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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.AbstractMap;
import java.util.HashMap;
import java.util.List;
import java.util.Map;
import java.util.Spliterator;
import java.util.concurrent.CompletableFuture;
import java.util.function.Consumer;

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

/**
 * @author Grzegorz Piwowarek
 */
final class CompletionOrderSpliterator implements Spliterator {

    private final Map>> indexedFutures;

    CompletionOrderSpliterator(List> futures) {
        indexedFutures = toIndexedFutures(futures);
    }

    @Override
    public boolean tryAdvance(Consumer action) {
        if (!indexedFutures.isEmpty()) {
            action.accept(nextCompleted());
            return true;
        } else {
            return false;
        }
    }

    private T nextCompleted() {
        return anyOf(indexedFutures.values().toArray(new CompletableFuture[0]))
          .thenApply(result -> ((Map.Entry) result))
          .thenApply(result -> {
              indexedFutures.remove(result.getKey());
              return result.getValue();
          }).join();
    }

    @Override
    public Spliterator trySplit() {
        return null;
    }

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

    @Override
    public int characteristics() {
        return SIZED & IMMUTABLE & NONNULL;
    }

    private static  Map>> toIndexedFutures(List> futures) {
        Map>> map = new HashMap<>(futures.size(), 1);

        int counter = 0;
        for (CompletableFuture future : futures) {
            int index = counter++;
            map.put(index, future.thenApply(value -> new AbstractMap.SimpleEntry<>(index, value)));
        }
        return map;
    }
}