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package java.util;
import java.math.BigInteger;
import java.util.stream.Stream;
/**
* This interface is designed to provide a common protocol for objects
* that generate sequences of pseudorandom numbers (or Boolean values)
* and furthermore can easily not only jump but also <i>leap</i> to
* a very distant point in the state cycle.
*
* Typically one will construct a series of {@code LeapableRng} objects
* by iterative leaping from a single original {@code LeapableRng}
* object, and then for each such object produce a subseries of objects
* by iterative jumping. There is little conceptual difference between
* leaping and jumping, but typically a leap will be a very long jump
* in the state cycle (perhaps distance 2<sup>128</sup> or so).
*
* <p>Ideally, all {@code LeapableRng} objects produced by iterative
* leaping and jumping from a single original {@code LeapableRng} object
* are statistically independent of one another and individually uniform.
* In practice, one must settle for some approximation to independence
* and uniformity. In particular, a specific implementation may
* assume that each generator in a stream produced by the {@code leaps}
* method is used to produce (by jumping) a number of objects no larger
* than 2<sup>64</sup>. Implementors are advised to use algorithms
* whose period is at least 2<sup>191</sup>.
*
* <p>Methods are provided to perform a single leap operation and also
* to produce a stream of generators produced from the original by
* iterative copying and leaping of internal state. The generators
* produced must implement the {@code JumpableRng} interface but need
* not also implement the {@code LeapableRng} interface. A typical
* strategy for a multithreaded application is to create a single
* {@code LeapableRng} object, calls its {@code leaps} method exactly
* once, and then parcel out generators from the resulting stream, one
* to each thread. Then the {@code jumps} method of each such generator
* be called to produce a substream of generator objects.
*
* <p>An implementation of the {@code LeapableRng} interface must provide
* concrete definitions for the methods {@code nextInt()}, {@code nextLong},
* {@code period()}, {@code copy()}, {@code jump()}, {@code defaultJumpDistance()},
* {@code leap()}, and {@code defaultLeapDistance()}.
* Default implementations are provided for all other methods.
*
* <p>Objects that implement {@code java.util.LeapableRng} are
* typically not cryptographically secure. Consider instead using
* {@link java.security.SecureRandom} to get a cryptographically
* secure pseudo-random number generator for use by
* security-sensitive applications.
*
* @author Guy Steele
* @since 1.9
*/
public interface LeapableRng extends JumpableRng {
/**
* Returns a new generator whose internal state is an exact copy
* of this generator (therefore their future behavior should be
* identical if subjected to the same series of operations).
*
* @return a new object that is a copy of this generator
*/
LeapableRng copy();
/**
* Alter the state of this pseudorandom number generator so as to
* leap forward a large, fixed distance (typically 2<sup>96</sup>
* or more) within its state cycle.
*/
void leap();
/**
* Returns the distance by which the {@code leap()} method will leap
* forward within the state cycle of this generator object.
*
* @return the default leap distance (as a {@code double} value)
*/
double defaultLeapDistance();
/**
* Returns an effectively unlimited stream of new pseudorandom
* number generators, each of which implements the {@code JumpableRng}
* interface.
*
* @implNote It is permitted to implement this method in a manner
* equivalent to {@code leaps(Long.MAX_VALUE)}.
*
* @implNote The default implementation produces a sequential stream
* that repeatedly calls {@code copy()} and {@code leap()} on this generator,
* and the copies become the generators produced by the stream.
*
* @return a stream of objects that implement the {@code JumpableRng} interface
*/
default Stream<JumpableRng> leaps() {
return Stream.generate(this::copyAndLeap).sequential();
}
/**
* Returns a stream producing the given {@code streamSize} number of
* new pseudorandom number generators, each of which implements the
* {@code JumpableRng} interface.
*
* @implNote The default implementation produces a sequential stream
* that repeatedly calls {@code copy()} and {@code leap()} on this generator,
* and the copies become the generators produced by the stream.
*
* @param streamSize the number of generators to generate
* @return a stream of objects that implement the {@code JumpableRng} interface
* @throws IllegalArgumentException if {@code streamSize} is
* less than zero
*/
default Stream<JumpableRng> leaps(long streamSize) {
return leaps().limit(streamSize);
}
/**
* Copy this generator, leap this generator forward, then return the copy.
*
* @return a copy of this generator object before the leap occurred
*/
default JumpableRng copyAndLeap() {
JumpableRng result = copy();
leap();
return result;
}
}