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Streams Utils is a set of operations written on Java 8 Streams. It allows several basic operations that are not available in the the Java 8, and that have proven to be very useful in some cases.

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/*
 * Copyright (C) 2017 José Paumard
 *
 * 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 org.paumard.spliterators;

import org.paumard.streams.StreamsUtils;

import java.util.Map;
import java.util.Objects;
import java.util.Spliterator;
import java.util.concurrent.atomic.AtomicReference;
import java.util.function.BinaryOperator;
import java.util.function.Consumer;

/**
 * See the documentation and patterns to be used in this class in the {@link StreamsUtils} factory class.
 * 
 * Created by José
 */
public class AccumulatingEntriesSpliterator implements Spliterator> {

    private BinaryOperator operator;
    private final Spliterator> spliterator;
    private AtomicReference accumulator;

    public static  AccumulatingEntriesSpliterator of(Spliterator> spliterator, BinaryOperator operator) {

        Objects.requireNonNull(spliterator);
        Objects.requireNonNull(operator);

        if ((spliterator.characteristics() & Spliterator.ORDERED) == 0) {
            throw new IllegalArgumentException(("Why would you try to accumulate a non-ORDERED spliterator?"));
        }

        return new AccumulatingEntriesSpliterator<>(spliterator, operator);
    }

    private AccumulatingEntriesSpliterator(Spliterator> spliterator, BinaryOperator operator) {
        this.spliterator = spliterator;
        this.operator = operator;
    }

    @Override
    public boolean tryAdvance(Consumer> action) {
        boolean hasMore = spliterator.tryAdvance(
                entry -> {
                    if (accumulator == null) {
                        accumulator = new AtomicReference<>(entry.getValue());
                        action.accept(entry);
                    } else {
                        entry.setValue(accumulator.accumulateAndGet(entry.getValue(), operator));
                        action.accept(entry);
                    }
                }
        );

        return hasMore;
    }

    @Override
    public Spliterator> trySplit() {
        Spliterator> splitSpliterator = spliterator.trySplit();
        return splitSpliterator == null ? null : new AccumulatingEntriesSpliterator<>(splitSpliterator, operator);
    }

    @Override
    public long estimateSize() {
        return spliterator.estimateSize();
    }

    @Override
    public int characteristics() {
        return this.spliterator.characteristics();
    }
}