/*
 * Copyright (C) 2008 The Guava Authors
 *
 * 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.
 */

package com.google.common.collect;

import static com.google.common.base.Preconditions.checkNotNull;

import java.util.Collection;
import java.util.Comparator;
import java.util.Iterator;
import java.util.List;
import java.util.SortedSet;

import javax.annotation.CheckReturnValue;
import javax.annotation.Nullable;

import com.google.common.annotations.Beta;
import com.google.common.annotations.GwtCompatible;
import com.google.common.annotations.GwtIncompatible;
import com.google.common.base.Function;
import com.google.common.base.Optional;
import com.google.common.base.Predicate;

/**
 * {@code FluentIterable} provides a rich interface for manipulating
 * {@code Iterable} instances in a chained fashion. A {@code FluentIterable} can
 * be created from an {@code Iterable}, or from a set of elements. The following
 * types of methods are provided on {@code FluentIterable}:
 * <ul>
 * <li>chained methods which return a new {@code FluentIterable} based in some
 * way on the contents of the current one (for example {@link #transform})
 * <li>conversion methods which copy the {@code FluentIterable}'s contents into
 * a new collection or array (for example {@link #toList})
 * <li>element extraction methods which facilitate the retrieval of certain
 * elements (for example {@link #last})
 * <li>query methods which answer questions about the {@code FluentIterable}'s
 * contents (for example {@link #anyMatch})
 * </ul>
 *
 * <p>
 * Here is an example that merges the lists returned by two separate database
 * calls, transforms it by invoking {@code toString()} on each element, and
 * returns the first 10 elements as an {@code ImmutableList}:
 * 
 * <pre>
 *    {@code
 *
 *   FluentIterable
 *       .from(database.getClientList())
 *       .filter(activeInLastMonth())
 *       .transform(Functions.toStringFunction())
 *       .limit(10)
 *       .toList();}
 * </pre>
 *
 * <p>
 * Anything which can be done using {@code FluentIterable} could be done in a
 * different fashion (often with {@link Iterables}), however the use of
 * {@code FluentIterable} makes many sets of operations significantly more
 * concise.
 *
 * @author Marcin Mikosik
 * @since 12.0
 */
@GwtCompatible(emulated = true)
public abstract class FluentIterable<E> implements Iterable<E> {
	// We store 'iterable' and use it instead of 'this' to allow Iterables to
	// perform instanceof
	// checks on the _original_ iterable when FluentIterable.from is used.
	private final Iterable<E> iterable;

	/** Constructor for use by subclasses. */
	protected FluentIterable() {
		this.iterable = this;
	}

	FluentIterable(Iterable<E> iterable) {
		this.iterable = checkNotNull(iterable);
	}

	/**
	 * Returns a fluent iterable that wraps {@code iterable}, or {@code iterable}
	 * itself if it is already a {@code FluentIterable}.
	 */
	public static <E> FluentIterable<E> from(final Iterable<E> iterable) {
		return (iterable instanceof FluentIterable) ? (FluentIterable<E>) iterable : new FluentIterable<E>(iterable) {
			@Override
			public Iterator<E> iterator() {
				return iterable.iterator();
			}
		};
	}

	/**
	 * Construct a fluent iterable from another fluent iterable. This is obviously
	 * never necessary, but is intended to help call out cases where one migration
	 * from {@code Iterable} to {@code FluentIterable} has obviated the need to
	 * explicitly convert to a {@code FluentIterable}.
	 *
	 * @deprecated instances of {@code FluentIterable} don't need to be converted to
	 *             {@code FluentIterable}
	 */
	@Deprecated
	public static <E> FluentIterable<E> from(FluentIterable<E> iterable) {
		return checkNotNull(iterable);
	}

	/**
	 * Returns a string representation of this fluent iterable, with the format
	 * {@code [e1, e2, ..., en]}.
	 */
	@Override
	public String toString() {
		return Iterables.toString(iterable);
	}

	/**
	 * Returns the number of elements in this fluent iterable.
	 */
	public final int size() {
		return Iterables.size(iterable);
	}

	/**
	 * Returns {@code true} if this fluent iterable contains any object for which
	 * {@code equals(element)} is true.
	 */
	public final boolean contains(@Nullable Object element) {
		return Iterables.contains(iterable, element);
	}

	/**
	 * Returns a fluent iterable whose {@code Iterator} cycles indefinitely over the
	 * elements of this fluent iterable.
	 *
	 * <p>
	 * That iterator supports {@code remove()} if {@code iterable.iterator()} does.
	 * After {@code remove()} is called, subsequent cycles omit the removed element,
	 * which is no longer in this fluent iterable. The iterator's {@code hasNext()}
	 * method returns {@code true} until this fluent iterable is empty.
	 *
	 * <p>
	 * <b>Warning:</b> Typical uses of the resulting iterator may produce an
	 * infinite loop. You should use an explicit {@code break} or be certain that
	 * you will eventually remove all the elements.
	 */
	@CheckReturnValue
	public final FluentIterable<E> cycle() {
		return from(Iterables.cycle(iterable));
	}

	/**
	 * Returns the elements from this fluent iterable that satisfy a predicate. The
	 * resulting fluent iterable's iterator does not support {@code remove()}.
	 */
	@CheckReturnValue
	public final FluentIterable<E> filter(Predicate<? super E> predicate) {
		return from(Iterables.filter(iterable, predicate));
	}

	/**
	 * Returns the elements from this fluent iterable that are instances of class
	 * {@code type}.
	 *
	 * @param type the type of elements desired
	 */
	@GwtIncompatible("Class.isInstance")
	@CheckReturnValue
	public final <T> FluentIterable<T> filter(Class<T> type) {
		return from(Iterables.filter(iterable, type));
	}

	/**
	 * Returns {@code true} if any element in this fluent iterable satisfies the
	 * predicate.
	 */
	public final boolean anyMatch(Predicate<? super E> predicate) {
		return Iterables.any(iterable, predicate);
	}

	/**
	 * Returns {@code true} if every element in this fluent iterable satisfies the
	 * predicate. If this fluent iterable is empty, {@code true} is returned.
	 */
	public final boolean allMatch(Predicate<? super E> predicate) {
		return Iterables.all(iterable, predicate);
	}

	/**
	 * Returns an {@link Optional} containing the first element in this fluent
	 * iterable that satisfies the given predicate, if such an element exists.
	 *
	 * <p>
	 * <b>Warning:</b> avoid using a {@code predicate} that matches {@code null}. If
	 * {@code null} is matched in this fluent iterable, a
	 * {@link NullPointerException} will be thrown.
	 */
	public final Optional<E> firstMatch(Predicate<? super E> predicate) {
		return Iterables.tryFind(iterable, predicate);
	}

	/**
	 * Returns a fluent iterable that applies {@code function} to each element of
	 * this fluent iterable.
	 *
	 * <p>
	 * The returned fluent iterable's iterator supports {@code remove()} if this
	 * iterable's iterator does. After a successful {@code remove()} call, this
	 * fluent iterable no longer contains the corresponding element.
	 */
	public final <T> FluentIterable<T> transform(Function<? super E, T> function) {
		return from(Iterables.transform(iterable, function));
	}

	/**
	 * Applies {@code function} to each element of this fluent iterable and returns
	 * a fluent iterable with the concatenated combination of results.
	 * {@code function} returns an Iterable of results.
	 *
	 * <p>
	 * The returned fluent iterable's iterator supports {@code remove()} if this
	 * function-returned iterables' iterator does. After a successful
	 * {@code remove()} call, the returned fluent iterable no longer contains the
	 * corresponding element.
	 *
	 * @since 13.0 (required {@code Function<E, Iterable<T>>} until 14.0)
	 */
	public <T> FluentIterable<T> transformAndConcat(Function<? super E, ? extends Iterable<? extends T>> function) {
		return from(Iterables.concat(transform(function)));
	}

	/**
	 * Returns an {@link Optional} containing the first element in this fluent
	 * iterable. If the iterable is empty, {@code Optional.absent()} is returned.
	 *
	 * @throws NullPointerException if the first element is null; if this is a
	 *                              possibility, use {@code iterator().next()} or
	 *                              {@link Iterables#getFirst} instead.
	 */
	public final Optional<E> first() {
		Iterator<E> iterator = iterable.iterator();
		return iterator.hasNext() ? Optional.of(iterator.next()) : Optional.<E>absent();
	}

	/**
	 * Returns an {@link Optional} containing the last element in this fluent
	 * iterable. If the iterable is empty, {@code Optional.absent()} is returned.
	 *
	 * @throws NullPointerException if the last element is null; if this is a
	 *                              possibility, use {@link Iterables#getLast}
	 *                              instead.
	 */
	public final Optional<E> last() {
		// Iterables#getLast was inlined here so we don't have to throw/catch a NSEE

		// TODO(kevinb): Support a concurrently modified collection?
		if (iterable instanceof List) {
			List<E> list = (List<E>) iterable;
			if (list.isEmpty()) {
				return Optional.absent();
			}
			return Optional.of(list.get(list.size() - 1));
		}
		Iterator<E> iterator = iterable.iterator();
		if (!iterator.hasNext()) {
			return Optional.absent();
		}

		/*
		 * TODO(kevinb): consider whether this "optimization" is worthwhile. Users with
		 * SortedSets tend to know they are SortedSets and probably would not call this
		 * method.
		 */
		if (iterable instanceof SortedSet) {
			SortedSet<E> sortedSet = (SortedSet<E>) iterable;
			return Optional.of(sortedSet.last());
		}

		while (true) {
			E current = iterator.next();
			if (!iterator.hasNext()) {
				return Optional.of(current);
			}
		}
	}

	/**
	 * Returns a view of this fluent iterable that skips its first
	 * {@code numberToSkip} elements. If this fluent iterable contains fewer than
	 * {@code numberToSkip} elements, the returned fluent iterable skips all of its
	 * elements.
	 *
	 * <p>
	 * Modifications to this fluent iterable before a call to {@code iterator()} are
	 * reflected in the returned fluent iterable. That is, the its iterator skips
	 * the first {@code numberToSkip} elements that exist when the iterator is
	 * created, not when {@code skip()} is called.
	 *
	 * <p>
	 * The returned fluent iterable's iterator supports {@code remove()} if the
	 * {@code Iterator} of this fluent iterable supports it. Note that it is
	 * <i>not</i> possible to delete the last skipped element by immediately calling
	 * {@code remove()} on the returned fluent iterable's iterator, as the
	 * {@code Iterator} contract states that a call to {@code * remove()} before a
	 * call to {@code next()} will throw an {@link IllegalStateException}.
	 */
	@CheckReturnValue
	public final FluentIterable<E> skip(int numberToSkip) {
		return from(Iterables.skip(iterable, numberToSkip));
	}

	/**
	 * Creates a fluent iterable with the first {@code size} elements of this fluent
	 * iterable. If this fluent iterable does not contain that many elements, the
	 * returned fluent iterable will have the same behavior as this fluent iterable.
	 * The returned fluent iterable's iterator supports {@code remove()} if this
	 * fluent iterable's iterator does.
	 *
	 * @param size the maximum number of elements in the returned fluent iterable
	 * @throws IllegalArgumentException if {@code size} is negative
	 */
	@CheckReturnValue
	public final FluentIterable<E> limit(int size) {
		return from(Iterables.limit(iterable, size));
	}

	/**
	 * Determines whether this fluent iterable is empty.
	 */
	public final boolean isEmpty() {
		return !iterable.iterator().hasNext();
	}

	/**
	 * Returns an {@code ImmutableList} containing all of the elements from this
	 * fluent iterable in proper sequence.
	 *
	 * @since 14.0 (since 12.0 as {@code toImmutableList()}).
	 */
	public final ImmutableList<E> toList() {
		return ImmutableList.copyOf(iterable);
	}

	/**
	 * Returns an {@code ImmutableList} containing all of the elements from this
	 * {@code
	 * FluentIterable} in the order specified by {@code comparator}. To produce an
	 * {@code
	 * ImmutableList} sorted by its natural ordering, use
	 * {@code toSortedList(Ordering.natural())}.
	 *
	 * @param comparator the function by which to sort list elements
	 * @throws NullPointerException if any element is null
	 * @since 14.0 (since 13.0 as {@code toSortedImmutableList()}).
	 */
	@Beta
	public final ImmutableList<E> toSortedList(Comparator<? super E> comparator) {
		return Ordering.from(comparator).immutableSortedCopy(iterable);
	}

	/**
	 * Returns an {@code ImmutableSet} containing all of the elements from this
	 * fluent iterable with duplicates removed.
	 *
	 * @since 14.0 (since 12.0 as {@code toImmutableSet()}).
	 */
	public final ImmutableSet<E> toSet() {
		return ImmutableSet.copyOf(iterable);
	}

	/**
	 * Returns an {@code ImmutableSortedSet} containing all of the elements from
	 * this {@code
	 * FluentIterable} in the order specified by {@code comparator}, with duplicates
	 * (determined by {@code comparator.compare(x, y) == 0}) removed. To produce an
	 * {@code ImmutableSortedSet} sorted by its natural ordering, use
	 * {@code toSortedSet(Ordering.natural())}.
	 *
	 * @param comparator the function by which to sort set elements
	 * @throws NullPointerException if any element is null
	 * @since 14.0 (since 12.0 as {@code toImmutableSortedSet()}).
	 */
	public final ImmutableSortedSet<E> toSortedSet(Comparator<? super E> comparator) {
		return ImmutableSortedSet.copyOf(comparator, iterable);
	}

	/**
	 * Returns an immutable map for which the elements of this
	 * {@code FluentIterable} are the keys in the same order, mapped to values by
	 * the given function. If this iterable contains duplicate elements, the
	 * returned map will contain each distinct element once in the order it first
	 * appears.
	 *
	 * @throws NullPointerException if any element of this iterable is {@code null},
	 *                              or if {@code
	 *     valueFunction}        produces {@code null} for any key
	 * @since 14.0
	 */
	public final <V> ImmutableMap<E, V> toMap(Function<? super E, V> valueFunction) {
		return Maps.toMap(iterable, valueFunction);
	}

	/**
	 * Creates an index {@code ImmutableListMultimap} that contains the results of
	 * applying a specified function to each item in this {@code FluentIterable} of
	 * values. Each element of this iterable will be stored as a value in the
	 * resulting multimap, yielding a multimap with the same size as this iterable.
	 * The key used to store that value in the multimap will be the result of
	 * calling the function on that value. The resulting multimap is created as an
	 * immutable snapshot. In the returned multimap, keys appear in the order they
	 * are first encountered, and the values corresponding to each key appear in the
	 * same order as they are encountered.
	 *
	 * @param keyFunction the function used to produce the key for each value
	 * @throws NullPointerException if any of the following cases is true:
	 *                              <ul>
	 *                              <li>{@code keyFunction} is null
	 *                              <li>An element in this fluent iterable is null
	 *                              <li>{@code keyFunction} returns {@code null} for
	 *                              any element of this iterable
	 *                              </ul>
	 * @since 14.0
	 */
	public final <K> ImmutableListMultimap<K, E> index(Function<? super E, K> keyFunction) {
		return Multimaps.index(iterable, keyFunction);
	}

	/**
	 * Returns an immutable map for which the {@link java.util.Map#values} are the
	 * elements of this {@code FluentIterable} in the given order, and each key is
	 * the product of invoking a supplied function on its corresponding value.
	 *
	 * @param keyFunction the function used to produce the key for each value
	 * @throws IllegalArgumentException if {@code keyFunction} produces the same key
	 *                                  for more than one value in this fluent
	 *                                  iterable
	 * @throws NullPointerException     if any element of this fluent iterable is
	 *                                  null, or if {@code keyFunction} produces
	 *                                  {@code null} for any value
	 * @since 14.0
	 */
	public final <K> ImmutableMap<K, E> uniqueIndex(Function<? super E, K> keyFunction) {
		return Maps.uniqueIndex(iterable, keyFunction);
	}

	/**
	 * Returns an array containing all of the elements from this fluent iterable in
	 * iteration order.
	 *
	 * @param type the type of the elements
	 * @return a newly-allocated array into which all the elements of this fluent
	 *         iterable have been copied
	 */
	@GwtIncompatible("Array.newArray(Class, int)")
	public final E[] toArray(Class<E> type) {
		return Iterables.toArray(iterable, type);
	}

	/**
	 * Copies all the elements from this fluent iterable to {@code collection}. This
	 * is equivalent to calling {@code Iterables.addAll(collection, this)}.
	 *
	 * @param collection the collection to copy elements to
	 * @return {@code collection}, for convenience
	 * @since 14.0
	 */
	public final <C extends Collection<? super E>> C copyInto(C collection) {
		checkNotNull(collection);
		if (iterable instanceof Collection) {
			collection.addAll(Collections2.cast(iterable));
		} else {
			for (E item : iterable) {
				collection.add(item);
			}
		}
		return collection;
	}

	/**
	 * Returns the element at the specified position in this fluent iterable.
	 *
	 * @param position position of the element to return
	 * @return the element at the specified position in this fluent iterable
	 * @throws IndexOutOfBoundsException if {@code position} is negative or greater
	 *                                   than or equal to the size of this fluent
	 *                                   iterable
	 */
	public final E get(int position) {
		return Iterables.get(iterable, position);
	}

	/**
	 * Function that transforms {@code Iterable<E>} into a fluent iterable.
	 */
	private static class FromIterableFunction<E> implements Function<Iterable<E>, FluentIterable<E>> {
		@Override
		public FluentIterable<E> apply(Iterable<E> fromObject) {
			return FluentIterable.from(fromObject);
		}
	}
}