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package java.util.random;

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 {@link JumpableRNG} objects produced by iterative jumping from a single original
 * {@link 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 {@link 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<RandomNumberGenerator>} rather than {@code Stream<JumpableRNG>}, even though the actual
 * generator objects in that stream likely do also implement the {@link JumpableRNG} interface.
 * <p>
 * An implementation of the {@link 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 {@link 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.
 *
 * @since 14
 */
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 {@link RandomNumberGenerator} interface.
     *
     * @return a stream of objects that implement the {@link RandomNumberGenerator} 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.
     */
    default Stream<RandomNumberGenerator> 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 {@link RandomNumberGenerator} interface.
     *
     * @param streamSize the number of generators to generate
     *
     * @return a stream of objects that implement the {@link RandomNumberGenerator} interface
     *
     * @throws IllegalArgumentException if {@code streamSize} is less than zero
     * @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.
     */
    default Stream<RandomNumberGenerator> jumps(long streamSize) {
        return jumps().limit(streamSize);
    }

    /**
     * Returns an effectively unlimited stream of new pseudorandom number generators, each of which
     * implements the {@link RandomNumberGenerator} interface.  Ideally the generators in the stream
     * will appear to be statistically independent.
     *
     * @return a stream of objects that implement the {@link RandomNumberGenerator} interface
     *
     * @implNote The default implementation calls {@code jumps()}.
     */
    default Stream<RandomNumberGenerator> rngs() {
        return this.jumps();
    }

    /**
     * Returns a stream producing the given {@code streamSize} number of new pseudorandom number
     * generators, each of which implements the {@link RandomNumberGenerator} interface.  Ideally
     * the generators in the stream will appear to be statistically independent.
     *
     * @param streamSize the number of generators to generate
     *
     * @return a stream of objects that implement the {@link RandomNumberGenerator} interface
     *
     * @throws IllegalArgumentException if {@code streamSize} is less than zero
     * @implNote The default implementation calls {@code jumps(streamSize)}.
     */
    default Stream<RandomNumberGenerator> 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 RandomNumberGenerator copyAndJump() {
        RandomNumberGenerator result = copy();
        jump();
        return result;
    }

}