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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 (output shape, intermediate operations, stream flags, parallelism,

 * etc) in one place.

 *

 * <p>

 * A {@code PipelineHelper} describes the initial segment of a stream pipeline,

 * including its source, intermediate operations, and may additionally

 * incorporate 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, java.util.Spliterator)},

 * {@link TerminalOp#evaluateSequential(PipelineHelper, java.util.Spliterator)},

 * and {@link AbstractPipeline#opEvaluateParallel(PipelineHelper, java.util.Spliterator,

 * java.util.function.IntFunction)}, methods, which can use the

 * {@code PipelineHelper} to access information about the pipeline such as

 * input shape, output shape, stream flags, and size, and use the helper methods

 * such as {@link #wrapAndCopyInto(Sink, Spliterator)},

 * {@link #copyInto(Sink, Spliterator)}, and {@link #wrapSink(Sink)} to execute

 * pipeline operations.

 *

 * @param <P_OUT> type of output elements from the pipeline

 * @since 1.8

 */

abstract class PipelineHelper<P_OUT> {

    /**

     * Gets the combined stream and operation flags for the output of the described

     * 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

     * @see StreamOpFlag

     */

    abstract int getStreamAndOpFlags();

    /**

     * 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

     */

    abstract<P_IN> long exactOutputSizeIfKnown(Spliterator<P_IN> spliterator);

    /**

     * Applies 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>{@code

     *     intoWrapped(wrapSink(sink), spliterator);

     * }</pre>

     *

     * @param sink the {@code Sink} to receive the results

     * @param spliterator the spliterator describing the source input to process

     */

    abstract<P_IN, S extends Sink<P_OUT>> S wrapAndCopyInto(S sink, Spliterator<P_IN> spliterator);

    /**

     * Pushes 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}), the

     * {@link Sink#cancellationRequested()} is checked after each

     * element, 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.

     *

     * @param wrappedSink the destination {@code Sink}

     * @param spliterator the source {@code Spliterator}

     */

    abstract<P_IN> void copyInto(Sink<P_IN> wrappedSink, Spliterator<P_IN> spliterator);

    /**

     * Pushes 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}

     */

    abstract <P_IN> void copyIntoWithCancel(Sink<P_IN> wrappedSink, Spliterator<P_IN> spliterator);

    /**

     * Takes 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}

     */

    abstract<P_IN> Sink<P_IN> wrapSink(Sink<P_OUT> sink);

    /**

     * Constructs 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 after the builder has reached capacity.

     * @param generator a factory function for array instances

     * @return a {@code Node.Builder} compatible with the output shape of this

     *         {@code PipelineHelper}

     */

    abstract Node.Builder<P_OUT> makeNodeBuilder(long exactSizeIfKnown,

                                                 IntFunction<P_OUT[]> generator);

    /**

     * Collects all output elements resulting from applying the pipeline stages

     * to the source {@code Spliterator} into a {@code Node}.

     *

     * @implNote

     * 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 spliterator the source {@code Spliterator}

     * @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.

     * @param generator a factory function for array instances

     * @return the {@code Node} containing all output elements

     */

    abstract<P_IN> Node<P_OUT> evaluate(Spliterator<P_IN> spliterator,

                                        boolean flatten,

                                        IntFunction<P_OUT[]> generator);

}