/*
 * Licensed to the Apache Software Foundation (ASF) under one or more
 * contributor license agreements.  See the NOTICE file distributed with
 * this work for additional information regarding copyright ownership.
 * The ASF licenses this file to You 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.
 */
package org.apache.commons.lang3.stream;

import java.lang.reflect.Array;
import java.util.ArrayList;
import java.util.Collection;
import java.util.Collections;
import java.util.List;
import java.util.Set;
import java.util.function.BiConsumer;
import java.util.function.BinaryOperator;
import java.util.function.Consumer;
import java.util.function.Function;
import java.util.function.Predicate;
import java.util.function.Supplier;
import java.util.stream.Collector;
import java.util.stream.Collectors;
import java.util.stream.Stream;

import org.apache.commons.lang3.function.Failable;
import org.apache.commons.lang3.function.FailableConsumer;
import org.apache.commons.lang3.function.FailableFunction;
import org.apache.commons.lang3.function.FailablePredicate;

/**
 * Provides utility functions, and classes for working with the
 * {@code java.util.stream} package, or more generally, with Java 8 lambdas.
 * More specifically, it attempts to address the fact that lambdas are supposed
 * not to throw Exceptions, at least not checked Exceptions, AKA instances of
 * {@link Exception}. This enforces the use of constructs like
 * 
 * <pre>
 * Consumer&lt;java.lang.reflect.Method&gt; consumer = m -&gt; {
 * 	try {
 * 		m.invoke(o, args);
 * 	} catch (Throwable t) {
 * 		throw Failable.rethrow(t);
 * 	}
 * };
 * stream.forEach(consumer);
 * </pre>
 * 
 * Using a {@link FailableStream}, this can be rewritten as follows:
 * 
 * <pre>
 * Streams.failable(stream).forEach((m) -&gt; m.invoke(o, args));
 * </pre>
 * 
 * Obviously, the second version is much more concise and the spirit of Lambda
 * expressions is met better than in the first version.
 *
 * @see Stream
 * @see Failable
 * @since 3.11
 */
public class Streams {

	/**
	 * A Collector type for arrays.
	 *
	 * @param <O> The array type.
	 */
	public static class ArrayCollector<O> implements Collector<O, List<O>, O[]> {
		private static final Set<Characteristics> characteristics = Collections.emptySet();
		private final Class<O> elementType;

		/**
		 * Constructs a new instance for the given element type.
		 *
		 * @param elementType The element type.
		 */
		public ArrayCollector(final Class<O> elementType) {
			this.elementType = elementType;
		}

		@Override
		public BiConsumer<List<O>, O> accumulator() {
			return List::add;
		}

		@Override
		public Set<Characteristics> characteristics() {
			return characteristics;
		}

		@Override
		public BinaryOperator<List<O>> combiner() {
			return (left, right) -> {
				left.addAll(right);
				return left;
			};
		}

		@Override
		public Function<List<O>, O[]> finisher() {
			return list -> {
				@SuppressWarnings("unchecked")
				final O[] array = (O[]) Array.newInstance(elementType, list.size());
				return list.toArray(array);
			};
		}

		@Override
		public Supplier<List<O>> supplier() {
			return ArrayList::new;
		}
	}

	/**
	 * A reduced, and simplified version of a {@link Stream} with failable method
	 * signatures.
	 *
	 * @param <O> The streams element type.
	 */
	public static class FailableStream<O extends Object> {

		private Stream<O> stream;
		private boolean terminated;

		/**
		 * Constructs a new instance with the given {@code stream}.
		 *
		 * @param stream The stream.
		 */
		public FailableStream(final Stream<O> stream) {
			this.stream = stream;
		}

		/**
		 * Returns whether all elements of this stream match the provided predicate. May
		 * not evaluate the predicate on all elements if not necessary for determining
		 * the result. If the stream is empty then {@code true} is returned and the
		 * predicate is not evaluated.
		 *
		 * <p>
		 * This is a short-circuiting terminal operation.
		 *
		 * Note This method evaluates the <em>universal quantification</em> of the
		 * predicate over the elements of the stream (for all x P(x)). If the stream is
		 * empty, the quantification is said to be <em>vacuously satisfied</em> and is
		 * always {@code true} (regardless of P(x)).
		 *
		 * @param predicate A non-interfering, stateless predicate to apply to elements
		 *                  of this stream
		 * @return {@code true} If either all elements of the stream match the provided
		 *         predicate or the stream is empty, otherwise {@code false}.
		 */
		public boolean allMatch(final FailablePredicate<O, ?> predicate) {
			assertNotTerminated();
			return stream().allMatch(Failable.asPredicate(predicate));
		}

		/**
		 * Returns whether any elements of this stream match the provided predicate. May
		 * not evaluate the predicate on all elements if not necessary for determining
		 * the result. If the stream is empty then {@code false} is returned and the
		 * predicate is not evaluated.
		 *
		 * <p>
		 * This is a short-circuiting terminal operation.
		 *
		 * Note This method evaluates the <em>existential quantification</em> of the
		 * predicate over the elements of the stream (for some x P(x)).
		 *
		 * @param predicate A non-interfering, stateless predicate to apply to elements
		 *                  of this stream
		 * @return {@code true} if any elements of the stream match the provided
		 *         predicate, otherwise {@code false}
		 */
		public boolean anyMatch(final FailablePredicate<O, ?> predicate) {
			assertNotTerminated();
			return stream().anyMatch(Failable.asPredicate(predicate));
		}

		protected void assertNotTerminated() {
			if (terminated) {
				throw new IllegalStateException("This stream is already terminated.");
			}
		}

		/**
		 * Performs a mutable reduction operation on the elements of this stream using a
		 * {@code Collector}. A {@code Collector} encapsulates the functions used as
		 * arguments to {@link #collect(Supplier, BiConsumer, BiConsumer)}, allowing for
		 * reuse of collection strategies and composition of collect operations such as
		 * multiple-level grouping or partitioning.
		 *
		 * <p>
		 * If the underlying stream is parallel, and the {@code Collector} is
		 * concurrent, and either the stream is unordered or the collector is unordered,
		 * then a concurrent reduction will be performed (see {@link Collector} for
		 * details on concurrent reduction.)
		 *
		 * <p>
		 * This is a terminal operation.
		 *
		 * <p>
		 * When executed in parallel, multiple intermediate results may be instantiated,
		 * populated, and merged so as to maintain isolation of mutable data structures.
		 * Therefore, even when executed in parallel with non-thread-safe data
		 * structures (such as {@code ArrayList}), no additional synchronization is
		 * needed for a parallel reduction.
		 *
		 * Note The following will accumulate strings into an ArrayList:
		 *
		 * <pre>
		 * {
		 * 	&#64;code
		 * 	List<String> asList = stringStream.collect(Collectors.toList());
		 * }
		 * </pre>
		 *
		 * <p>
		 * The following will classify {@code Person} objects by city:
		 *
		 * <pre>
		 * {
		 * 	&#64;code
		 * 	Map<String, List<Person>> peopleByCity = personStream.collect(Collectors.groupingBy(Person::getCity));
		 * }
		 * </pre>
		 *
		 * <p>
		 * The following will classify {@code Person} objects by state and city,
		 * cascading two {@code Collector}s together:
		 *
		 * <pre>
		 * {
		 * 	&#64;code
		 * 	Map<String, Map<String, List<Person>>> peopleByStateAndCity = personStream
		 * 			.collect(Collectors.groupingBy(Person::getState, Collectors.groupingBy(Person::getCity)));
		 * }
		 * </pre>
		 *
		 * @param <R>       the type of the result
		 * @param <A>       the intermediate accumulation type of the {@code Collector}
		 * @param collector the {@code Collector} describing the reduction
		 * @return the result of the reduction
		 * @see #collect(Supplier, BiConsumer, BiConsumer)
		 * @see Collectors
		 */
		public <A, R> R collect(final Collector<? super O, A, R> collector) {
			makeTerminated();
			return stream().collect(collector);
		}

		/**
		 * Performs a mutable reduction operation on the elements of this
		 * FailableStream. A mutable reduction is one in which the reduced value is a
		 * mutable result container, such as an {@code ArrayList}, and elements are
		 * incorporated by updating the state of the result rather than by replacing the
		 * result. This produces a result equivalent to:
		 *
		 * <pre>
		 * {@code
		 *     R result = supplier.get();
		 *     for (T element : this stream)
		 *         accumulator.accept(result, element);
		 *     return result;
		 * }
		 * </pre>
		 *
		 * <p>
		 * Like {@link #reduce(Object, BinaryOperator)}, {@code collect} operations can
		 * be parallelized without requiring additional synchronization.
		 *
		 * <p>
		 * This is a terminal operation.
		 *
		 * Note There are many existing classes in the JDK whose signatures are
		 * well-suited for use with method references as arguments to {@code collect()}.
		 * For example, the following will accumulate strings into an {@code ArrayList}:
		 *
		 * <pre>
		 * {
		 * 	&#64;code
		 * 	List<String> asList = stringStream.collect(ArrayList::new, ArrayList::add, ArrayList::addAll);
		 * }
		 * </pre>
		 *
		 * <p>
		 * The following will take a stream of strings and concatenates them into a
		 * single string:
		 *
		 * <pre>
		 * {
		 * 	&#64;code
		 * 	String concat = stringStream.collect(StringBuilder::new, StringBuilder::append, StringBuilder::append)
		 * 			.toString();
		 * }
		 * </pre>
		 *
		 * @param <R>         type of the result
		 * @param <A>         Type of the accumulator.
		 * @param pupplier    a function that creates a new result container. For a
		 *                    parallel execution, this function may be called multiple
		 *                    times and must return a fresh value each time.
		 * @param accumulator An associative, non-interfering, stateless function for
		 *                    incorporating an additional element into a result
		 * @param combiner    An associative, non-interfering, stateless function for
		 *                    combining two values, which must be compatible with the
		 *                    accumulator function
		 * @return The result of the reduction
		 */
		public <A, R> R collect(final Supplier<R> pupplier, final BiConsumer<R, ? super O> accumulator,
				final BiConsumer<R, R> combiner) {
			makeTerminated();
			return stream().collect(pupplier, accumulator, combiner);
		}

		/**
		 * Returns a FailableStream consisting of the elements of this stream that match
		 * the given FailablePredicate.
		 *
		 * <p>
		 * This is an intermediate operation.
		 *
		 * @param predicate a non-interfering, stateless predicate to apply to each
		 *                  element to determine if it should be included.
		 * @return the new stream
		 */
		public FailableStream<O> filter(final FailablePredicate<O, ?> predicate) {
			assertNotTerminated();
			stream = stream.filter(Failable.asPredicate(predicate));
			return this;
		}

		/**
		 * Performs an action for each element of this stream.
		 *
		 * <p>
		 * This is a terminal operation.
		 *
		 * <p>
		 * The behavior of this operation is explicitly nondeterministic. For parallel
		 * stream pipelines, this operation does <em>not</em> guarantee to respect the
		 * encounter order of the stream, as doing so would sacrifice the benefit of
		 * parallelism. For any given element, the action may be performed at whatever
		 * time and in whatever thread the library chooses. If the action accesses
		 * shared state, it is responsible for providing the required synchronization.
		 *
		 * @param action a non-interfering action to perform on the elements
		 */
		public void forEach(final FailableConsumer<O, ?> action) {
			makeTerminated();
			stream().forEach(Failable.asConsumer(action));
		}

		protected void makeTerminated() {
			assertNotTerminated();
			terminated = true;
		}

		/**
		 * Returns a stream consisting of the results of applying the given function to
		 * the elements of this stream.
		 *
		 * <p>
		 * This is an intermediate operation.
		 *
		 * @param <R>    The element type of the new stream
		 * @param mapper A non-interfering, stateless function to apply to each element
		 * @return the new stream
		 */
		public <R> FailableStream<R> map(final FailableFunction<O, R, ?> mapper) {
			assertNotTerminated();
			return new FailableStream<>(stream.map(Failable.asFunction(mapper)));
		}

		/**
		 * Performs a reduction on the elements of this stream, using the provided
		 * identity value and an associative accumulation function, and returns the
		 * reduced value. This is equivalent to:
		 *
		 * <pre>
		 * {@code
		 *     T result = identity;
		 *     for (T element : this stream)
		 *         result = accumulator.apply(result, element)
		 *     return result;
		 * }
		 * </pre>
		 *
		 * but is not constrained to execute sequentially.
		 *
		 * <p>
		 * The {@code identity} value must be an identity for the accumulator function.
		 * This means that for all {@code t}, {@code accumulator.apply(identity, t)} is
		 * equal to {@code t}. The {@code accumulator} function must be an associative
		 * function.
		 *
		 * <p>
		 * This is a terminal operation.
		 *
		 * Note Sum, min, max, average, and string concatenation are all special cases
		 * of reduction. Summing a stream of numbers can be expressed as:
		 *
		 * <pre>
		 * {
		 * 	&#64;code
		 * 	Integer sum = integers.reduce(0, (a, b) -> a + b);
		 * }
		 * </pre>
		 *
		 * or:
		 *
		 * <pre>
		 * {
		 * 	&#64;code
		 * 	Integer sum = integers.reduce(0, Integer::sum);
		 * }
		 * </pre>
		 *
		 * <p>
		 * While this may seem a more roundabout way to perform an aggregation compared
		 * to simply mutating a running total in a loop, reduction operations
		 * parallelize more gracefully, without needing additional synchronization and
		 * with greatly reduced risk of data races.
		 *
		 * @param identity    the identity value for the accumulating function
		 * @param accumulator an associative, non-interfering, stateless function for
		 *                    combining two values
		 * @return the result of the reduction
		 */
		public O reduce(final O identity, final BinaryOperator<O> accumulator) {
			makeTerminated();
			return stream().reduce(identity, accumulator);
		}

		/**
		 * Converts the FailableStream into an equivalent stream.
		 *
		 * @return A stream, which will return the same elements, which this
		 *         FailableStream would return.
		 */
		public Stream<O> stream() {
			return stream;
		}
	}

	/**
	 * Converts the given {@link Collection} into a {@link FailableStream}. This is
	 * basically a simplified, reduced version of the {@link Stream} class, with the
	 * same underlying element stream, except that failable objects, like
	 * {@link FailablePredicate}, {@link FailableFunction}, or
	 * {@link FailableConsumer} may be applied, instead of {@link Predicate},
	 * {@link Function}, or {@link Consumer}. The idea is to rewrite a code snippet
	 * like this:
	 *
	 * <pre>
	 * final List&lt;O&gt; list;
	 * final Method m;
	 * final Function&lt;O, String&gt; mapper = (o) -&gt; {
	 * 	try {
	 * 		return (String) m.invoke(o);
	 * 	} catch (Throwable t) {
	 * 		throw Failable.rethrow(t);
	 * 	}
	 * };
	 * final List&lt;String&gt; strList = list.stream().map(mapper).collect(Collectors.toList());
	 * </pre>
	 *
	 * as follows:
	 *
	 * <pre>
	 * final List&lt;O&gt; list;
	 * final Method m;
	 * final List&lt;String&gt; strList = Failable.stream(list.stream()).map((o) -&gt; (String) m.invoke(o))
	 * 		.collect(Collectors.toList());
	 * </pre>
	 *
	 * While the second version may not be <em>quite</em> as efficient (because it
	 * depends on the creation of additional, intermediate objects, of type
	 * FailableStream), it is much more concise, and readable, and meets the spirit
	 * of Lambdas better than the first version.
	 *
	 * @param <O>    The streams element type.
	 * @param stream The stream, which is being converted.
	 * @return The {@link FailableStream}, which has been created by converting the
	 *         stream.
	 */
	public static <O> FailableStream<O> stream(final Collection<O> stream) {
		return stream(stream.stream());
	}

	/**
	 * Converts the given {@link Stream stream} into a {@link FailableStream}. This
	 * is basically a simplified, reduced version of the {@link Stream} class, with
	 * the same underlying element stream, except that failable objects, like
	 * {@link FailablePredicate}, {@link FailableFunction}, or
	 * {@link FailableConsumer} may be applied, instead of {@link Predicate},
	 * {@link Function}, or {@link Consumer}. The idea is to rewrite a code snippet
	 * like this:
	 *
	 * <pre>
	 * final List&lt;O&gt; list;
	 * final Method m;
	 * final Function&lt;O, String&gt; mapper = (o) -&gt; {
	 * 	try {
	 * 		return (String) m.invoke(o);
	 * 	} catch (Throwable t) {
	 * 		throw Failable.rethrow(t);
	 * 	}
	 * };
	 * final List&lt;String&gt; strList = list.stream().map(mapper).collect(Collectors.toList());
	 * </pre>
	 *
	 * as follows:
	 *
	 * <pre>
	 * final List&lt;O&gt; list;
	 * final Method m;
	 * final List&lt;String&gt; strList = Failable.stream(list.stream()).map((o) -&gt; (String) m.invoke(o))
	 * 		.collect(Collectors.toList());
	 * </pre>
	 *
	 * While the second version may not be <em>quite</em> as efficient (because it
	 * depends on the creation of additional, intermediate objects, of type
	 * FailableStream), it is much more concise, and readable, and meets the spirit
	 * of Lambdas better than the first version.
	 *
	 * @param <O>    The streams element type.
	 * @param stream The stream, which is being converted.
	 * @return The {@link FailableStream}, which has been created by converting the
	 *         stream.
	 */
	public static <O> FailableStream<O> stream(final Stream<O> stream) {
		return new FailableStream<>(stream);
	}

	/**
	 * Returns a {@code Collector} that accumulates the input elements into a new
	 * array.
	 *
	 * @param pElementType Type of an element in the array.
	 * @param <O>          the type of the input elements
	 * @return a {@code Collector} which collects all the input elements into an
	 *         array, in encounter order
	 */
	public static <O extends Object> Collector<O, ?, O[]> toArray(final Class<O> pElementType) {
		return new ArrayCollector<>(pElementType);
	}
}
