/*
 * Java Genetic Algorithm Library (jenetics-8.0.0).
 * Copyright (c) 2007-2024 Franz Wilhelmstötter
 *
 * Licensed under the Apache License, Version 2.0 (the "License");
 * you may not use this file except in compliance with the License.
 * You may obtain a copy of the License at
 *
 *      http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 *
 * Author:
 *    Franz Wilhelmstötter (franz.wilhelmstoetter@gmail.com)
 */
package io.jenetics.util;

import static java.time.Clock.systemUTC;
import static java.util.Objects.requireNonNull;

import java.time.Clock;
import java.time.Duration;
import java.util.Comparator;
import java.util.function.BinaryOperator;
import java.util.function.Function;
import java.util.stream.Stream;

/**
 * This class contains factory methods for (flat) mapping stream elements. The
 * functions of this class can be used in the following way.
 *
 * {@snippet lang="java":
 * final ISeq<Integer> values = new Random().ints(0, 100).boxed()
 *     .limit(100)
 *     .flatMap(Streams.toIntervalMax(13))
 *     .collect(ISeq.toISeq());
 * }
 *
 * @author <a href="mailto:franz.wilhelmstoetter@gmail.com">Franz Wilhelmstötter</a>
 * @since 6.0
 * @version 6.0
 */
public final class Streams {
        private Streams() {}


        /**
         * Return a new flat-mapper function, which guarantees a strictly increasing
         * stream, from an arbitrarily ordered source stream. Note that this
         * function doesn't sort the stream. It <em>just</em> skips the <em>out of
         * order</em> elements.
         *
         * <pre>{@code
         *     +----3--2--5--4--7--7--4--9----|
         *        toStrictlyIncreasing()
         *     +----3-----5-----7--------9----|
         * }</pre>
         *
         * {@snippet lang="java":
         * final ISeq<Integer> values = new Random().ints(0, 100)
         *     .boxed()
         *     .limit(100)
         *     .flatMap(Streams.toStrictlyIncreasing())
         *     .collect(ISeq.toISeq());
         *
         * System.out.println(values);
         * // [6,47,65,78,96,96,99]
         * }
         *
         *
         * @param <C> the comparable type
         * @return a new flat-mapper function
         */
        public static <C extends Comparable<? super C>>
        Function<C, Stream<C>> toStrictlyIncreasing() {
                return strictlyImproving(Streams::max);
        }

        /**
         * Return a new flat-mapper function, which guarantees a strictly decreasing
         * stream, from an arbitrarily ordered source stream. Note that this
         * function doesn't sort the stream. It <em>just</em> skips the <em>out of
         * order</em> elements.
         *
         * <pre>{@code
         *     +----9--8--9--5--6--6--2--9----|
         *        toStrictlyDecreasing()
         *     +----9--8-----5--------2-------|
         * }</pre>
         *
         * {@snippet lang="java":
         * final ISeq<Integer> values = new Random().ints(0, 100)
         *     .boxed()
         *     .limit(100)
         *     .flatMap(Streams.toStrictlyDecreasing())
         *     .collect(ISeq.toISeq());
         *
         * System.out.println(values);
         * // [45,32,15,12,3,1]
         * }
         *
         * @param <C> the comparable type
         * @return a new flat-mapper function
         */
        public static <C extends Comparable<? super C>>
        Function<C, Stream<C>> toStrictlyDecreasing() {
                return strictlyImproving(Streams::min);
        }

        /**
         * Return a new flat-mapper function, which guarantees a strictly improving
         * stream, from an arbitrarily ordered source stream. Note that this
         * function doesn't sort the stream. It <em>just</em> skips the <em>out of
         * order</em> elements.
         *
         * {@snippet lang="java":
         * final ISeq<Integer> values = new Random().ints(0, 100)
         *     .boxed()
         *     .limit(100)
         *     .flatMap(Streams.toStrictlyImproving(Comparator.naturalOrder()))
         *     .collect(ISeq.toISeq());
         *
         * System.out.println(values);
         * // [6,47,65,78,96,96,99]
         * }
         *
         * @see #toStrictlyIncreasing()
         * @see #toStrictlyDecreasing()
         *
         * @param <T> the element type
         * @param comparator the comparator used for testing the elements
         * @return a new flat-mapper function
         */
        public static <T> Function<T, Stream<T>>
        toStrictlyImproving(final Comparator<? super T> comparator) {
                return strictlyImproving((a, b) -> best(comparator, a, b));
        }

        private static <C> Function<C, Stream<C>>
        strictlyImproving(final BinaryOperator<C> comparator) {
                requireNonNull(comparator);

                return new Function<>() {
                        private C _best;

                        @Override
                        public Stream<C> apply(final C result) {
                                final C best = comparator.apply(_best, result);

                                final Stream<C> stream = best == _best
                                        ? Stream.empty()
                                        : Stream.of(best);

                                _best = best;

                                return stream;
                        }
                };
        }

        private static <T extends Comparable<? super T>> T max(final T a, final T b) {
                return best(Comparator.naturalOrder(), a, b);
        }

        private static <T extends Comparable<? super T>> T min(final T a, final T b) {
                return best(Comparator.reverseOrder(), a, b);
        }

        private static <T>
        T best(final Comparator<? super T> comparator, final T a, final T b) {
                if (a == null && b == null) return null;
                if (a == null) return b;
                if (b == null) return a;
                return comparator.compare(a, b) >= 0 ? a : b;
        }

        /**
         * Return a new flat-mapper function which returns (emits) the maximal value
         * of the last <em>n</em> elements.
         *
         * <pre>{@code
         *          +----3---+----3---+
         *          |        |        |
         *     +----9--8--3--3--5--4--2--9----|
         *        toIntervalMax(3)
         *     +----------9--------5----------|
         * }</pre>
         *
         * @param size the size of the slice
         * @param <C> the element type
         * @return a new flat-mapper function
         * @throws IllegalArgumentException if the given size is smaller than one
         */
        public static <C extends Comparable<? super C>>
        Function<C, Stream<C>> toIntervalMax(final int size) {
                return sliceBest(Streams::max, size);
        }

        /**
         * Return a new flat-mapper function which returns (emits) the minimal value
         * of the last <em>n</em> elements.
         *
         * <pre>{@code
         *          +----3---+----3---+
         *          |        |        |
         *     +----9--8--3--3--1--4--2--9----|
         *        toIntervalMin(3)
         *     +----------3--------1----------|
         * }</pre>
         *
         * @param size the size of the slice
         * @param <C> the element type
         * @return a new flat-mapper function
         * @throws IllegalArgumentException if the given size is smaller than one
         */
        public static <C extends Comparable<? super C>>
        Function<C, Stream<C>> toIntervalMin(final int size) {
                return sliceBest(Streams::min, size);
        }

        /**
         * Return a new flat-mapper function which returns (emits) the minimal value
         * of the last <em>n</em> elements.
         *
         * @see #toIntervalMax(int)
         * @see #toIntervalMin(int)
         *
         * @param <C> the element type
         * @param size the size of the slice
         * @param comparator the comparator used for testing the elements
         * @return a new flat-mapper function
         * @throws IllegalArgumentException if the given size is smaller than one
         * @throws NullPointerException if the given {@code comparator} is
         *         {@code null}
         */
        public static <C> Function<C, Stream<C>>
        toIntervalBest(final Comparator<? super C> comparator, final int size) {
                requireNonNull(comparator);
                return sliceBest((a, b) -> best(comparator, a, b), size);
        }

        private static <C> Function<C, Stream<C>> sliceBest(
                final BinaryOperator<C> comp,
                final int rangeSize
        ) {
                requireNonNull(comp);
                if (rangeSize < 1) {
                        throw new IllegalArgumentException(
                                "Range size must be at least one: " + rangeSize
                        );
                }

                return new Function<>() {
                        private int _count = 0;
                        private C _best;

                        @Override
                        public Stream<C> apply(final C value) {
                                ++_count;
                                _best = comp.apply(_best, value);

                                final Stream<C> result;
                                if (_count >= rangeSize) {
                                        result = Stream.of(_best);
                                        _count = 0;
                                        _best = null;
                                } else {
                                        result = Stream.empty();
                                }

                                return result;
                        }
                };
        }

        /**
         * Return a new flat-mapper function which returns (emits) the maximal value
         * of the elements emitted within the given {@code timespan}.
         *
         * <pre>{@code
         *          +---3s---+---3s---+
         *          |        |        |
         *     +----9--8--3--3--5--4--2--9----|
         *        toIntervalMax(3s)
         *     +----------9--------5----------|
         * }</pre>
         *
         * @see #toIntervalMax(Duration, Clock)
         *
         * @param <C> the element type
         * @param timespan the timespan the elements are collected for the
         *        calculation slice
         * @return a new flat-mapper function
         * @throws IllegalArgumentException if the given size is smaller than one
         * @throws NullPointerException if the given {@code timespan} is {@code null}
         */
        public static <C extends Comparable<? super C>>
        Function<C, Stream<C>> toIntervalMax(final Duration timespan) {
                return sliceBest(Streams::max, timespan, systemUTC());
        }

        /**
         * Return a new flat-mapper function which returns (emits) the maximal value
         * of the elements emitted within the given {@code timespan}.
         *
         * <pre>{@code
         *          +---3s---+---3s---+
         *          |        |        |
         *     +----9--8--3--3--5--4--2--9----|
         *        toIntervalMax(3s)
         *     +----------9--------5----------|
         * }</pre>
         *
         * @see #toIntervalMax(Duration)
         *
         * @param <C> the element type
         * @param timespan the timespan the elements are collected for the
         *        calculation slice
         * @param clock the {@code clock} used for measuring the {@code timespan}
         * @return a new flat-mapper function
         * @throws IllegalArgumentException if the given size is smaller than one
         * @throws NullPointerException if one of the arguments is {@code null}
         */
        public static <C extends Comparable<? super C>>
        Function<C, Stream<C>> toIntervalMax(final Duration timespan, final Clock clock) {
                return sliceBest(Streams::max, timespan, clock);
        }

        /**
         * Return a new flat-mapper function which returns (emits) the minimal value
         * of the elements emitted within the given {@code timespan}.
         *
         * <pre>{@code
         *          +---3s---+---3s---+
         *          |        |        |
         *     +----9--8--3--3--1--4--2--9----|
         *        toIntervalMin(3s)
         *     +----------3--------1----------|
         * }</pre>
         *
         * @see #toIntervalMin(Duration, Clock)
         *
         * @param <C> the element type
         * @param timespan the timespan the elements are collected for the
         *        calculation slice
         * @return a new flat-mapper function
         * @throws IllegalArgumentException if the given size is smaller than one
         * @throws NullPointerException if the given {@code timespan} is {@code null}
         */
        public static <C extends Comparable<? super C>>
        Function<C, Stream<C>> toIntervalMin(final Duration timespan) {
                return sliceBest(Streams::min, timespan, systemUTC());
        }

        /**
         * Return a new flat-mapper function which returns (emits) the minimal value
         * of the elements emitted within the given {@code timespan}.
         *
         * <pre>{@code
         *          +---3s---+---3s---+
         *          |        |        |
         *     +----9--8--3--3--1--4--2--9----|
         *        toIntervalMin(3s)
         *     +----------3--------1----------|
         * }</pre>
         *
         * @see #toIntervalMin(Duration)
         *
         * @param <C> the element type
         * @param timespan the timespan the elements are collected for the
         *        calculation slice
         * @param clock the {@code clock} used for measuring the {@code timespan}
         * @return a new flat-mapper function
         * @throws IllegalArgumentException if the given size is smaller than one
         * @throws NullPointerException if one of the arguments is {@code null}
         */
        public static <C extends Comparable<? super C>>
        Function<C, Stream<C>> toIntervalMin(final Duration timespan, final Clock clock) {
                return sliceBest(Streams::min, timespan, clock);
        }

        /**
         * Return a new flat-mapper function which returns (emits) the minimal value
         * of the elements emitted within the given {@code timespan}.
         *
         * @see #toIntervalMin(Duration)
         * @see #toIntervalMax(Duration)
         *
         * @param <C> the element type
         * @param comparator the comparator used for testing the elements
         * @param timespan the timespan the elements are collected for the
         *        calculation slice
         * @return a new flat-mapper function
         * @throws IllegalArgumentException if the given size is smaller than one
         @throws NullPointerException if one of the arguments is {@code null}
         */
        public static <C> Function<C, Stream<C>>
        toIntervalBest(final Comparator<? super C> comparator, final Duration timespan) {
                requireNonNull(comparator);
                return sliceBest((a, b) -> best(comparator, a, b), timespan, systemUTC());
        }

        /**
         * Return a new flat-mapper function which returns (emits) the minimal value
         * of the elements emitted within the given {@code timespan}.
         *
         * @param <C> the element type
         * @param comparator the comparator used for testing the elements
         * @param timespan the timespan the elements are collected for the
         *        calculation slice
         * @param clock the {@code clock} used for measuring the {@code timespan}
         * @return a new flat-mapper function
         * @throws IllegalArgumentException if the given size is smaller than one
         @throws NullPointerException if one of the arguments is {@code null}
         */
        public static <C> Function<C, Stream<C>>
        toIntervalBest(
                final Comparator<? super C> comparator,
                final Duration timespan,
                final Clock clock
        ) {
                requireNonNull(comparator);
                return sliceBest((a, b) -> best(comparator, a, b), timespan, clock);
        }

        private static <C> Function<C, Stream<C>> sliceBest(
                final BinaryOperator<C> comp,
                final Duration timespan,
                final Clock clock
        ) {
                requireNonNull(comp);
                requireNonNull(timespan);

                return new Function<>() {
                        private final long _timespan  = timespan.toMillis();

                        private long _start = 0;
                        private C _best;

                        @Override
                        public Stream<C> apply(final C value) {
                                if (_start == 0) {
                                        _start = clock.millis();
                                }

                                _best = comp.apply(_best, value);
                                long end = clock.millis();

                                final Stream<C> result;
                                if (end - _start >= _timespan) {
                                        result = Stream.of(_best);
                                        _start = 0;
                                        _best = null;
                                } else {
                                        result = Stream.empty();
                                }

                                return result;
                        }
                };
        }

}