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regarding copyright ownership.  The ASF licenses this file
to you 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.apache.edgent.topology;

import java.util.List;

import org.apache.edgent.function.BiFunction;
import org.apache.edgent.function.Function;

/**
 * Partitioned window of tuples. Logically a window
 * represents an continuously updated ordered list of tuples according to the
 * criteria that created it. For example {@link TStream#last(int, Function) s.last(10, zero())}
 * declares a window with a single partition that at any time contains the last ten tuples seen on
 * stream {@code s}.
 * <P>
 * Windows are partitioned which means the window's configuration
 * is independently maintained for each key seen on the stream.
 * For example with a window created using {@link TStream#last(int, Function) last(3, tuple -> tuple.getId())}
 * then each key has its own window containing the last
 * three tuples with the same key obtained from the tuple's identity using {@code getId()}.
 * </P>
 *
 * @param <T> Tuple type
 * @param <K> Partition key type
 * 
 * @see TStream#last(int, Function) Count based window
 * @see TStream#last(long, java.util.concurrent.TimeUnit, Function) Time based window
 */
public interface TWindow<T, K> extends TopologyElement {
    /**
     * Declares a stream that is a continuous, sliding, aggregation of
     * partitions in this window.
     * <P>
     * Changes in a partition's contents trigger an invocation of
     * {@code aggregator.apply(tuples, key)}, where {@code tuples} is
     * a {@code List<T>} containing all the tuples in the partition in
     * insertion order from oldest to newest.  The list is stable
     * during the aggregator invocation.
     * <UL>
     * <LI>Count-based window: the aggregator is called after each
     * tuple added to a partition.  When an addition results in a tuple
     * being evicted, the eviction occurs before the aggregator is called.</LI>
     * <LI>Time-based window: the aggregator is called after each tuple
     * added to a partition. The aggregator is also called
     * each time one or more tuples are evicted from a partition 
     * (multiple tuples may be evicted at once).  The list will be
     * empty if the eviction results in an empty partition.</LI>
     * </UL>
     * A non-null {@code aggregator} result is added to the returned stream.
     * </P>
     * <P>
     * Thus the returned stream will contain a sequence of tuples where the
     * most recent tuple represents the most up to date aggregation of a
     * partition.
     *
     * @param <U> Tuple type
     * @param aggregator
     *            Logic to aggregation a partition.
     * @return A stream that contains the latest aggregations of partitions in this window.
     */
    <U> TStream<U> aggregate(BiFunction<List<T>, K, U> aggregator);
    
    /**
     * Declares a stream that represents a batched aggregation of
     * partitions in this window. 
     * <P>
     * Each partition "batch" triggers an invocation of
     * {@code batcher.apply(tuples, key)}, where {@code tuples} is
     * a {@code List<T>} containing all the tuples in the partition in
     * insertion order from oldest to newest  The list is stable
     * during the batcher invocation.
     * <UL>
     * <LI>Count-based window: a batch occurs when the partition is full.</LI>
     * <LI>Time-based window: a batch occurs every "time" period units.  The
     * list will be empty if no tuples have been received during the period.</LI>
     * </UL>
     * A non-null {@code batcher} result is added to the returned stream.
     * The partition's contents are cleared after a batch is processed.
     * </P>
     * <P>
     * Thus the returned stream will contain a sequence of tuples where the
     * most recent tuple represents the most up to date aggregation of a
     * partition.
     * 
     * @param <U> Tuple type
     * @param batcher
     *            Logic to aggregation a partition.
     * @return A stream that contains the latest aggregations of partitions in this window.
     */
    <U> TStream<U> batch(BiFunction<List<T>, K, U> batcher);
    
    /**
     * Returns the key function used to map tuples to partitions.
     * @return Key function used to map tuples to partitions.
     */
    Function<T, K> getKeyFunction();
    
    /**
     * Get the stream that feeds this window.
     * @return stream that feeds this window.
     */
    TStream<T> feeder();
}