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async-enumerable developed by David Karnok
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/*
 * Copyright 2017 David Karnok
 *
 * 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 hu.akarnokd.asyncenum;

import java.util.concurrent.*;
import java.util.concurrent.atomic.AtomicInteger;
import java.util.function.*;

final class AsyncSkipWhile implements AsyncEnumerable {

    final AsyncEnumerable source;

    final Predicate predicate;

    AsyncSkipWhile(AsyncEnumerable source, Predicate predicate) {
        this.source = source;
        this.predicate = predicate;
    }

    @Override
    public AsyncEnumerator enumerator() {
        return new SkipWhileEnumerator<>(source.enumerator(), predicate);
    }

    static final class SkipWhileEnumerator
            extends AtomicInteger
            implements AsyncEnumerator, BiConsumer {

        final AsyncEnumerator source;

        final Predicate predicate;

        boolean passThrough;

        CompletableFuture completable;

        T current;

        SkipWhileEnumerator(AsyncEnumerator source, Predicate predicate) {
            this.source = source;
            this.predicate = predicate;
        }

        @Override
        public CompletionStage moveNext() {
            current = null;
            CompletableFuture cf = new CompletableFuture<>();
            completable = cf;
            nextSource();
            return cf;
        }

        void nextSource() {
            if (getAndIncrement() == 0) {
                do {
                    source.moveNext().whenComplete(this);
                } while (decrementAndGet() != 0);
            }
        }

        @Override
        public T current() {
            return current;
        }

        @Override
        public void cancel() {
            source.cancel();
        }

        @Override
        public void accept(Boolean aBoolean, Throwable throwable) {
            if (throwable != null) {
                completable.completeExceptionally(throwable);
                return;
            }

            if (aBoolean) {
                T v = source.current();
                if (!passThrough) {
                    if (predicate.test(v)) {
                        nextSource();
                        return;
                    } else {
                        passThrough = true;
                    }
                }
                current = v;
                completable.complete(true);
            } else {
                completable.complete(false);
            }
        }
    }
}