/*
 * Copyright 2017-2020 the original author or 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
 *
 *      https://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.springframework.data.repository.config;

import lombok.RequiredArgsConstructor;

import java.util.Collection;
import java.util.Optional;
import java.util.function.Function;
import java.util.function.Predicate;
import java.util.stream.Collectors;

Allows filtering of a collection in order to select a unique element. Once a unique element is found all further
filters are ignored.
Author:
Jens Schauder, Oliver Gierke
Since:
2.0/**
 * Allows filtering of a collection in order to select a unique element. Once a unique element is found all further
 * filters are ignored.
 *
 * @author Jens Schauder
 * @author Oliver Gierke
 * @since 2.0
 */
@RequiredArgsConstructor(staticName = "of")
class SelectionSet<T> {

	private final Collection<T> collection;
	private final Function<Collection<T>, Optional<T>> fallback;

	
creates a SelectionSet with a default fallback of null, when no element is found and an IllegalStateException when no element is found. /**
	 * creates a {@link SelectionSet} with a default fallback of {@literal null}, when no element is found and an
	 * {@link IllegalStateException} when no element is found.
	 */
	static <T> SelectionSet<T> of(Collection<T> collection) {
		return new SelectionSet<>(collection, defaultFallback());
	}

	
If this SelectionSet contains exactly one element it gets returned. If no unique result can be identified the fallback function passed in at the constructor gets called and its return value becomes the return value of this method. 
Returns:
a unique result, or the result of the callback provided in the constructor./**
	 * If this {@code SelectionSet} contains exactly one element it gets returned. If no unique result can be identified
	 * the fallback function passed in at the constructor gets called and its return value becomes the return value of
	 * this method.
	 *
	 * @return a unique result, or the result of the callback provided in the constructor.
	 */
	Optional<T> uniqueResult() {

		Optional<T> uniqueResult = findUniqueResult();

		return uniqueResult.isPresent() ? uniqueResult : fallback.apply(collection);
	}

	
Filters the collection with the predicate if there are still more then one elements in the collection.
Params:
predicate – To be used for filtering./**
	 * Filters the collection with the predicate if there are still more then one elements in the collection.
	 *
	 * @param predicate To be used for filtering.
	 */
	SelectionSet<T> filterIfNecessary(Predicate<T> predicate) {

		return findUniqueResult().map(it -> this).orElseGet(
				() -> new SelectionSet<T>(collection.stream().filter(predicate).collect(Collectors.toList()), fallback));
	}

	private static <S> Function<Collection<S>, Optional<S>> defaultFallback() {

		return c -> {
			if (c.isEmpty()) {
				return Optional.empty();
			} else {
				throw new IllegalStateException("More then one element in collection.");
			}
		};
	}

	private Optional<T> findUniqueResult() {
		return Optional.ofNullable(collection.size() == 1 ? collection.iterator().next() : null);
	}
}