--- a/src/java.base/share/classes/java/util/random/L64X256MixRandom.java Thu Aug 29 11:33:26 2019 -0300
+++ b/src/java.base/share/classes/java/util/random/L64X256MixRandom.java Thu Nov 14 08:54:56 2019 -0400
@@ -28,7 +28,7 @@
import java.math.BigInteger;
import java.util.concurrent.atomic.AtomicLong;
import java.util.random.RandomGenerator.SplittableGenerator;
-import java.util.random.RandomSupport.AbstractSplittableGenerator;
+import java.util.random.RandomSupport.AbstractSplittableWithBrineGenerator;
/**
* A generator of uniform pseudorandom values applicable for use in
@@ -54,9 +54,10 @@
* {@link L64X256MixRandom} is a specific member of the LXM family of algorithms
* for pseudorandom number generators. Every LXM generator consists of two
* subgenerators; one is an LCG (Linear Congruential Generator) and the other is
- * an Xorshift generator. Each output of an LXM generator is the sum of one
- * output from each subgenerator, possibly processed by a final mixing function
- * (and {@link L64X256MixRandom} does use a mixing function).
+ * an Xorshift generator. Each output of an LXM generator is the result of
+ * combining state from the LCG with state from the Xorshift generator by
+ * using a Mixing function (and then the state of the LCG and the state of the
+ * Xorshift generator are advanced).
* <p>
* The LCG subgenerator for {@link L64X256MixRandom} has an update step of the
* form {@code s = m * s + a}, where {@code s}, {@code m}, and {@code a} are all
@@ -73,7 +74,8 @@
* and {@code x3}, which can take on any values provided that they are not all zero.
* The period of this subgenerator is 2<sup>256</sup>-1.
* <p>
- * The mixing function for {@link L64X256MixRandom} is the 64-bit MurmurHash3 finalizer.
+ * The mixing function for {@link L64X256MixRandom} is {@link RandomSupport.mixLea64}
+ * applied to the argument {@code (s + x0)}.
* <p>
* Because the periods 2<sup>64</sup> and 2<sup>256</sup>-1 of the two subgenerators
* are relatively prime, the <em>period</em> of any single {@link L64X256MixRandom} object
@@ -98,8 +100,8 @@
* 2<sup>256</sup> subsequence values, nearly all of them (2<sup>256</sup>-2<sup>64</sup>)
* occur 2<sup>64</sup> times over the course of the entire cycle, and the other
* 2<sup>64</sup> subsequence values occur only 2<sup>64</sup>-1 times. So the ratio
- * of the probability of getting one of the less common subsequence values and the
- * probability of getting one of the more common subsequence values is 1-2<sup>-64</sup>.
+ * of the probability of getting any specific one of the less common subsequence values and the
+ * probability of getting any specific one of the more common subsequence values is 1-2<sup>-64</sup>.
* (Note that the set of 2<sup>64</sup> less-common subsequence values will differ from
* one instance of {@link L64X256MixRandom} to another, as a function of the additive
* parameter of the LCG.) The values produced by the {@code nextInt()}, {@code nextFloat()},
@@ -136,7 +138,7 @@
*
* @since 14
*/
-public final class L64X256MixRandom extends AbstractSplittableGenerator {
+public final class L64X256MixRandom extends AbstractSplittableWithBrineGenerator {
/*
* Implementation Overview.
@@ -180,14 +182,13 @@
BigInteger.ONE.shiftLeft(256).subtract(BigInteger.ONE).shiftLeft(64);
/*
- * Multiplier used in the LCG portion of the algorithm, taken from
- * Pierre L'Ecuyer, Tables of linear congruential generators of
- * different sizes and good lattice structure, <em>Mathematics of
- * Computation</em> 68, 225 (January 1999), pages 249-260,
- * Table 4 (first multiplier for size 2<sup>64</sup>).
+ * Multiplier used in the LCG portion of the algorithm.
+ * Chosen based on research by Sebastiano Vigna and Guy Steele (2019).
+ * The spectral scores for dimensions 2 through 8 for the multiplier 0xd1342543de82ef95
+ * are [0.958602, 0.937479, 0.870757, 0.822326, 0.820405, 0.813065, 0.760215].
*/
- private static final long M = 2862933555777941757L;
+ private static final long M = 0xd1342543de82ef95L;
/* ---------------- instance fields ---------------- */
@@ -227,11 +228,12 @@
this.x3 = x3;
// If x0, x1, x2, and x3 are all zero, we must choose nonzero values.
if ((x0 | x1 | x2 | x3) == 0) {
+ long v = s;
// At least three of the four values generated here will be nonzero.
- this.x0 = RandomSupport.mixStafford13(s += RandomSupport.GOLDEN_RATIO_64);
- this.x1 = RandomSupport.mixStafford13(s += RandomSupport.GOLDEN_RATIO_64);
- this.x2 = RandomSupport.mixStafford13(s += RandomSupport.GOLDEN_RATIO_64);
- this.x3 = RandomSupport.mixStafford13(s + RandomSupport.GOLDEN_RATIO_64);
+ this.x0 = RandomSupport.mixStafford13(v += RandomSupport.GOLDEN_RATIO_64);
+ this.x1 = RandomSupport.mixStafford13(v += RandomSupport.GOLDEN_RATIO_64);
+ this.x2 = RandomSupport.mixStafford13(v += RandomSupport.GOLDEN_RATIO_64);
+ this.x3 = RandomSupport.mixStafford13(v + RandomSupport.GOLDEN_RATIO_64);
}
}
@@ -252,7 +254,7 @@
// The seed is hashed by mixStafford13 to produce the initial `x0`,
// which will then be used to produce the first generated value.
// The other x values are filled in as if by a SplitMix PRNG with
- // GOLDEN_RATIO_64 as the gamma value and Stafford13 as the mixer.
+ // GOLDEN_RATIO_64 as the gamma value and mixStafford13 as the mixer.
this(RandomSupport.mixMurmur64(seed ^= RandomSupport.SILVER_RATIO_64),
1,
RandomSupport.mixStafford13(seed),
@@ -295,27 +297,28 @@
/* ---------------- public methods ---------------- */
/**
- * Constructs and returns a new instance of {@link L64X256MixRandom}
- * that shares no mutable state with this instance.
+ * Given 63 bits of "brine", constructs and returns a new instance of
+ * {@code L64X256MixRandom} that shares no mutable state with this instance.
* However, with very high probability, the set of values collectively
* generated by the two objects has the same statistical properties as if
* same the quantity of values were generated by a single thread using
- * a single {@link L64X256MixRandom} object. Either or both of the two
+ * a single {@code L64X256MixRandom} object. Either or both of the two
* objects may be further split using the {@code split} method,
* and the same expected statistical properties apply to the
* entire set of generators constructed by such recursive splitting.
*
- * @param source a {@link SplittableGenerator} instance to be used instead
+ * @param source a {@code SplittableGenerator} instance to be used instead
* of this one as a source of pseudorandom bits used to
* initialize the state of the new ones.
- *
- * @return a new instance of {@link L64X256MixRandom}
+ * @param brine a long value, of which the low 63 bits are used to choose
+ * the {@code a} parameter for the new instance.
+ * @return a new instance of {@code L64X256MixRandom}
*/
- public L64X256MixRandom split(SplittableGenerator source) {
- // Literally pick a new instance "at random".
- return new L64X256MixRandom(source.nextLong(), source.nextLong(),
- source.nextLong(), source.nextLong(),
- source.nextLong(), source.nextLong());
+ public SplittableGenerator split(SplittableGenerator source, long brine) {
+ // Pick a new instance "at random", but use the brine for `a`.
+ return new L64X256MixRandom(brine << 1, source.nextLong(),
+ source.nextLong(), source.nextLong(),
+ source.nextLong(), source.nextLong());
}
/**
@@ -324,8 +327,12 @@
* @return a pseudorandom {@code long} value
*/
public long nextLong() {
- final long z = s + x0;
- s = M * s + a; // LCG
+ // Compute the result based on current state information
+ // (this allows the computation to be overlapped with state update).
+ final long result = RandomSupport.mixLea64(s + x0);
+ // Update the LCG subgenerator
+ s = M * s + a;
+ // Update the Xorshift subgenerator
long q0 = x0, q1 = x1, q2 = x2, q3 = x3;
{ // xoshiro256 1.0
long t = q1 << 17;
@@ -337,9 +344,15 @@
q3 = Long.rotateLeft(q3, 45);
}
x0 = q0; x1 = q1; x2 = q2; x3 = q3;
- return RandomSupport.mixLea64(z); // mixing function
+ return result;
}
+ /**
+ * Returns the period of this random generator.
+ *
+ * @return a {@link BigInteger} whose value is the number of distinct possible states of this
+ * {@link RandomGenerator} object (2<sup>64</sup>(2<sup>256</sup>-1)).
+ */
public BigInteger period() {
return PERIOD;
}