/*
 * Copyright (c) 2012, Oracle and/or its affiliates. All rights reserved.
 * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
 *
 * This code is free software; you can redistribute it and/or modify it
 * under the terms of the GNU General Public License version 2 only, as
 * published by the Free Software Foundation.  Oracle designates this
 * particular file as subject to the "Classpath" exception as provided
 * by Oracle in the LICENSE file that accompanied this code.
 *
 * This code is distributed in the hope that it will be useful, but WITHOUT
 * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
 * FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public License
 * version 2 for more details (a copy is included in the LICENSE file that
 * accompanied this code).
 *
 * You should have received a copy of the GNU General Public License version
 * 2 along with this work; if not, write to the Free Software Foundation,
 * Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA.
 *
 * Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA
 * or visit www.oracle.com if you need additional information or have any
 * questions.
 */
package java.util.stream;

import java.util.Spliterator;
import java.util.function.IntFunction;

/**
 * Helper class for executing  <a href="package-summary.html#StreamPipelines">stream pipelines</a>, capturing
 * all of the information about a stream pipeline (source, output shape, stream flags, parallelism, etc)
 * in one place.
 *
 * @apiNote
 * A stream pipeline consists of a source, zero or more intermediate operations, and a terminal operation.
 * Execution of the stream pipeline begins when the terminal operation is executed.
 * A {@code PipelineHelper} describes the portion of a stream pipeline including its source, some or all of its
 * intermediate operations, and certain information about the terminal (or stateful) operation which follows
 * the last intermediate operation described by this {@code PipelineHelper}.The
 * {@code PipelineHelper} is passed to the {@link TerminalOp#evaluateParallel(PipelineHelper)},
 * {@link TerminalOp#evaluateSequential(PipelineHelper)}, and {@link StatefulOp#evaluateParallel(PipelineHelper)},
 * methods, which can use the {@code PipelineHelper} to access the source {@code Spliterator} for the pipeline,
 * information about the pipeline such as input shape, output shape, stream flags, and size, and use the helper
 * methods such as {@link #into(Sink, Spliterator)}, {@link #intoWrapped(Sink, Spliterator)},
 * and {@link #wrapSink(Sink)} to execute pipeline operations.
 *
 * @param <P_IN>  Type of input elements to the pipeline
 * @param <P_OUT> Type of output elements from the pipeline
 * @since 1.8
 */
interface PipelineHelper<P_IN, P_OUT> {

    /**
     * Get the {@code StreamShape} describing the input shape of the pipeline
     * @return The input shape of the pipeline
     */
    StreamShape getInputShape();

    /**
     * Get the {@code StreamShape} describing the output shape of the pipeline
     * @return The output shape of the pipeline
     */
    StreamShape getOutputShape();

    /**
     * Get the combined stream and operation flags for the output of the pipeline.  This will incorporate
     * stream flags from the stream source, all the intermediate operations and the terminal operation.
     *
     * @return the combined stream and operation flags for the output of the pipeline
     * @see StreamOpFlag
     */
    int getStreamAndOpFlags();

    /**
     * Get the operation flags for the terminal operation.
     *
     * @return the operation flags for the terminal operation.
     * @see StreamOpFlag
     */
    // @@@ Specifying this concisely is somewhat complicated since since the actual terminal operation flags
    //     are masked by StreamOpFlag.UPSTREAM_TERMINAL_OP_MASK as the flags propagate upstream through parallel
    //     pipeline chunks
    int getTerminalOpFlags();

    /**
     * Returns whether this pipeline is parallel or sequential
     *
     * @return true if the pipeline is a parallel pipeline, otherwise false
     */
    boolean isParallel();

    /**
     * Get the {@code Spliterator} for the source of the pipeline.  This {@code Spliterator} reflects
     * only the source elements, not the actions of any of the intermediate stages.
     *
     * @return the source spliterator
     */
    Spliterator<P_IN> sourceSpliterator();

    /**
     * Returns the exact output size of the portion of the output resulting from applying the pipeline stages
     * described by this {@code PipelineHelper} to the the portion of the input described by the provided
     * {@code Spliterator}, if known.  If not known or known infinite, will return {@code -1}.
     *
     * @apiNote
     * The exact output size is known if the {@code Spliterator} has the {@code SIZED} characteristic,
     * and the operation flags {@link StreamOpFlag#SIZED} is known on the combined stream and operation
     * flags.
     *
     * @param spliterator the spliterator describing the relevant portion of the source data
     * @return the exact size if known, or -1 if infinite or unknown
     */
    long exactOutputSizeIfKnown(Spliterator<P_IN> spliterator);

    /**
     * Apply the pipeline stages described by this {@code PipelineHelper} to the provided {@code Spliterator} and
     * send the results to the provided {@code Sink}.
     *
     * @implSpec
     * The implementation behaves as if:
     * <pre>
     *     intoWrapped(wrapSink(sink), spliterator);
     * </pre>
     *
     * @param sink the {@code Sink} to receive the results
     * @param spliterator the spliterator describing the portion of the source input to process
     */
    <S extends Sink<P_OUT>> S into(S sink, Spliterator<P_IN> spliterator);

    /**
     * Push elements obtained from the {@code Spliterator} into the provided {@code Sink}.  If the stream
     * pipeline is known to have short-circuiting stages in it (see {@link StreamOpFlag#SHORT_CIRCUIT}), then the
     * elements are delivered as per {@link #intoWrappedWithCancel(Sink, Spliterator)}.
     *
     * @implSpec
     * This method conforms to the {@code Sink} protocol of calling {@code Sink.begin} before pushing
     * elements, via {@code Sink.accept}, and calling {@code Sink.end} after all elements have
     * been pushed.
     *
     * @param wrappedSink the destination {@code Sink}
     * @param spliterator the source {@code Spliterator}
     */
    void intoWrapped(Sink<P_IN> wrappedSink, Spliterator<P_IN> spliterator);

    /**
     * Push elements obtained from the {@code Spliterator} into the provided {@code Sink}, checking
     * {@link Sink#cancellationRequested()} after each element, and stopping if cancellation is requested.
     *
     * @implSpec
     * This method conforms to the {@code Sink} protocol of calling {@code Sink.begin} before pushing
     * elements, via {@code Sink.accept}, and calling {@code Sink.end} after all elements have
     * been pushed or if cancellation is requested.
     *
     * @param wrappedSink the destination {@code Sink}
     * @param spliterator the source {@code Spliterator}
     */
    void intoWrappedWithCancel(Sink<P_IN> wrappedSink, Spliterator<P_IN> spliterator);

    /**
     * Take a {@code Sink} that accepts elements of the output type of the {@code PipelineHelper}, and wrap it with
     * a {@code Sink} that accepts elements of the input type and implements all the intermediate operations described
     * by this {@code PipelineHelper}, delivering the result into the provided {@code Sink}.
     *
     * @param sink the {@code Sink} to receive the results
     * @return a {@code Sink} that implements the pipeline stages and sends results to the provided {@code Sink}
     */
    Sink<P_IN> wrapSink(Sink<P_OUT> sink);

    /**
     * Construct a @{link Node.Builder} compatible with the output shape of this {@code PipelineHelper}
     *
     * @param exactSizeIfKnown if >=0 then a builder will be created that has a fixed capacity of exactly
     *                         sizeIfKnown elements; if < 0 then the builder has variable capacity.
     *                         A fixed capacity builder will fail if an element is added and the builder has reached
     *                         capacity.
     * @return A {@code Node.Builder} compatible with the output shape of this {@code PipelineHelper}
     */
    Node.Builder<P_OUT> makeNodeBuilder(long exactSizeIfKnown);

    /**
     * Collect all output elements resulting from applying the pipeline stages to the source {@code Spliterator}
     * into a {@code Node}.
     *
     * @implSpec
     * If the pipeline has no intermediate operations and the source is backed by a {@code Node} then
     * that {@code Node} will be returned or flattened and then returned. This reduces copying for a pipeline
     * consisting of a stateful operation followed by a terminal operation that returns an array, such as:
     * <pre>{@code
     *     stream.sorted().toArray();
     * }</pre>
     *
     * @param flatten if true and the pipeline is a parallel pipeline then the {@code Node} returned
     *                will contain no children, otherwise the {@code Node} may represent the root in a
     *                tree that reflects the shape of the computation tree.
     * @return the {@code Node} containing all output elements
     */
    Node<P_OUT> collectOutput(boolean flatten);

    /**
     * Get an array factory associated with the output type of this pipeline.
     *
     * @return a factory for arrays of the output type of this pipeline.
     */
    IntFunction<P_OUT[]> arrayGenerator();
}