net.morimekta.collect.collectors.InBatchesOfCollector Maven / Gradle / Ivy

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package net.morimekta.collect.collectors;

import net.morimekta.collect.UnmodifiableList;

import java.util.ArrayList;
import java.util.LinkedList;
import java.util.List;
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;

import static net.morimekta.collect.UnmodifiableList.toList;
import static net.morimekta.collect.UnmodifiableSet.setOf;

public class InBatchesOfCollector implements Collector>, List>> {
    private final int itemsPerBatch;

    public InBatchesOfCollector(int itemsPerBatch) {
        this.itemsPerBatch = itemsPerBatch;
    }

    @Override
    public Supplier>> supplier() {
        return LinkedList::new;
    }

    @Override
    public BiConsumer>, T> accumulator() {
        return (l, i) -> {
            List last = l.peekLast();
            if (last == null || last.size() >= itemsPerBatch) {
                last = new ArrayList<>(itemsPerBatch);
                l.add(last);
            }
            last.add(i);
        };
    }

    @Override
    public BinaryOperator>> combiner() {
        return (a, b) -> {
            // Merge the two lists so the batches matches the order
            // of the non-parallel inBatchesOf with (a1..an) + (b1..bn)
            // as the set of items. It's not extremely efficient, but
            // works fine as this is not optimized for parallel streams.
            List last = a.peekLast();
            while (!b.isEmpty()) {
                for (T i : b.peekFirst()) {
                    if (last == null || last.size() >= itemsPerBatch) {
                        last = new ArrayList<>(itemsPerBatch);
                        a.add(last);
                    }
                    last.add(i);
                }
                b.pollFirst();
            }
            return a;
        };
    }

    @Override
    public Function>, List>> finisher() {
        return batches -> batches.stream()
                                 .map(UnmodifiableList::asList)
                                 .collect(toList());
    }

    @Override
    public Set characteristics() {
        return setOf(Characteristics.UNORDERED);
    }
}