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Parallel collection processing with customizable thread pools

There is a newer version: 3.2.0

package com.pivovarit.collectors;

import java.util.Collection;
import java.util.EnumSet;
import java.util.List;
import java.util.Set;
import java.util.concurrent.CompletableFuture;
import java.util.concurrent.Executor;
import java.util.function.BiConsumer;
import java.util.function.Function;
import java.util.function.Supplier;

import static java.util.concurrent.CompletableFuture.completedFuture;

/**
 * @author Grzegorz Piwowarek
 */
final class AsyncParallelCollector>
  extends AbstractParallelCollector>
  implements AutoCloseable {

    private final Dispatcher dispatcher;

    private final Function operation;
    private final Supplier collectionFactory;

    AsyncParallelCollector(
      Function operation,
      Supplier collection,
      Executor executor,
      int parallelism) {
        this.dispatcher = new ThrottlingDispatcher<>(executor, parallelism);
        this.collectionFactory = collection;
        this.operation = operation;
    }

    AsyncParallelCollector(
      Function operation,
      Supplier collection,
      Executor executor) {
        this.dispatcher = new UnboundedDispatcher<>(executor);
        this.collectionFactory = collection;
        this.operation = operation;
    }

    @Override
    public BiConsumer>, T> accumulator() {
        return (acc, e) -> acc.add(dispatcher.enqueue(() -> operation.apply(e)));
    }

    @Override
    public Function>, CompletableFuture> finisher() {
        if (dispatcher.getWorkingQueue().size() != 0) {
            dispatcher.start();
            return foldLeftFutures(collectionFactory).andThen(f -> supplyWithResources(() -> f, dispatcher::close));
        } else {
            return supplyWithResources(() -> (__) -> completedFuture(collectionFactory
              .get()), dispatcher::close);
        }
    }

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

    @Override
    public void close() {
        dispatcher.close();
    }
}