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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 pseudorandom sequences of numbers (or Boolean values)

 * and furthermore can easily <i>jump</i> forward (by a fixed amount)

 * to a distant point in the state cycle.

 *

 * <p>Ideally, all {@code JumpableRng} objects produced by iterative

 * jumping from a single original {@code JumpableRng} 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 jumps}

 * method is used to produce a number of values no larger than either

 * 2<sup>64</sup> or the square root of its period.  Implementors are

 * advised to use algorithms whose period is at least 2<sup>127</sup>.

 *

 * <p>Methods are provided to perform a single jump operation and also

 * to produce a stream of generators produced from the original by

 * iterative copying and jumping of internal state.  A typical

 * strategy for a multithreaded application is to create a single

 * {@code JumpableRng} object, calls its {@code jumps} method exactly

 * once, and then parcel out generators from the resulting stream, one

 * to each thread.  It is generally not a good idea to call {@code jump}

 * on a generator that was itself produced by the {@code jumps} method,

 * because the result may be a generator identical to another

 * generator already produce by that call to the {@code jumps} method.

 * For this reason, the return type of the {@code jumps} method is

 * {@code Stream<Rng>} rather than {@code Stream<JumpableRng>}, even

 * though the actual generator objects in that stream likely do also

 * implement the {@code JumpableRng} interface.

 *

 * <p>An implementation of the {@code JumpableRng} interface must provide

 * concrete definitions for the methods {@code nextInt()}, {@code nextLong},

 * {@code period()}, {@code copy()}, {@code jump()}, and {@code defaultJumpDistance()}.

 * Default implementations are provided for all other methods.

 *

 * <p>Objects that implement {@code java.util.JumpableRng} 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 JumpableRng extends StreamableRng {

    /**

     * 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

     */

    JumpableRng copy();

    /**

     * Alter the state of this pseudorandom number generator so as to

     * jump forward a large, fixed distance (typically 2<sup>64</sup>

     * or more) within its state cycle.

     */

    void jump();

    /**

     * Returns the distance by which the {@code jump()} method will jump

     * forward within the state cycle of this generator object.

     *

     * @return the default jump distance (as a {@code double} value)

     */

    double defaultJumpDistance();

    /**

     * Returns an effectively unlimited stream of new pseudorandom

     * number generators, each of which implements the {@code Rng}

     * interface.

     *

     * @implNote It is permitted to implement this method in a manner

     * equivalent to {@code jumps(Long.MAX_VALUE)}.

     *

     * @implNote The default implementation produces a sequential stream

     * that  repeatedly calls {@code copy()} and {@code jump()} on this generator,

     * and the copies become the generators produced by the stream.

     *

     * @return a stream of objects that implement the {@code Rng} interface

     */

    default Stream<Rng> jumps() {

	return Stream.generate(this::copyAndJump).sequential();

    }

    /**

     * Returns a stream producing the given {@code streamSize} number of

     * new pseudorandom number generators, each of which implements the

     * {@code Rng} interface.

     *

     * @implNote The default implementation produces a sequential stream

     * that  repeatedly calls {@code copy()} and {@code jump()} 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 Rng} interface

     * @throws IllegalArgumentException if {@code streamSize} is

     *         less than zero

     */

    default Stream<Rng> jumps(long streamSize) {

        return jumps().limit(streamSize);

    }

    /**

     * Returns an effectively unlimited stream of new pseudorandom

     * number generators, each of which implements the {@code Rng}

     * interface.  Ideally the generators in the stream will appear

     * to be statistically independent.

     *

     * @implNote The default implementation calls {@code jumps()}.

     *

     * @return a stream of objects that implement the {@code Rng} interface

     */

    default Stream<Rng> rngs() {

	return this.jumps();

    }

    /**

     * Returns a stream producing the given {@code streamSize} number of

     * new pseudorandom number generators, each of which implements the

     * {@code Rng} interface.  Ideally the generators in the stream will

     * appear to be statistically independent.

     *

     * @implNote The default implementation calls {@code jumps(streamSize)}.

     *

     * @param streamSize the number of generators to generate

     * @return a stream of objects that implement the {@code Rng} interface

     * @throws IllegalArgumentException if {@code streamSize} is

     *         less than zero

     */

    default Stream<Rng> rngs(long streamSize) {

	return this.jumps(streamSize);

    }

    /**

     * Copy this generator, jump this generator forward, then return the copy.

     *

     * @return a copy of this generator object before the jump occurred

     */

    default Rng copyAndJump() {

	Rng result = copy();

	jump();

	return result;

    }

}