--- jsr166/src/main/java/util/SplittableRandom.java	2013/07/11 13:40:42	1.4
+++ jsr166/src/main/java/util/SplittableRandom.java	2015/09/19 21:07:10	1.28
@@ -26,23 +26,23 @@
 package java.util;
 
 import java.util.concurrent.atomic.AtomicLong;
-import java.util.Spliterator;
+import java.util.function.DoubleConsumer;
 import java.util.function.IntConsumer;
 import java.util.function.LongConsumer;
-import java.util.function.DoubleConsumer;
-import java.util.stream.StreamSupport;
+import java.util.stream.DoubleStream;
 import java.util.stream.IntStream;
 import java.util.stream.LongStream;
-import java.util.stream.DoubleStream;
-
+import java.util.stream.StreamSupport;
 
 /**
  * A generator of uniform pseudorandom values applicable for use in
  * (among other contexts) isolated parallel computations that may
- * generate subtasks. Class SplittableRandom supports methods for
+ * generate subtasks. Class {@code SplittableRandom} supports methods for
  * producing pseudorandom numbers of type {@code int}, {@code long},
  * and {@code double} with similar usages as for class
- * {@link java.util.Random} but differs in the following ways: <ul>
+ * {@link java.util.Random} but differs in the following ways:
+ *
+ * <ul>
  *
  * <li>Series of generated values pass the DieHarder suite testing
  * independence and uniformity properties of random number generators.
@@ -50,15 +50,17 @@ 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> Method {@link #split} constructs and returns a new
+ * <li>Method {@link #split} constructs and returns a new
  * SplittableRandom instance that shares no mutable state with the
- * current instance. However, with very high probability, the set of
- * values collectively generated by the two objects has the same
+ * current instance. However, with very high probability, the
+ * values collectively generated by the two objects have the same
  * statistical properties as if the same quantity of values were
  * generated by a single thread using a single {@code
- * SplittableRandom} object.  </li>
+ * SplittableRandom} object.
  *
  * <li>Instances of SplittableRandom are <em>not</em> thread-safe.
  * They are designed to be split, not shared, across threads. For
@@ -69,25 +71,22 @@ import java.util.stream.DoubleStream;
  *
  * <li>This class provides additional methods for generating random
  * streams, that employ the above techniques when used in {@code
- * stream.parallel()} mode.</li>
+ * stream.parallel()} mode.
  *
  * </ul>
  *
+ * <p>Instances of {@code SplittableRandom} are not cryptographically
+ * secure.  Consider instead using {@link java.security.SecureRandom}
+ * in security-sensitive applications. Additionally,
+ * default-constructed instances do not use a cryptographically random
+ * seed unless the {@linkplain System#getProperty system property}
+ * {@code java.util.secureRandomSeed} is set to {@code true}.
+ *
  * @author  Guy Steele
  * @author  Doug Lea
  * @since   1.8
  */
-public class SplittableRandom {
-
-    /*
-     * File organization: First the non-public methods that constitute
-     * the main algorithm, then the main public methods, followed by
-     * some custom spliterator classes needed for stream methods.
-     *
-     * Credits: Primary algorithm and code by Guy Steele.  Stream
-     * support methods by Doug Lea.  Documentation jointly produced
-     * with additional help from Brian Goetz.
-     */
+public final class SplittableRandom {
 
     /*
      * Implementation Overview.
@@ -95,191 +94,156 @@ public class SplittableRandom {
      * This algorithm was inspired by the "DotMix" algorithm by
      * Leiserson, Schardl, and Sukha "Deterministic Parallel
      * Random-Number Generation for Dynamic-Multithreading Platforms",
-     * PPoPP 2012, but improves and extends it in several ways.
+     * PPoPP 2012, as well as those in "Parallel random numbers: as
+     * easy as 1, 2, 3" by Salmon, Morae, Dror, and Shaw, SC 2011.  It
+     * differs mainly in simplifying and cheapening operations.
+     *
+     * The primary update step (method nextSeed()) is to add a
+     * constant ("gamma") to the current (64 bit) seed, forming a
+     * simple sequence.  The seed and the gamma values for any two
+     * SplittableRandom instances are highly likely to be different.
+     *
+     * Methods nextLong, nextInt, and derivatives do not return the
+     * sequence (seed) values, but instead a hash-like bit-mix of
+     * their bits, producing more independently distributed sequences.
+     * For nextLong, the mix64 function is based on David Stafford's
+     * (http://zimbry.blogspot.com/2011/09/better-bit-mixing-improving-on.html)
+     * "Mix13" variant of the "64-bit finalizer" function in Austin
+     * Appleby's MurmurHash3 algorithm (see
+     * http://code.google.com/p/smhasher/wiki/MurmurHash3). The mix32
+     * function is based on Stafford's Mix04 mix function, but returns
+     * the upper 32 bits cast as int.
+     *
+     * The split operation uses the current generator to form the seed
+     * and gamma for another SplittableRandom.  To conservatively
+     * avoid potential correlations between seed and value generation,
+     * gamma selection (method mixGamma) uses different
+     * (Murmurhash3's) mix constants.  To avoid potential weaknesses
+     * in bit-mixing transformations, we restrict gammas to odd values
+     * with at least 24 0-1 or 1-0 bit transitions.  Rather than
+     * rejecting candidates with too few or too many bits set, method
+     * mixGamma flips some bits (which has the effect of mapping at
+     * most 4 to any given gamma value).  This reduces the effective
+     * set of 64bit odd gamma values by about 2%, and serves as an
+     * automated screening for sequence constant selection that is
+     * left as an empirical decision in some other hashing and crypto
+     * algorithms.
+     *
+     * The resulting generator thus transforms a sequence in which
+     * (typically) many bits change on each step, with an inexpensive
+     * mixer with good (but less than cryptographically secure)
+     * avalanching.
+     *
+     * The default (no-argument) constructor, in essence, invokes
+     * split() for a common "defaultGen" SplittableRandom.  Unlike
+     * other cases, this split must be performed in a thread-safe
+     * manner, so we use an AtomicLong to represent the seed rather
+     * than use an explicit SplittableRandom. To bootstrap the
+     * defaultGen, we start off using a seed based on current time
+     * unless the java.util.secureRandomSeed property is set. This
+     * serves as a slimmed-down (and insecure) variant of SecureRandom
+     * that also avoids stalls that may occur when using /dev/random.
+     *
+     * It is a relatively simple matter to apply the basic design here
+     * to use 128 bit seeds. However, emulating 128bit arithmetic and
+     * carrying around twice the state add more overhead than appears
+     * warranted for current usages.
      *
-     * The primary update step is simply to add a constant ("gamma")
-     * to the current seed, modulo a prime ("George"). However, the
-     * nextLong and nextInt methods do not return this value, but
-     * instead the results of bit-mixing transformations that produce
-     * more uniformly distributed sequences.
-     *
-     * "George" is the otherwise nameless (because it cannot be
-     * represented) prime number 2^64+13. Using a prime number larger
-     * than can fit in a long ensures that all possible long values
-     * can occur, plus 13 others that just get skipped over when they
-     * 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.
-     *
-     * 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.
-     *
-     * Support for the default (no-argument) constructor relies on an
-     * AtomicLong (defaultSeedGenerator) to help perform the
-     * equivalent of a split of a statically constructed
-     * 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
-     * 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
-     * this root generator, even though the root is not explicitly
-     * represented as a 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.
-     */
-    private static final long GAMMA_GAMMA = 0xF2281E2DBA6606F3L;
-
-    /**
-     * 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.
-     */
-    private static final long DEFAULT_SEED_GAMMA = 0xBD24B73A95FB84D9L;
+     * File organization: First the non-public methods that constitute
+     * the main algorithm, then the main public methods, followed by
+     * some custom spliterator classes needed for stream methods.
+     */
 
     /**
-     * The next seed for default constructors.
+     * The golden ratio scaled to 64bits, used as the initial gamma
+     * value for (unsplit) SplittableRandoms.
      */
-    private static final AtomicLong defaultSeedGenerator =
-        new AtomicLong(System.nanoTime());
+    private static final long GOLDEN_GAMMA = 0x9e3779b97f4a7c15L;
 
     /**
-     * The seed, updated only via method nextSeed.
+     * The least non-zero value returned by nextDouble(). This value
+     * is scaled by a random value of 53 bits to produce a result.
      */
-    private long seed;
+    private static final double DOUBLE_UNIT = 0x1.0p-53; // 1.0 / (1L << 53);
 
     /**
-     * The constant value added to seed (mod George) on each update.
+     * The seed. Updated only via method nextSeed.
      */
-    private final long gamma;
+    private long seed;
 
     /**
-     * The next seed to use for splits. Propagated using
-     * addGammaModGeorge across instances.
+     * The step value.
      */
-    private final long nextSplit;
+    private final long gamma;
 
     /**
-     * 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
-     * nextSplit to use by this instance.
+     * Internal constructor used by all others except default constructor.
      */
-    private SplittableRandom(long seed, long splitSeed) {
+    private SplittableRandom(long seed, long gamma) {
         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);
-        this.gamma = g;
-        this.nextSplit = s;
+        this.gamma = gamma;
     }
 
     /**
-     * Adds the given gamma value, g, to the given seed value s, mod
-     * George (2^64+13). We regard s and g as unsigned values
-     * (ranging from 0 to 2^64-1). We add g to s either once or twice
-     * (mod George) as necessary to produce an (unsigned) result less
-     * than 2^64.  We require that g must be at least 13. This
-     * guarantees that if (s+g) mod George >= 2^64 then (s+g+g) mod
-     * 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
-     * @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);
+     * Computes Stafford variant 13 of 64bit mix function.
+     */
+    private static long mix64(long z) {
+        z = (z ^ (z >>> 30)) * 0xbf58476d1ce4e5b9L;
+        z = (z ^ (z >>> 27)) * 0x94d049bb133111ebL;
+        return z ^ (z >>> 31);
     }
 
     /**
-     * Updates in-place and returns seed.
-     * See above for explanation.
+     * Returns the 32 high bits of Stafford variant 4 mix64 function as int.
      */
-    private long nextSeed() {
-        return seed = addGammaModGeorge(seed, gamma);
+    private static int mix32(long z) {
+        z = (z ^ (z >>> 33)) * 0x62a9d9ed799705f5L;
+        return (int)(((z ^ (z >>> 28)) * 0xcb24d0a5c88c35b3L) >>> 32);
     }
 
     /**
-     * Returns a bit-mixed transformation of its argument.
-     * See above for explanation.
+     * Returns the gamma value to use for a new split instance.
      */
-    private static long mix64(long z) {
-        z ^= (z >>> 33);
-        z *= 0xff51afd7ed558ccdL;
-        z ^= (z >>> 33);
-        z *= 0xc4ceb9fe1a85ec53L;
-        z ^= (z >>> 33);
-        return z;
+    private static long mixGamma(long z) {
+        z = (z ^ (z >>> 33)) * 0xff51afd7ed558ccdL; // MurmurHash3 mix constants
+        z = (z ^ (z >>> 33)) * 0xc4ceb9fe1a85ec53L;
+        z = (z ^ (z >>> 33)) | 1L;                  // force to be odd
+        int n = Long.bitCount(z ^ (z >>> 1));       // ensure enough transitions
+        return (n < 24) ? z ^ 0xaaaaaaaaaaaaaaaaL : z;
     }
 
     /**
-     * Returns a bit-mixed int transformation of its argument.
-     * See above for explanation.
+     * Adds gamma to seed.
      */
-    private static int mix32(long z) {
-        z ^= (z >>> 33);
-        z *= 0xc4ceb9fe1a85ec53L;
-        return (int)(z >>> 32);
+    private long nextSeed() {
+        return seed += gamma;
     }
 
     /**
-     * Atomically updates and returns next seed for default constructor
+     * The seed generator for default constructors.
      */
-    private static long nextDefaultSeed() {
-        long oldSeed, newSeed;
-        do {
-            oldSeed = defaultSeedGenerator.get();
-            newSeed = addGammaModGeorge(oldSeed, DEFAULT_SEED_GAMMA);
-        } while (!defaultSeedGenerator.compareAndSet(oldSeed, newSeed));
-        return mix64(newSeed);
+    private static final AtomicLong defaultGen = new AtomicLong(initialSeed());
+
+    private static long initialSeed() {
+        String pp = java.security.AccessController.doPrivileged(
+                new sun.security.action.GetPropertyAction(
+                        "java.util.secureRandomSeed"));
+        if (pp != null && pp.equalsIgnoreCase("true")) {
+            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;
+        }
+        return (mix64(System.currentTimeMillis()) ^
+                mix64(System.nanoTime()));
     }
 
+    // IllegalArgumentException messages
+    static final String BAD_BOUND = "bound must be positive";
+    static final String BAD_RANGE = "bound must be greater than origin";
+    static final String BAD_SIZE  = "size must be non-negative";
+
     /*
      * Internal versions of nextX methods used by streams, as well as
      * the public nextX(origin, bound) methods.  These exist mainly to
@@ -326,16 +290,16 @@ public class SplittableRandom {
         long r = mix64(nextSeed());
         if (origin < bound) {
             long n = bound - origin, m = n - 1;
-            if ((n & m) == 0L) // power of two
+            if ((n & m) == 0L)  // power of two
                 r = (r & m) + origin;
-            else if (n > 0) { // reject over-represented candidates
+            else if (n > 0L) {  // reject over-represented candidates
                 for (long u = r >>> 1;            // ensure nonnegative
-                     u + m - (r = u % n) < 0L;    // reject
+                     u + m - (r = u % n) < 0L;    // rejection check
                      u = mix64(nextSeed()) >>> 1) // retry
                     ;
                 r += origin;
             }
-            else {             // range not representable as long
+            else {              // range not representable as long
                 while (r < origin || r >= bound)
                     r = mix64(nextSeed());
             }
@@ -355,11 +319,11 @@ 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;
-                     u + m - (r = u % n) < 0L;
+                     u + m - (r = u % n) < 0;
                      u = mix32(nextSeed()) >>> 1)
                     ;
                 r += origin;
@@ -380,11 +344,10 @@ public class SplittableRandom {
      * @return a pseudorandom value
      */
     final double internalNextDouble(double origin, double bound) {
-        long bits = (1023L << 52) | (nextLong() >>> 12);
-        double r = Double.longBitsToDouble(bits) - 1.0;
+        double r = (nextLong() >>> 11) * DOUBLE_UNIT;
         if (origin < bound) {
             r = r * (bound - origin) + origin;
-            if (r == bound) // correct for rounding
+            if (r >= bound) // correct for rounding
                 r = Double.longBitsToDouble(Double.doubleToLongBits(bound) - 1);
         }
         return r;
@@ -393,14 +356,14 @@ public class SplittableRandom {
     /* ---------------- public methods ---------------- */
 
     /**
-     * Creates a new SplittableRandom instance using the given initial
-     * seed. Two SplittableRandom instances created with the same seed
-     * generate identical sequences of values.
+     * Creates a new SplittableRandom instance using the specified
+     * initial seed. SplittableRandom instances created with the same
+     * seed in the same program generate identical sequences of values.
      *
      * @param seed the initial seed
      */
     public SplittableRandom(long seed) {
-        this(seed, 0);
+        this(seed, GOLDEN_GAMMA);
     }
 
     /**
@@ -409,8 +372,10 @@ public class SplittableRandom {
      * of those of any other instances in the current program; and
      * may, and typically does, vary across program invocations.
      */
-    public SplittableRandom() {
-        this(nextDefaultSeed(), GAMMA_GAMMA);
+    public SplittableRandom() { // emulate defaultGen.split()
+        long s = defaultGen.getAndAdd(2 * GOLDEN_GAMMA);
+        this.seed = mix64(s);
+        this.gamma = mixGamma(s + GOLDEN_GAMMA);
     }
 
     /**
@@ -428,39 +393,38 @@ public class SplittableRandom {
      * @return the new SplittableRandom instance
      */
     public SplittableRandom split() {
-        return new SplittableRandom(nextSeed(), nextSplit);
+        return new SplittableRandom(nextLong(), mixGamma(nextSeed()));
     }
 
     /**
      * Returns a pseudorandom {@code int} value.
      *
-     * @return a pseudorandom value
+     * @return a pseudorandom {@code int} value
      */
     public int nextInt() {
         return mix32(nextSeed());
     }
 
     /**
-     * Returns a pseudorandom {@code int} value between 0 (inclusive)
+     * Returns a pseudorandom {@code int} value between zero (inclusive)
      * and the specified bound (exclusive).
      *
-     * @param bound the bound on the random number to be returned.  Must be
-     *        positive.
-     * @return a pseudorandom {@code int} value between {@code 0}
-     *         (inclusive) and the bound (exclusive).
-     * @exception IllegalArgumentException if the bound is not positive
+     * @param bound the upper bound (exclusive).  Must be positive.
+     * @return a pseudorandom {@code int} value between zero
+     *         (inclusive) and the bound (exclusive)
+     * @throws IllegalArgumentException if {@code bound} is not positive
      */
     public int nextInt(int bound) {
         if (bound <= 0)
-            throw new IllegalArgumentException("bound must be positive");
+            throw new IllegalArgumentException(BAD_BOUND);
         // 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;
-                 u + m - (r = u % bound) < 0L;
+                 u + m - (r = u % bound) < 0;
                  u = mix32(nextSeed()) >>> 1)
                 ;
         }
@@ -474,38 +438,37 @@ 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).
-     * @exception IllegalArgumentException if {@code origin} is greater than
+     *         (inclusive) and the bound (exclusive)
+     * @throws IllegalArgumentException if {@code origin} is greater than
      *         or equal to {@code bound}
      */
     public int nextInt(int origin, int bound) {
         if (origin >= bound)
-            throw new IllegalArgumentException("bound must be greater than origin");
+            throw new IllegalArgumentException(BAD_RANGE);
         return internalNextInt(origin, bound);
     }
 
     /**
      * Returns a pseudorandom {@code long} value.
      *
-     * @return a pseudorandom value
+     * @return a pseudorandom {@code long} value
      */
     public long nextLong() {
         return mix64(nextSeed());
     }
 
     /**
-     * Returns a pseudorandom {@code long} value between 0 (inclusive)
+     * Returns a pseudorandom {@code long} value between zero (inclusive)
      * and the specified bound (exclusive).
      *
-     * @param bound the bound on the random number to be returned.  Must be
-     *        positive.
-     * @return a pseudorandom {@code long} value between {@code 0}
-     *         (inclusive) and the bound (exclusive).
-     * @exception IllegalArgumentException if the bound is not positive
+     * @param bound the upper bound (exclusive).  Must be positive.
+     * @return a pseudorandom {@code long} value between zero
+     *         (inclusive) and the bound (exclusive)
+     * @throws IllegalArgumentException if {@code bound} is not positive
      */
     public long nextLong(long bound) {
         if (bound <= 0)
-            throw new IllegalArgumentException("bound must be positive");
+            throw new IllegalArgumentException(BAD_BOUND);
         // Specialize internalNextLong for origin 0
         long r = mix64(nextSeed());
         long m = bound - 1;
@@ -527,78 +490,86 @@ 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).
-     * @exception IllegalArgumentException if {@code origin} is greater than
+     *         (inclusive) and the bound (exclusive)
+     * @throws IllegalArgumentException if {@code origin} is greater than
      *         or equal to {@code bound}
      */
     public long nextLong(long origin, long bound) {
         if (origin >= bound)
-            throw new IllegalArgumentException("bound must be greater than origin");
+            throw new IllegalArgumentException(BAD_RANGE);
         return internalNextLong(origin, bound);
     }
 
     /**
-     * Returns a pseudorandom {@code double} value between {@code 0.0}
-     * (inclusive) and {@code 1.0} (exclusive).
+     * Returns a pseudorandom {@code double} value between zero
+     * (inclusive) and one (exclusive).
      *
-     * @return a pseudorandom value between {@code 0.0}
-     * (inclusive) and {@code 1.0} (exclusive)
+     * @return a pseudorandom {@code double} value between zero
+     *         (inclusive) and one (exclusive)
      */
     public double nextDouble() {
-        long bits = (1023L << 52) | (nextLong() >>> 12);
-        return Double.longBitsToDouble(bits) - 1.0;
+        return (mix64(nextSeed()) >>> 11) * DOUBLE_UNIT;
     }
 
     /**
      * Returns a pseudorandom {@code double} value between 0.0
      * (inclusive) and the specified bound (exclusive).
      *
-     * @param bound the bound on the random number to be returned.  Must be
-     *        positive.
-     * @return a pseudorandom {@code double} value between {@code 0.0}
-     *         (inclusive) and the bound (exclusive).
+     * @param bound the upper bound (exclusive).  Must be positive.
+     * @return a pseudorandom {@code double} value between zero
+     *         (inclusive) and the bound (exclusive)
      * @throws IllegalArgumentException if {@code bound} is not positive
      */
     public double nextDouble(double bound) {
-        if (bound <= 0.0)
-            throw new IllegalArgumentException("bound must be positive");
-        double result = nextDouble() * bound;
+        if (!(bound > 0.0))
+            throw new IllegalArgumentException(BAD_BOUND);
+        double result = (mix64(nextSeed()) >>> 11) * DOUBLE_UNIT * bound;
         return (result < bound) ?  result : // correct for rounding
             Double.longBitsToDouble(Double.doubleToLongBits(bound) - 1);
     }
 
     /**
-     * Returns a pseudorandom {@code double} value between the given
+     * Returns a pseudorandom {@code double} value between the specified
      * origin (inclusive) and bound (exclusive).
      *
      * @param origin the least value returned
-     * @param bound the upper bound
+     * @param bound the upper bound (exclusive)
      * @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}
      */
     public double nextDouble(double origin, double bound) {
-        if (origin >= bound)
-            throw new IllegalArgumentException("bound must be greater than origin");
+        if (!(origin < bound))
+            throw new IllegalArgumentException(BAD_RANGE);
         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.
 
     /**
-     * Returns a stream with the given {@code streamSize} number of
-     * pseudorandom {@code int} values.
+     * Returns a stream producing the given {@code streamSize} number
+     * of pseudorandom {@code int} values from this generator and/or
+     * one split from it.
      *
      * @param streamSize the number of values to generate
      * @return a stream of pseudorandom {@code int} values
      * @throws IllegalArgumentException if {@code streamSize} is
-     * less than zero
+     *         less than zero
      */
     public IntStream ints(long streamSize) {
         if (streamSize < 0L)
-            throw new IllegalArgumentException("negative Stream size");
+            throw new IllegalArgumentException(BAD_SIZE);
         return StreamSupport.intStream
             (new RandomIntsSpliterator
              (this, 0L, streamSize, Integer.MAX_VALUE, 0),
@@ -607,7 +578,7 @@ public class SplittableRandom {
 
     /**
      * Returns an effectively unlimited stream of pseudorandom {@code int}
-     * values
+     * values from this generator and/or one split from it.
      *
      * @implNote This method is implemented to be equivalent to {@code
      * ints(Long.MAX_VALUE)}.
@@ -622,26 +593,26 @@ public class SplittableRandom {
     }
 
     /**
-     * Returns a stream with the given {@code streamSize} number of
-     * pseudorandom {@code int} values, each conforming to the given
-     * origin and bound.
+     * Returns a stream producing the given {@code streamSize} number
+     * of pseudorandom {@code int} values from this generator and/or one split
+     * from it; each value conforms to the given origin (inclusive) and bound
+     * (exclusive).
      *
      * @param streamSize the number of values to generate
-     * @param randomNumberOrigin the origin of each random value
-     * @param randomNumberBound the bound of each random value
+     * @param randomNumberOrigin the origin (inclusive) of each random value
+     * @param randomNumberBound the bound (exclusive) of each random value
      * @return a stream of pseudorandom {@code int} values,
-     * each with the given origin and bound.
+     *         each with the given origin (inclusive) and bound (exclusive)
      * @throws IllegalArgumentException if {@code streamSize} is
-     * less than zero.
-     * @throws IllegalArgumentException if {@code randomNumberOrigin}
+     *         less than zero, or {@code randomNumberOrigin}
      *         is greater than or equal to {@code randomNumberBound}
      */
     public IntStream ints(long streamSize, int randomNumberOrigin,
                           int randomNumberBound) {
         if (streamSize < 0L)
-            throw new IllegalArgumentException("negative Stream size");
+            throw new IllegalArgumentException(BAD_SIZE);
         if (randomNumberOrigin >= randomNumberBound)
-            throw new IllegalArgumentException("bound must be greater than origin");
+            throw new IllegalArgumentException(BAD_RANGE);
         return StreamSupport.intStream
             (new RandomIntsSpliterator
              (this, 0L, streamSize, randomNumberOrigin, randomNumberBound),
@@ -650,21 +621,22 @@ public class SplittableRandom {
 
     /**
      * Returns an effectively unlimited stream of pseudorandom {@code
-     * int} values, each conforming to the given origin and bound.
+     * int} values from this generator and/or one split from it; each value
+     * conforms to the given origin (inclusive) and bound (exclusive).
      *
      * @implNote This method is implemented to be equivalent to {@code
      * ints(Long.MAX_VALUE, randomNumberOrigin, randomNumberBound)}.
      *
-     * @param randomNumberOrigin the origin of each random value
-     * @param randomNumberBound the bound of each random value
+     * @param randomNumberOrigin the origin (inclusive) of each random value
+     * @param randomNumberBound the bound (exclusive) of each random value
      * @return a stream of pseudorandom {@code int} values,
-     * each with the given origin and bound.
+     *         each with the given origin (inclusive) and bound (exclusive)
      * @throws IllegalArgumentException if {@code randomNumberOrigin}
      *         is greater than or equal to {@code randomNumberBound}
      */
     public IntStream ints(int randomNumberOrigin, int randomNumberBound) {
         if (randomNumberOrigin >= randomNumberBound)
-            throw new IllegalArgumentException("bound must be greater than origin");
+            throw new IllegalArgumentException(BAD_RANGE);
         return StreamSupport.intStream
             (new RandomIntsSpliterator
              (this, 0L, Long.MAX_VALUE, randomNumberOrigin, randomNumberBound),
@@ -672,17 +644,18 @@ public class SplittableRandom {
     }
 
     /**
-     * Returns a stream with the given {@code streamSize} number of
-     * pseudorandom {@code long} values.
+     * Returns a stream producing the given {@code streamSize} number
+     * of pseudorandom {@code long} values from this generator and/or
+     * one split from it.
      *
      * @param streamSize the number of values to generate
-     * @return a stream of {@code long} values
+     * @return a stream of pseudorandom {@code long} values
      * @throws IllegalArgumentException if {@code streamSize} is
-     * less than zero
+     *         less than zero
      */
     public LongStream longs(long streamSize) {
         if (streamSize < 0L)
-            throw new IllegalArgumentException("negative Stream size");
+            throw new IllegalArgumentException(BAD_SIZE);
         return StreamSupport.longStream
             (new RandomLongsSpliterator
              (this, 0L, streamSize, Long.MAX_VALUE, 0L),
@@ -690,8 +663,8 @@ public class SplittableRandom {
     }
 
     /**
-     * Returns an effectively unlimited stream of pseudorandom {@code long}
-     * values.
+     * Returns an effectively unlimited stream of pseudorandom {@code
+     * long} values from this generator and/or one split from it.
      *
      * @implNote This method is implemented to be equivalent to {@code
      * longs(Long.MAX_VALUE)}.
@@ -706,26 +679,26 @@ public class SplittableRandom {
     }
 
     /**
-     * Returns a stream with the given {@code streamSize} number of
-     * pseudorandom {@code long} values, each conforming to the
-     * given origin and bound.
+     * Returns a stream producing the given {@code streamSize} number of
+     * pseudorandom {@code long} values from this generator and/or one split
+     * from it; each value conforms to the given origin (inclusive) and bound
+     * (exclusive).
      *
      * @param streamSize the number of values to generate
-     * @param randomNumberOrigin the origin of each random value
-     * @param randomNumberBound the bound of each random value
+     * @param randomNumberOrigin the origin (inclusive) of each random value
+     * @param randomNumberBound the bound (exclusive) of each random value
      * @return a stream of pseudorandom {@code long} values,
-     * each with the given origin and bound.
+     *         each with the given origin (inclusive) and bound (exclusive)
      * @throws IllegalArgumentException if {@code streamSize} is
-     * less than zero.
-     * @throws IllegalArgumentException if {@code randomNumberOrigin}
+     *         less than zero, or {@code randomNumberOrigin}
      *         is greater than or equal to {@code randomNumberBound}
      */
     public LongStream longs(long streamSize, long randomNumberOrigin,
                             long randomNumberBound) {
         if (streamSize < 0L)
-            throw new IllegalArgumentException("negative Stream size");
+            throw new IllegalArgumentException(BAD_SIZE);
         if (randomNumberOrigin >= randomNumberBound)
-            throw new IllegalArgumentException("bound must be greater than origin");
+            throw new IllegalArgumentException(BAD_RANGE);
         return StreamSupport.longStream
             (new RandomLongsSpliterator
              (this, 0L, streamSize, randomNumberOrigin, randomNumberBound),
@@ -734,21 +707,22 @@ public class SplittableRandom {
 
     /**
      * Returns an effectively unlimited stream of pseudorandom {@code
-     * long} values, each conforming to the given origin and bound.
+     * long} values from this generator and/or one split from it; each value
+     * conforms to the given origin (inclusive) and bound (exclusive).
      *
      * @implNote This method is implemented to be equivalent to {@code
      * longs(Long.MAX_VALUE, randomNumberOrigin, randomNumberBound)}.
      *
-     * @param randomNumberOrigin the origin of each random value
-     * @param randomNumberBound the bound of each random value
+     * @param randomNumberOrigin the origin (inclusive) of each random value
+     * @param randomNumberBound the bound (exclusive) of each random value
      * @return a stream of pseudorandom {@code long} values,
-     * each with the given origin and bound.
+     *         each with the given origin (inclusive) and bound (exclusive)
      * @throws IllegalArgumentException if {@code randomNumberOrigin}
      *         is greater than or equal to {@code randomNumberBound}
      */
     public LongStream longs(long randomNumberOrigin, long randomNumberBound) {
         if (randomNumberOrigin >= randomNumberBound)
-            throw new IllegalArgumentException("bound must be greater than origin");
+            throw new IllegalArgumentException(BAD_RANGE);
         return StreamSupport.longStream
             (new RandomLongsSpliterator
              (this, 0L, Long.MAX_VALUE, randomNumberOrigin, randomNumberBound),
@@ -756,18 +730,18 @@ public class SplittableRandom {
     }
 
     /**
-     * Returns a stream with the given {@code streamSize} number of
-     * pseudorandom {@code double} values, each between {@code 0.0}
-     * (inclusive) and {@code 1.0} (exclusive).
+     * Returns a stream producing the given {@code streamSize} number of
+     * pseudorandom {@code double} values from this generator and/or one split
+     * from it; each value is between zero (inclusive) and one (exclusive).
      *
      * @param streamSize the number of values to generate
      * @return a stream of {@code double} values
      * @throws IllegalArgumentException if {@code streamSize} is
-     * less than zero
+     *         less than zero
      */
     public DoubleStream doubles(long streamSize) {
         if (streamSize < 0L)
-            throw new IllegalArgumentException("negative Stream size");
+            throw new IllegalArgumentException(BAD_SIZE);
         return StreamSupport.doubleStream
             (new RandomDoublesSpliterator
              (this, 0L, streamSize, Double.MAX_VALUE, 0.0),
@@ -776,8 +750,8 @@ public class SplittableRandom {
 
     /**
      * Returns an effectively unlimited stream of pseudorandom {@code
-     * double} values, each between {@code 0.0} (inclusive) and {@code
-     * 1.0} (exclusive).
+     * double} values from this generator and/or one split from it; each value
+     * is between zero (inclusive) and one (exclusive).
      *
      * @implNote This method is implemented to be equivalent to {@code
      * doubles(Long.MAX_VALUE)}.
@@ -792,26 +766,27 @@ public class SplittableRandom {
     }
 
     /**
-     * Returns a stream with the given {@code streamSize} number of
-     * pseudorandom {@code double} values, each conforming to the
-     * given origin and bound.
+     * Returns a stream producing the given {@code streamSize} number of
+     * pseudorandom {@code double} values from this generator and/or one split
+     * from it; each value conforms to the given origin (inclusive) and bound
+     * (exclusive).
      *
      * @param streamSize the number of values to generate
-     * @param randomNumberOrigin the origin of each random value
-     * @param randomNumberBound the bound of each random value
+     * @param randomNumberOrigin the origin (inclusive) of each random value
+     * @param randomNumberBound the bound (exclusive) of each random value
      * @return a stream of pseudorandom {@code double} values,
-     * each with the given origin and bound.
+     *         each with the given origin (inclusive) and bound (exclusive)
      * @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}
      */
     public DoubleStream doubles(long streamSize, double randomNumberOrigin,
                                 double randomNumberBound) {
         if (streamSize < 0L)
-            throw new IllegalArgumentException("negative Stream size");
-        if (randomNumberOrigin >= randomNumberBound)
-            throw new IllegalArgumentException("bound must be greater than origin");
+            throw new IllegalArgumentException(BAD_SIZE);
+        if (!(randomNumberOrigin < randomNumberBound))
+            throw new IllegalArgumentException(BAD_RANGE);
         return StreamSupport.doubleStream
             (new RandomDoublesSpliterator
              (this, 0L, streamSize, randomNumberOrigin, randomNumberBound),
@@ -820,21 +795,22 @@ public class SplittableRandom {
 
     /**
      * Returns an effectively unlimited stream of pseudorandom {@code
-     * double} values, each conforming to the given origin and bound.
+     * double} values from this generator and/or one split from it; each value
+     * conforms to the given origin (inclusive) and bound (exclusive).
      *
      * @implNote This method is implemented to be equivalent to {@code
      * doubles(Long.MAX_VALUE, randomNumberOrigin, randomNumberBound)}.
      *
-     * @param randomNumberOrigin the origin of each random value
-     * @param randomNumberBound the bound of each random value
+     * @param randomNumberOrigin the origin (inclusive) of each random value
+     * @param randomNumberBound the bound (exclusive) of each random value
      * @return a stream of pseudorandom {@code double} values,
-     * each with the given origin and bound.
+     *         each with the given origin (inclusive) and bound (exclusive)
      * @throws IllegalArgumentException if {@code randomNumberOrigin}
      *         is greater than or equal to {@code randomNumberBound}
      */
     public DoubleStream doubles(double randomNumberOrigin, double randomNumberBound) {
-        if (randomNumberOrigin >= randomNumberBound)
-            throw new IllegalArgumentException("bound must be greater than origin");
+        if (!(randomNumberOrigin < randomNumberBound))
+            throw new IllegalArgumentException(BAD_RANGE);
         return StreamSupport.doubleStream
             (new RandomDoublesSpliterator
              (this, 0L, Long.MAX_VALUE, randomNumberOrigin, randomNumberBound),
@@ -843,13 +819,13 @@ public class SplittableRandom {
 
     /**
      * Spliterator for int streams.  We multiplex the four int
-     * versions into one class by treating and bound < origin as
+     * versions into one class by treating a bound less than origin as
      * unbounded, and also by treating "infinite" as equivalent to
      * Long.MAX_VALUE. For splits, it uses the standard divide-by-two
      * 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;
@@ -892,9 +868,10 @@ public class SplittableRandom {
             long i = index, f = fence;
             if (i < f) {
                 index = f;
+                SplittableRandom r = rng;
                 int o = origin, b = bound;
                 do {
-                    consumer.accept(rng.internalNextInt(o, b));
+                    consumer.accept(r.internalNextInt(o, b));
                 } while (++i < f);
             }
         }
@@ -903,7 +880,7 @@ 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;
@@ -946,9 +923,10 @@ public class SplittableRandom {
             long i = index, f = fence;
             if (i < f) {
                 index = f;
+                SplittableRandom r = rng;
                 long o = origin, b = bound;
                 do {
-                    consumer.accept(rng.internalNextLong(o, b));
+                    consumer.accept(r.internalNextLong(o, b));
                 } while (++i < f);
             }
         }
@@ -958,7 +936,7 @@ 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;
@@ -1001,13 +979,13 @@ public class SplittableRandom {
             long i = index, f = fence;
             if (i < f) {
                 index = f;
+                SplittableRandom r = rng;
                 double o = origin, b = bound;
                 do {
-                    consumer.accept(rng.internalNextDouble(o, b));
+                    consumer.accept(r.internalNextDouble(o, b));
                 } while (++i < f);
             }
         }
     }
 
 }
-