7 @@ public class SplittableRandom {
      * @param bound the bound on the random number to be returned.  Must be
      *        positive.
      * @return a pseudorandom {@code int} value between zero
-     *         (inclusive) and the bound (exclusive).
+     *         (inclusive) and the bound (exclusive)
      * @throws IllegalArgumentException if the bound is less than zero
      */
     public int nextInt(int bound) {
@@ -464,7 +496,7 @@ public class SplittableRandom {
         // Specialize internalNextInt for origin 0
         int r = mix32(nextSeed());
         int m = bound - 1;
-        if ((bound & m) == 0L) // power of two
+        if ((bound & m) == 0) // power of two
             r &= m;
         else { // reject over-represented candidates
             for (int u = r >>> 1;
@@ -482,7 +514,7 @@ public class SplittableRandom {
      * @param origin the least value returned
      * @param bound the upper bound (exclusive)
      * @return a pseudorandom {@code int} value between the origin
-     *         (inclusive) and the bound (exclusive).
+     *         (inclusive) and the bound (exclusive)
      * @throws IllegalArgumentException if {@code origin} is greater than
      *         or equal to {@code bound}
      */
@@ -508,7 +540,7 @@ public class SplittableRandom {
      * @param bound the bound on the random number to be returned.  Must be
      *        positive.
      * @return a pseudorandom {@code long} value between zero
-     *         (inclusive) and the bound (exclusive).
+     *         (inclusive) and the bound (exclusive)
      * @throws IllegalArgumentException if {@code bound} is less than zero
      */
     public long nextLong(long bound) {
@@ -535,7 +567,7 @@ public class SplittableRandom {
      * @param origin the least value returned
      * @param bound the upper bound (exclusive)
      * @return a pseudorandom {@code long} value between the origin
-     *         (inclusive) and the bound (exclusive).
+     *         (inclusive) and the bound (exclusive)
      * @throws IllegalArgumentException if {@code origin} is greater than
      *         or equal to {@code bound}
      */
@@ -553,7 +585,7 @@ public class SplittableRandom {
      * (inclusive) and one (exclusive)
      */
     public double nextDouble() {
-        return (nextLong() >>> 11) * DOUBLE_UNIT;
+        return (mix64(nextSeed()) >>> 11) * DOUBLE_UNIT;
     }
 
     /**
@@ -563,13 +595,13 @@ public class SplittableRandom {
      * @param bound the bound on the random number to be returned.  Must be
      *        positive.
      * @return a pseudorandom {@code double} value between zero
-     *         (inclusive) and the bound (exclusive).
+     *         (inclusive) and the bound (exclusive)
      * @throws IllegalArgumentException if {@code bound} is less than zero
      */
     public double nextDouble(double bound) {
         if (!(bound > 0.0))
             throw new IllegalArgumentException("bound must be positive");
-        double result = nextDouble() * bound;
+        double result = (mix64(nextSeed()) >>> 11) * DOUBLE_UNIT * bound;
         return (result < bound) ?  result : // correct for rounding
             Double.longBitsToDouble(Double.doubleToLongBits(bound) - 1);
     }
@@ -581,7 +613,7 @@ public class SplittableRandom {
      * @param origin the least value returned
      * @param bound the upper bound
      * @return a pseudorandom {@code double} value between the origin
-     *         (inclusive) and the bound (exclusive).
+     *         (inclusive) and the bound (exclusive)
      * @throws IllegalArgumentException if {@code origin} is greater than
      *         or equal to {@code bound}
      */
@@ -591,6 +623,15 @@ public class SplittableRandom {
         return internalNextDouble(origin, bound);
     }
 
+    /**
+     * Returns a pseudorandom {@code boolean} value.
+     *
+     * @return a pseudorandom {@code boolean} value
+     */
+    public boolean nextBoolean() {
+        return mix32(nextSeed()) < 0;
+    }
+
     // stream methods, coded in a way intended to better isolate for
     // maintenance purposes the small differences across forms.
 
@@ -614,7 +655,7 @@ public class SplittableRandom {
 
     /**
      * Returns an effectively unlimited stream of pseudorandom {@code int}
-     * values
+     * values.
      *
      * @implNote This method is implemented to be equivalent to {@code
      * ints(Long.MAX_VALUE)}.
@@ -637,7 +678,7 @@ public class SplittableRandom {
      * @param randomNumberOrigin the origin of each random value
      * @param randomNumberBound the bound of each random value
      * @return a stream of pseudorandom {@code int} values,
-     *         each with the given origin and bound.
+     *         each with the given origin and bound
      * @throws IllegalArgumentException if {@code streamSize} is
      *         less than zero, or {@code randomNumberOrigin}
      *         is greater than or equal to {@code randomNumberBound}
@@ -664,7 +705,7 @@ public class SplittableRandom {
      * @param randomNumberOrigin the origin of each random value
      * @param randomNumberBound the bound of each random value
      * @return a stream of pseudorandom {@code int} values,
-     *         each with the given origin and bound.
+     *         each with the given origin and bound
      * @throws IllegalArgumentException if {@code randomNumberOrigin}
      *         is greater than or equal to {@code randomNumberBound}
      */
@@ -720,7 +761,7 @@ public class SplittableRandom {
      * @param randomNumberOrigin the origin of each random value
      * @param randomNumberBound the bound of each random value
      * @return a stream of pseudorandom {@code long} values,
-     *         each with the given origin and bound.
+     *         each with the given origin and bound
      * @throws IllegalArgumentException if {@code streamSize} is
      *         less than zero, or {@code randomNumberOrigin}
      *         is greater than or equal to {@code randomNumberBound}
@@ -747,7 +788,7 @@ public class SplittableRandom {
      * @param randomNumberOrigin the origin of each random value
      * @param randomNumberBound the bound of each random value
      * @return a stream of pseudorandom {@code long} values,
-     *         each with the given origin and bound.
+     *         each with the given origin and bound
      * @throws IllegalArgumentException if {@code randomNumberOrigin}
      *         is greater than or equal to {@code randomNumberBound}
      */
@@ -805,9 +846,9 @@ public class SplittableRandom {
      * @param randomNumberOrigin the origin of each random value
      * @param randomNumberBound the bound of each random value
      * @return a stream of pseudorandom {@code double} values,
-     * each with the given origin and bound.
+     * each with the given origin and bound
      * @throws IllegalArgumentException if {@code streamSize} is
-     * less than zero.
+     * less than zero
      * @throws IllegalArgumentException if {@code randomNumberOrigin}
      *         is greater than or equal to {@code randomNumberBound}
      */
@@ -833,7 +874,7 @@ public class SplittableRandom {
      * @param randomNumberOrigin the origin of each random value
      * @param randomNumberBound the bound of each random value
      * @return a stream of pseudorandom {@code double} values,
-     * each with the given origin and bound.
+     * each with the given origin and bound
      * @throws IllegalArgumentException if {@code randomNumberOrigin}
      *         is greater than or equal to {@code randomNumberBound}
      */
@@ -854,7 +895,7 @@ public class SplittableRandom {
      * approach. The long and double versions of this class are
      * identical except for types.
      */
-    static class RandomIntsSpliterator implements Spliterator.OfInt {
+    static final class RandomIntsSpliterator implements Spliterator.OfInt {
         final SplittableRandom rng;
         long index;
         final long fence;
@@ -908,7 +949,7 @@ public class SplittableRandom {
     /**
      * Spliterator for long streams.
      */
-    static class RandomLongsSpliterator implements Spliterator.OfLong {
+    static final class RandomLongsSpliterator implements Spliterator.OfLong {
         final SplittableRandom rng;
         long index;
         final long fence;
@@ -963,7 +1004,7 @@ public class SplittableRandom {
     /**
      * Spliterator for double streams.
      */
-    static class RandomDoublesSpliterator implements Spliterator.OfDouble {
+    static final class RandomDoublesSpliterator implements Spliterator.OfDouble {
         final SplittableRandom rng;
         long index;
         final long fence;