/*-
 * #%L
 * HAPI FHIR - Core Library
 * %%
 * Copyright (C) 2014 - 2023 Smile CDR, Inc.
 * %%
 * 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.
 * #L%
 */
package ca.uhn.fhir.util;

import com.google.common.collect.ArrayListMultimap;
import com.google.common.collect.ImmutableSet;
import com.google.common.collect.ListMultimap;

import java.util.Set;
import java.util.function.BiConsumer;
import java.util.function.BinaryOperator;
import java.util.function.Function;
import java.util.function.Supplier;
import java.util.stream.Collector;

/**
 * Copied from https://stackoverflow.com/questions/23003542/cleanest-way-to-create-a-guava-multimap-from-a-java-8-stream
 */
public class MultimapCollector<T, K, V> implements Collector<T, ListMultimap<K, V>, ListMultimap<K, V>> {

	private final Function<T, K> keyGetter;
	private final Function<T, V> valueGetter;

	public MultimapCollector(Function<T, K> keyGetter, Function<T, V> valueGetter) {
		this.keyGetter = keyGetter;
		this.valueGetter = valueGetter;
	}

	public static <T, K, V> MultimapCollector<T, K, V> toMultimap(
			Function<T, K> keyGetter, Function<T, V> valueGetter) {
		return new MultimapCollector<>(keyGetter, valueGetter);
	}

	public static <T, K, V> MultimapCollector<T, K, T> toMultimap(Function<T, K> keyGetter) {
		return new MultimapCollector<>(keyGetter, v -> v);
	}

	@Override
	public Supplier<ListMultimap<K, V>> supplier() {
		return ArrayListMultimap::create;
	}

	@Override
	public BiConsumer<ListMultimap<K, V>, T> accumulator() {
		return (map, element) -> map.put(keyGetter.apply(element), valueGetter.apply(element));
	}

	@Override
	public BinaryOperator<ListMultimap<K, V>> combiner() {
		return (map1, map2) -> {
			map1.putAll(map2);
			return map1;
		};
	}

	@Override
	public Function<ListMultimap<K, V>, ListMultimap<K, V>> finisher() {
		return map -> map;
	}

	@Override
	public Set<Characteristics> characteristics() {
		return ImmutableSet.of(Characteristics.IDENTITY_FINISH);
	}
}
