/*
* 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/publicdomain/zero/1.0/
*/
package java.util.concurrent;
import java.util.Random;
/**
* A random number generator isolated to the current thread. Like the
* global {@link java.util.Random} generator used by the {@link
* java.lang.Math} class, a {@code ThreadLocalRandom} is initialized
* with an internally generated seed that may not otherwise be
* modified. When applicable, use of {@code ThreadLocalRandom} rather
* than shared {@code Random} objects in concurrent programs will
* typically encounter much less overhead and contention. Use of
* {@code ThreadLocalRandom} is 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(...)} (where
* {@code X} is {@code Int}, {@code Long}, etc).
* When all usages are of this form, it is never possible to
* accidently share a {@code ThreadLocalRandom} across multiple threads.
*
* <p>This class also provides additional commonly used bounded random
* generation methods.
*
* @since 1.7
* @author Doug Lea
*/
public class ThreadLocalRandom extends Random {
// same constants as Random, but must be redeclared because private
private static final long multiplier = 0x5DEECE66DL;
private static final long addend = 0xBL;
private static final long mask = (1L << 48) - 1;
/**
* The random seed. We can't use super.seed.
*/
private long rnd;
/**
* Initialization flag to permit calls to setSeed to succeed only
* while executing the Random constructor. 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.
*/
ThreadLocalRandom() {
super();
initialized = true;
}
/**
* Returns the current thread's {@code ThreadLocalRandom}.
*
* @return the current thread's {@code ThreadLocalRandom}
*/
public static ThreadLocalRandom current() {
return localRandom.get();
}
/**
* Throws {@code UnsupportedOperationException}. Setting seeds in
* this generator is not supported.
*
* @throws UnsupportedOperationException always
*/
public void setSeed(long seed) {
if (initialized)
throw new UnsupportedOperationException();
rnd = (seed ^ multiplier) & mask;
}
protected int next(int bits) {
rnd = (rnd * multiplier + addend) & mask;
return (int) (rnd >>> (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)
* @return the next value
* @throws IllegalArgumentException if least greater than or equal
* to bound
*/
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;
}
private static final long serialVersionUID = -5851777807851030925L;
}