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/*
* Written by Doug Lea with assistance from members of JCP JSR-166
* Expert Group and released to the public domain, as explained at
* http://creativecommons.org/licenses/publicdomain
*/
package scala.concurrent.forkjoin;
import java.util.*;
/**
* A random number generator with the same properties as class {@link
* Random} but isolated to the current Thread. Like the global
* generator used by the {@link java.lang.Math} class, a
* ThreadLocalRandom is initialized with an internally generated seed
* that may not otherwise be modified. When applicable, use of
* ThreadLocalRandom rather than shared Random objects in concurrent
* programs will typically encounter much less overhead and
* contention. ThreadLocalRandoms are particularly appropriate when
* multiple tasks (for example, each a {@link ForkJoinTask}), use
* random numbers in parallel in thread pools.
*
* <p>Usages of this class should typically be of the form:
* <code>ThreadLocalRandom.current().nextX(...)</code> (where
* <code>X</code> is <code>Int</code>, <code>Long</code>, etc).
* When all usages are of this form, it is never possible to
* accidently share ThreadLocalRandoms across multiple threads.
*
* <p>This class also provides additional commonly used bounded random
* generation methods.
*/
public class ThreadLocalRandom extends Random {
// same constants as Random, but must be redeclared because private
private final static long multiplier = 0x5DEECE66DL;
private final static long addend = 0xBL;
private final static long mask = (1L << 48) - 1;
/**
* The random seed. We can't use super.seed
*/
private long rnd;
/**
* Initialization flag to permit the first and only allowed call
* to setSeed (inside Random constructor) to succeed. We can't
* allow others since it would cause setting seed in one part of a
* program to unintentionally impact other usages by the thread.
*/
boolean initialized;
// Padding to help avoid memory contention among seed updates in
// different TLRs in the common case that they are located near
// each other.
private long pad0, pad1, pad2, pad3, pad4, pad5, pad6, pad7;
/**
* The actual ThreadLocal
*/
private static final ThreadLocal<ThreadLocalRandom> localRandom =
new ThreadLocal<ThreadLocalRandom>() {
protected ThreadLocalRandom initialValue() {
return new ThreadLocalRandom();
}
};
/**
* Constructor called only by localRandom.initialValue.
* We rely on the fact that the superclass no-arg constructor
* invokes setSeed exactly once to initialize.
*/
ThreadLocalRandom() {
super();
}
/**
* Returns the current Thread's ThreadLocalRandom
* @return the current Thread's ThreadLocalRandom
*/
public static ThreadLocalRandom current() {
return localRandom.get();
}
/**
* Throws UnsupportedOperationException. Setting seeds in this
* generator is unsupported.
* @throws UnsupportedOperationException always
*/
public void setSeed(long seed) {
if (initialized)
throw new UnsupportedOperationException();
initialized = true;
rnd = (seed ^ multiplier) & mask;
}
protected int next(int bits) {
return (int)((rnd = (rnd * multiplier + addend) & mask) >>> (48-bits));
}
/**
* Returns a pseudorandom, uniformly distributed value between the
* given least value (inclusive) and bound (exclusive).
* @param least the least value returned
* @param bound the upper bound (exclusive)
* @throws IllegalArgumentException if least greater than or equal
* to bound
* @return the next value
*/
public int nextInt(int least, int bound) {
if (least >= bound)
throw new IllegalArgumentException();
return nextInt(bound - least) + least;
}
/**
* Returns a pseudorandom, uniformly distributed value
* between 0 (inclusive) and the specified value (exclusive)
* @param n the bound on the random number to be returned. Must be
* positive.
* @return the next value
* @throws IllegalArgumentException if n is not positive
*/
public long nextLong(long n) {
if (n <= 0)
throw new IllegalArgumentException("n must be positive");
// Divide n by two until small enough for nextInt. On each
// iteration (at most 31 of them but usually much less),
// randomly choose both whether to include high bit in result
// (offset) and whether to continue with the lower vs upper
// half (which makes a difference only if odd).
long offset = 0;
while (n >= Integer.MAX_VALUE) {
int bits = next(2);
long half = n >>> 1;
long nextn = ((bits & 2) == 0)? half : n - half;
if ((bits & 1) == 0)
offset += n - nextn;
n = nextn;
}
return offset + nextInt((int)n);
}
/**
* Returns a pseudorandom, uniformly distributed value between the
* given least value (inclusive) and bound (exclusive).
* @param least the least value returned
* @param bound the upper bound (exclusive)
* @return the next value
* @throws IllegalArgumentException if least greater than or equal
* to bound
*/
public long nextLong(long least, long bound) {
if (least >= bound)
throw new IllegalArgumentException();
return nextLong(bound - least) + least;
}
/**
* Returns a pseudorandom, uniformly distributed {@code double} value
* between 0 (inclusive) and the specified value (exclusive)
* @param n the bound on the random number to be returned. Must be
* positive.
* @return the next value
* @throws IllegalArgumentException if n is not positive
*/
public double nextDouble(double n) {
if (n <= 0)
throw new IllegalArgumentException("n must be positive");
return nextDouble() * n;
}
/**
* Returns a pseudorandom, uniformly distributed value between the
* given least value (inclusive) and bound (exclusive).
* @param least the least value returned
* @param bound the upper bound (exclusive)
* @return the next value
* @throws IllegalArgumentException if least greater than or equal
* to bound
*/
public double nextDouble(double least, double bound) {
if (least >= bound)
throw new IllegalArgumentException();
return nextDouble() * (bound - least) + least;
}
}
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