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package java.util;

import java.util.stream.Stream;

import java.util.stream.StreamSupport;

/**

 * This interface is designed to provide a common protocol for objects

 * that generate sequences of pseudorandom numbers (or Boolean values)

 * and furthermore can easily <i>jump</i> to an arbitrarily specified

 * distant point in the state cycle.

 *

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

 * iterative jumping from a single original {@code ArbtrarilyJumpableRng}

 * object are statistically independent of one another and

 * individually uniform, provided that they do not traverse

 * overlapping portions of the state cycle.  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 the jump distance

 * specified.  Implementors are advised to use algorithms whose period

 * is at least 2<sup>127</sup>.

 *

 * <p>For many applications, it suffices to jump forward by a power of

 * two or some small multiple of a power of two, but this power of two

 * may not be representable as a {@code long} value.  To avoid the

 * use of {@code BigInteger} values as jump distances, {@code double}

 * values are used instead.

 *

 * <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 ArbitrarilyJumpableRng} object, call its {@code jumps}

 * method exactly once, and then parcel out generators from the

 * resulting stream, one to each thread.  However, each generator

 * produced also has type {@code ArbitrarilyJumpableRng}; with care,

 * different jump distances can be used to traverse the entire

 * state cycle in various ways.

 *

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

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

 * {@code nextLong}, {@code period()}, {@code copy()}, {@code jump(double)},

 * {@code defaultJumpDistance()}, and {@code defaultLeapDistance()}.

 * Default implementations are provided for all other methods.

 * Perhaps the most convenient

 * way to implement this interface is to extend the abstract class

 * {@link java.util.AbstractArbitrarilyJumpableRng}, which provides

 * spliterator-based implementations of the methods {@code ints}, {@code longs},

 * {@code doubles}, {@code rngs}, {@code jumps}, and {@code leaps}.

 *

 * <p>Objects that implement {@code java.util.ArbitrarilyJumpableRng}

 * 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 ArbitrarilyJumpableRng extends LeapableRng {

    /**

     * 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

     */

    ArbitrarilyJumpableRng copy();

    /**

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

     * jump forward a distance equal to 2<sup>{@code logDistance}</sup>

     * within its state cycle.

     *

     * @param logDistance the base-2 logarithm of the distance to jump

     *        forward within the state cycle

     * @throws IllegalArgumentException if {@code logDistance} is NaN

     *         or negative, or if 2<sup>{@code logDistance}</sup> is

     *         greater than the period of this generator

     */

    void jumpPowerOfTwo(int logDistance);

    /**

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

     * jump forward a specified distance within its state cycle.

     *

     * @param distance the distance to jump forward within the state cycle

     * @throws IllegalArgumentException if {@code distance} is Nan,

     *         negative, or greater than the period of this generator

     */

    void jump(double distance);

    /**

     * 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.  The distance used is that

     * returned by method {@code defaultJumpDistance()}.

     */

    default void jump() { jump(defaultJumpDistance()); }

    /**

     * Returns an effectively unlimited stream of new pseudorandom

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

     * interface, produced by jumping copies of this generator

     * by different integer multiples of the specified jump distance.

     *

     * @implNote This method is implemented to be equivalent to

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

     *

     * @param distance a distance to jump forward within the state cycle

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

     */

    default Stream<ArbitrarilyJumpableRng> jumps(double distance) {

	return Stream.generate(() -> copyAndJump(distance)).sequential();

    }

    /**

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

     * new pseudorandom number generators, each of which implements the

     * {@code ArbitrarilyJumpableRng} interface, produced by jumping copies of this generator

     * by different integer multiples of the specified jump distance.

     *

     * @param streamSize the number of generators to generate

     * @param distance a distance to jump forward within the state cycle

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

     * @throws IllegalArgumentException if {@code streamSize} is

     *         less than zero

     */

    default Stream<ArbitrarilyJumpableRng> jumps(long streamSize, double distance) {

        return jumps(distance).limit(streamSize);

    }

    /**

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

     * jump forward a very large, fixed distance (typically 2<sup>128</sup>

     * or more) within its state cycle.  The distance used is that

     * returned by method {@code defaultJLeapDistance()}.

     */

    default void leap() { jump(defaultLeapDistance()); }

    /**

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

     *

     * @param distance a distance to jump forward within the state cycle

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

     */

    default ArbitrarilyJumpableRng copyAndJump(double distance) {

	ArbitrarilyJumpableRng result = copy();

	jump(distance);

	return result;

    }

}