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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;

    }

}