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Access to the Neo4j graph database through Java
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/*
 * Copyright (c) "Neo4j"
 * Neo4j Sweden AB [http://neo4j.com]
 *
 * This file is part of Neo4j.
 *
 * 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.neo4j.driver.internal.reactive;

import org.reactivestreams.Publisher;
import reactor.core.publisher.Mono;

import java.util.Optional;
import java.util.concurrent.CompletionStage;
import java.util.function.Supplier;

import org.neo4j.driver.internal.util.Futures;

public class RxUtils
{
    /**
     * The publisher created by this method will either succeed without publishing anything or fail with an error.
     * @param supplier supplies a {@link CompletionStage}.
     * @return A publisher that publishes nothing on completion or fails with an error.
     */
    public static  Publisher createEmptyPublisher( Supplier> supplier )
    {
        return Mono.create( sink -> supplier.get().whenComplete( ( ignore, completionError ) -> {
            Throwable error = Futures.completionExceptionCause( completionError );
            if ( error != null )
            {
                sink.error( error );
            }
            else
            {
                sink.success();
            }
        } ) );
    }

    /**
     * The publisher created by this method will either succeed with exactly one item or fail with an error.
     *
     * @param supplier                    supplies a {@link CompletionStage} that MUST produce a non-null result when completed successfully.
     * @param nullResultThrowableSupplier supplies a {@link Throwable} that is used as an error when the supplied completion stage completes successfully with
     *                                    null.
     * @param                          the type of the item to publish.
     * @return A publisher that succeeds exactly one item or fails with an error.
     */
    public static  Publisher createSingleItemPublisher( Supplier> supplier, Supplier nullResultThrowableSupplier )
    {
        return Mono.create( sink -> supplier.get().whenComplete(
                ( item, completionError ) ->
                {
                    if ( completionError == null )
                    {
                        if ( item != null )
                        {
                            sink.success( item );
                        }
                        else
                        {
                            sink.error( nullResultThrowableSupplier.get() );
                        }
                    }
                    else
                    {
                        Throwable error = Optional.ofNullable( Futures.completionExceptionCause( completionError ) ).orElse( completionError );
                        sink.error( error );
                    }
                } ) );
    }
}