1 /*
   2  * Copyright (c) 2012, 2013, Oracle and/or its affiliates. All rights reserved.
   3  * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
   4  *
   5  * This code is free software; you can redistribute it and/or modify it
   6  * under the terms of the GNU General Public License version 2 only, as
   7  * published by the Free Software Foundation.  Oracle designates this
   8  * particular file as subject to the "Classpath" exception as provided
   9  * by Oracle in the LICENSE file that accompanied this code.
  10  *
  11  * This code is distributed in the hope that it will be useful, but WITHOUT
  12  * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
  13  * FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public License
  14  * version 2 for more details (a copy is included in the LICENSE file that
  15  * accompanied this code).
  16  *
  17  * You should have received a copy of the GNU General Public License version
  18  * 2 along with this work; if not, write to the Free Software Foundation,
  19  * Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA.
  20  *
  21  * Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA
  22  * or visit www.oracle.com if you need additional information or have any
  23  * questions.
  24  */
  25 package java.util.stream;
  26 
  27 import java.util.Spliterator;
  28 import java.util.concurrent.CountedCompleter;
  29 import java.util.concurrent.ForkJoinPool;
  30 
  31 /**
  32  * Abstract base class for most fork-join tasks used to implement stream ops.
  33  * Manages splitting logic, tracking of child tasks, and intermediate results.
  34  * Each task is associated with a {@link Spliterator} that describes the portion
  35  * of the input associated with the subtree rooted at this task.
  36  * Tasks may be leaf nodes (which will traverse the elements of
  37  * the {@code Spliterator}) or internal nodes (which split the
  38  * {@code Spliterator} into multiple child tasks).
  39  *
  40  * @implNote
  41  * <p>This class is based on {@link CountedCompleter}, a form of fork-join task
  42  * where each task has a semaphore-like count of uncompleted children, and the
  43  * task is implicitly completed and notified when its last child completes.
  44  * Internal node tasks will likely override the {@code onCompletion} method from
  45  * {@code CountedCompleter} to merge the results from child tasks into the
  46  * current task's result.
  47  *
  48  * <p>Splitting and setting up the child task links is done by {@code compute()}
  49  * for internal nodes.  At {@code compute()} time for leaf nodes, it is
  50  * guaranteed that the parent's child-related fields (including sibling links
  51  * for the parent's children) will be set up for all children.
  52  *
  53  * <p>For example, a task that performs a reduce would override {@code doLeaf()}
  54  * to perform a reduction on that leaf node's chunk using the
  55  * {@code Spliterator}, and override {@code onCompletion()} to merge the results
  56  * of the child tasks for internal nodes:
  57  *
  58  * <pre>{@code
  59  *     protected S doLeaf() {
  60  *         spliterator.forEach(...);
  61  *         return localReductionResult;
  62  *     }
  63  *
  64  *     public void onCompletion(CountedCompleter caller) {
  65  *         if (!isLeaf()) {
  66  *             ReduceTask<P_IN, P_OUT, T, R> child = children;
  67  *             R result = child.getLocalResult();
  68  *             child = child.nextSibling;
  69  *             for (; child != null; child = child.nextSibling)
  70  *                 result = combine(result, child.getLocalResult());
  71  *             setLocalResult(result);
  72  *         }
  73  *     }
  74  * }</pre>
  75  *
  76  * <p>Serialization is not supported as there is no intention to serialize
  77  * tasks managed by stream ops.
  78  *
  79  * @param <P_IN> Type of elements input to the pipeline
  80  * @param <P_OUT> Type of elements output from the pipeline
  81  * @param <R> Type of intermediate result, which may be different from operation
  82  *        result type
  83  * @param <K> Type of parent, child and sibling tasks
  84  * @since 1.8
  85  */
  86 @SuppressWarnings("serial")
  87 abstract class AbstractTask<P_IN, P_OUT, R,
  88                             K extends AbstractTask<P_IN, P_OUT, R, K>>
  89         extends CountedCompleter<R> {
  90 
  91     /**
  92      * Default target factor of leaf tasks for parallel decomposition.
  93      * To allow load balancing, we over-partition, currently to approximately
  94      * four tasks per processor, which enables others to help out
  95      * if leaf tasks are uneven or some processors are otherwise busy.
  96      */
  97     static final int LEAF_TARGET = ForkJoinPool.getCommonPoolParallelism() << 2;
  98 
  99     /** The pipeline helper, common to all tasks in a computation */
 100     protected final PipelineHelper<P_OUT> helper;
 101 
 102     /**
 103      * The spliterator for the portion of the input associated with the subtree
 104      * rooted at this task
 105      */
 106     protected Spliterator<P_IN> spliterator;
 107 
 108     /** Target leaf size, common to all tasks in a computation */
 109     protected long targetSize; // may be lazily initialized
 110 
 111     /**
 112      * The left child.
 113      * null if no children
 114      * if non-null rightChild is non-null
 115      */
 116     protected K leftChild;
 117 
 118     /**
 119      * The right child.
 120      * null if no children
 121      * if non-null leftChild is non-null
 122      */
 123     protected K rightChild;
 124 
 125     /** The result of this node, if completed */
 126     private R localResult;
 127 
 128     /**
 129      * Constructor for root nodes.
 130      *
 131      * @param helper The {@code PipelineHelper} describing the stream pipeline
 132      *               up to this operation
 133      * @param spliterator The {@code Spliterator} describing the source for this
 134      *                    pipeline
 135      */
 136     protected AbstractTask(PipelineHelper<P_OUT> helper,
 137                            Spliterator<P_IN> spliterator) {
 138         super(null);
 139         this.helper = helper;
 140         this.spliterator = spliterator;
 141         this.targetSize = 0L;
 142     }
 143 
 144     /**
 145      * Constructor for non-root nodes.
 146      *
 147      * @param parent this node's parent task
 148      * @param spliterator {@code Spliterator} describing the subtree rooted at
 149      *        this node, obtained by splitting the parent {@code Spliterator}
 150      */
 151     protected AbstractTask(K parent,
 152                            Spliterator<P_IN> spliterator) {
 153         super(parent);
 154         this.spliterator = spliterator;
 155         this.helper = parent.helper;
 156         this.targetSize = parent.targetSize;
 157     }
 158 
 159     /**
 160      * Constructs a new node of type T whose parent is the receiver; must call
 161      * the AbstractTask(T, Spliterator) constructor with the receiver and the
 162      * provided Spliterator.
 163      *
 164      * @param spliterator {@code Spliterator} describing the subtree rooted at
 165      *        this node, obtained by splitting the parent {@code Spliterator}
 166      * @return newly constructed child node
 167      */
 168     protected abstract K makeChild(Spliterator<P_IN> spliterator);
 169 
 170     /**
 171      * Computes the result associated with a leaf node.  Will be called by
 172      * {@code compute()} and the result passed to @{code setLocalResult()}
 173      *
 174      * @return the computed result of a leaf node
 175      */
 176     protected abstract R doLeaf();
 177 
 178     /**
 179      * Returns a suggested target leaf size based on the initial size estimate.
 180      *
 181      * @return suggested target leaf size
 182      */
 183     public static long suggestTargetSize(long sizeEstimate) {
 184         long est = sizeEstimate / LEAF_TARGET;
 185         return est > 0L ? est : 1L;
 186     }
 187 
 188     /**
 189      * Returns the targetSize, initializing it via the supplied
 190      * size estimate if not already initialized.
 191      */
 192     protected final long getTargetSize(long sizeEstimate) {
 193         long s;
 194         return ((s = targetSize) != 0 ? s :
 195                 (targetSize = suggestTargetSize(sizeEstimate)));
 196     }
 197 
 198     /**
 199      * Returns the local result, if any. Subclasses should use
 200      * {@link #setLocalResult(Object)} and {@link #getLocalResult()} to manage
 201      * results.  This returns the local result so that calls from within the
 202      * fork-join framework will return the correct result.
 203      *
 204      * @return local result for this node previously stored with
 205      * {@link #setLocalResult}
 206      */
 207     @Override
 208     public R getRawResult() {
 209         return localResult;
 210     }
 211 
 212     /**
 213      * Does nothing; instead, subclasses should use
 214      * {@link #setLocalResult(Object)}} to manage results.
 215      *
 216      * @param result must be null, or an exception is thrown (this is a safety
 217      *        tripwire to detect when {@code setRawResult()} is being used
 218      *        instead of {@code setLocalResult()}
 219      */
 220     @Override
 221     protected void setRawResult(R result) {
 222         if (result != null)
 223             throw new IllegalStateException();
 224     }
 225 
 226     /**
 227      * Retrieves a result previously stored with {@link #setLocalResult}
 228      *
 229      * @return local result for this node previously stored with
 230      * {@link #setLocalResult}
 231      */
 232     protected R getLocalResult() {
 233         return localResult;
 234     }
 235 
 236     /**
 237      * Associates the result with the task, can be retrieved with
 238      * {@link #getLocalResult}
 239      *
 240      * @param localResult local result for this node
 241      */
 242     protected void setLocalResult(R localResult) {
 243         this.localResult = localResult;
 244     }
 245 
 246     /**
 247      * Indicates whether this task is a leaf node.  (Only valid after
 248      * {@link #compute} has been called on this node).  If the node is not a
 249      * leaf node, then children will be non-null and numChildren will be
 250      * positive.
 251      *
 252      * @return {@code true} if this task is a leaf node
 253      */
 254     protected boolean isLeaf() {
 255         return leftChild == null;
 256     }
 257 
 258     /**
 259      * Indicates whether this task is the root node
 260      *
 261      * @return {@code true} if this task is the root node.
 262      */
 263     protected boolean isRoot() {
 264         return getParent() == null;
 265     }
 266 
 267     /**
 268      * Returns the parent of this task, or null if this task is the root
 269      *
 270      * @return the parent of this task, or null if this task is the root
 271      */
 272     @SuppressWarnings("unchecked")
 273     protected K getParent() {
 274         return (K) getCompleter();
 275     }
 276 
 277     /**
 278      * Decides whether or not to split a task further or compute it
 279      * directly. If computing directly, calls {@code doLeaf} and pass
 280      * the result to {@code setRawResult}. Otherwise splits off
 281      * subtasks, forking one and continuing as the other.
 282      *
 283      * <p> The method is structured to conserve resources across a
 284      * range of uses.  The loop continues with one of the child tasks
 285      * when split, to avoid deep recursion. To cope with spliterators
 286      * that may be systematically biased toward left-heavy or
 287      * right-heavy splits, we alternate which child is forked versus
 288      * continued in the loop.
 289      */
 290     @Override
 291     public void compute() {
 292         Spliterator<P_IN> rs = spliterator, ls; // right, left spliterators
 293         long sizeEstimate = rs.estimateSize();
 294         long sizeThreshold = getTargetSize(sizeEstimate);
 295         boolean forkRight = false;
 296         @SuppressWarnings("unchecked") K task = (K) this;
 297         while (sizeEstimate > sizeThreshold && (ls = rs.trySplit()) != null) {
 298             K leftChild, rightChild, taskToFork;
 299             task.leftChild  = leftChild = task.makeChild(ls);
 300             task.rightChild = rightChild = task.makeChild(rs);
 301             task.setPendingCount(1);
 302             if (forkRight) {
 303                 forkRight = false;
 304                 rs = ls;
 305                 task = leftChild;
 306                 taskToFork = rightChild;
 307             }
 308             else {
 309                 forkRight = true;
 310                 task = rightChild;
 311                 taskToFork = leftChild;
 312             }
 313             taskToFork.fork();
 314             sizeEstimate = rs.estimateSize();
 315         }
 316         task.setLocalResult(task.doLeaf());
 317         task.tryComplete();
 318     }
 319 
 320     /**
 321      * {@inheritDoc}
 322      *
 323      * @implNote
 324      * Clears spliterator and children fields.  Overriders MUST call
 325      * {@code super.onCompletion} as the last thing they do if they want these
 326      * cleared.
 327      */
 328     @Override
 329     public void onCompletion(CountedCompleter<?> caller) {
 330         spliterator = null;
 331         leftChild = rightChild = null;
 332     }
 333 
 334     /**
 335      * Returns whether this node is a "leftmost" node -- whether the path from
 336      * the root to this node involves only traversing leftmost child links.  For
 337      * a leaf node, this means it is the first leaf node in the encounter order.
 338      *
 339      * @return {@code true} if this node is a "leftmost" node
 340      */
 341     protected boolean isLeftmostNode() {
 342         @SuppressWarnings("unchecked")
 343         K node = (K) this;
 344         while (node != null) {
 345             K parent = node.getParent();
 346             if (parent != null && parent.leftChild != node)
 347                 return false;
 348             node = parent;
 349         }
 350         return true;
 351     }
 352 }