--- a/jdk/src/share/classes/java/util/SplittableRandom.java Mon Oct 07 18:46:28 2013 -0700
+++ b/jdk/src/share/classes/java/util/SplittableRandom.java Tue Oct 08 11:17:15 2013 +0200
@@ -107,29 +107,25 @@
* 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 bit-mixing function 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." The mix32 function is
- * equivalent to (int)(mix64(seed) >>> 32), but faster because it
- * omits a step that doesn't contribute to result.
+ * 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 nextGamma) uses the "Mix13" constants
- * for MurmurHash3 described by David Stafford
- * (http://zimbry.blogspot.com/2011/09/better-bit-mixing-improving-on.html)
- * To avoid potential weaknesses in bit-mixing transformations, we
- * restrict gammas to odd values with at least 12 and no more than
- * 52 bits set. Rather than rejecting candidates with too few or
- * too many bits set, method nextGamma 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<sup>14</sup>, a very tiny percentage, and serves as an
+ * 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.
@@ -140,14 +136,15 @@
* avalanching.
*
* The default (no-argument) constructor, in essence, invokes
- * split() for a common "seeder" 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 seeder, we start
- * off using a seed based on current time and host 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.
+ * 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 and
+ * network interface address 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
@@ -160,17 +157,16 @@
*/
/**
- * The initial gamma value for (unsplit) SplittableRandoms. Must
- * be odd with at least 12 and no more than 52 bits set. Currently
- * set to the golden ratio scaled to 64bits.
+ * The golden ratio scaled to 64bits, used as the initial gamma
+ * value for (unsplit) SplittableRandoms.
*/
- private static final long INITIAL_GAMMA = 0x9e3779b97f4a7c15L;
+ private static final long GOLDEN_GAMMA = 0x9e3779b97f4a7c15L;
/**
* The least non-zero value returned by nextDouble(). This value
* is scaled by a random value of 53 bits to produce a result.
*/
- private static final double DOUBLE_UNIT = 1.0 / (1L << 53);
+ private static final double DOUBLE_UNIT = 0x1.0p-53; // 1.0 / (1L << 53);
/**
* The seed. Updated only via method nextSeed.
@@ -191,31 +187,31 @@
}
/**
- * Computes MurmurHash3 64bit mix function.
+ * Computes Stafford variant 13 of 64bit mix function.
*/
private static long mix64(long z) {
- z = (z ^ (z >>> 33)) * 0xff51afd7ed558ccdL;
- z = (z ^ (z >>> 33)) * 0xc4ceb9fe1a85ec53L;
- return z ^ (z >>> 33);
+ z = (z ^ (z >>> 30)) * 0xbf58476d1ce4e5b9L;
+ z = (z ^ (z >>> 27)) * 0x94d049bb133111ebL;
+ return z ^ (z >>> 31);
}
/**
- * Returns the 32 high bits of mix64(z) as int.
+ * Returns the 32 high bits of Stafford variant 4 mix64 function as int.
*/
private static int mix32(long z) {
- z = (z ^ (z >>> 33)) * 0xff51afd7ed558ccdL;
- return (int)(((z ^ (z >>> 33)) * 0xc4ceb9fe1a85ec53L) >>> 32);
+ z = (z ^ (z >>> 33)) * 0x62a9d9ed799705f5L;
+ return (int)(((z ^ (z >>> 28)) * 0xcb24d0a5c88c35b3L) >>> 32);
}
/**
* Returns the gamma value to use for a new split instance.
*/
- private static long nextGamma(long z) {
- z = (z ^ (z >>> 30)) * 0xbf58476d1ce4e5b9L; // Stafford "Mix13"
- z = (z ^ (z >>> 27)) * 0x94d049bb133111ebL;
- z = (z ^ (z >>> 31)) | 1L; // force to be odd
- int n = Long.bitCount(z); // ensure enough 0 and 1 bits
- return (n < 12 || n > 52) ? z ^ 0xaaaaaaaaaaaaaaaaL : 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;
}
/**
@@ -228,7 +224,7 @@
/**
* The seed generator for default constructors.
*/
- private static final AtomicLong seeder = new AtomicLong(initialSeed());
+ private static final AtomicLong defaultGen = new AtomicLong(initialSeed());
private static long initialSeed() {
String pp = java.security.AccessController.doPrivileged(
@@ -396,7 +392,7 @@
* @param seed the initial seed
*/
public SplittableRandom(long seed) {
- this(seed, INITIAL_GAMMA);
+ this(seed, GOLDEN_GAMMA);
}
/**
@@ -405,8 +401,10 @@
* of those of any other instances in the current program; and
* may, and typically does, vary across program invocations.
*/
- public SplittableRandom() { // emulate seeder.split()
- this.gamma = nextGamma(this.seed = seeder.addAndGet(INITIAL_GAMMA));
+ public SplittableRandom() { // emulate defaultGen.split()
+ long s = defaultGen.getAndAdd(2 * GOLDEN_GAMMA);
+ this.seed = mix64(s);
+ this.gamma = mixGamma(s + GOLDEN_GAMMA);
}
/**
@@ -424,8 +422,7 @@
* @return the new SplittableRandom instance
*/
public SplittableRandom split() {
- long s = nextSeed();
- return new SplittableRandom(s, nextGamma(s));
+ return new SplittableRandom(nextLong(), mixGamma(nextSeed()));
}
/**