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package java.util.stream;
import java.util.Spliterator;
import java.util.concurrent.CountedCompleter;
import java.util.concurrent.ForkJoinPool;
/**
* Abstract base class for most fork-join tasks used to implement stream ops.
* Manages splitting logic, tracking of child tasks, and intermediate results.
* Each task is associated with a {@link Spliterator} that describes the portion
* of the input associated with the subtree rooted at this task.
* Tasks may be leaf nodes (which will traverse the elements of
* the {@code Spliterator}) or internal nodes (which split the
* {@code Spliterator} into multiple child tasks).
*
* @implNote
* <p>This class is based on {@link CountedCompleter}, a form of fork-join task
* where each task has a semaphore-like count of uncompleted children, and the
* task is implicitly completed and notified when its last child completes.
* Internal node tasks will likely override the {@code onCompletion} method from
* {@code CountedCompleter} to merge the results from child tasks into the
* current task's result.
*
* <p>Splitting and setting up the child task links is done by {@code compute()}
* for internal nodes. At {@code compute()} time for leaf nodes, it is
* guaranteed that the parent's child-related fields (including sibling links
* for the parent's children) will be set up for all children.
*
* <p>For example, a task that performs a reduce would override {@code doLeaf()}
* to perform a reduction on that leaf node's chunk using the
* {@code Spliterator}, and override {@code onCompletion()} to merge the results
* of the child tasks for internal nodes:
*
* <pre>{@code
* protected S doLeaf() {
* spliterator.forEach(...);
* return localReductionResult;
* }
*
* public void onCompletion(CountedCompleter caller) {
* if (!isLeaf()) {
* ReduceTask<P_IN, P_OUT, T, R> child = children;
* R result = child.getLocalResult();
* child = child.nextSibling;
* for (; child != null; child = child.nextSibling)
* result = combine(result, child.getLocalResult());
* setLocalResult(result);
* }
* }
* }</pre>
*
* <p>Serialization is not supported as there is no intention to serialize
* tasks managed by stream ops.
*
* @param <P_IN> Type of elements input to the pipeline
* @param <P_OUT> Type of elements output from the pipeline
* @param <R> Type of intermediate result, which may be different from operation
* result type
* @param <K> Type of parent, child and sibling tasks
* @since 1.8
*/
@SuppressWarnings("serial")
abstract class AbstractTask<P_IN, P_OUT, R,
K extends AbstractTask<P_IN, P_OUT, R, K>>
extends CountedCompleter<R> {
/**
* Default target factor of leaf tasks for parallel decomposition.
* To allow load balancing, we over-partition, currently to approximately
* four tasks per processor, which enables others to help out
* if leaf tasks are uneven or some processors are otherwise busy.
*/
static final int LEAF_TARGET = ForkJoinPool.getCommonPoolParallelism() << 2;
/** The pipeline helper, common to all tasks in a computation */
protected final PipelineHelper<P_OUT> helper;
/**
* The spliterator for the portion of the input associated with the subtree
* rooted at this task
*/
protected Spliterator<P_IN> spliterator;
/** Target leaf size, common to all tasks in a computation */
protected long targetSize; // may be laziliy initialized
/**
* The left child.
* null if no children
* if non-null rightChild is non-null
*/
protected K leftChild;
/**
* The right child.
* null if no children
* if non-null leftChild is non-null
*/
protected K rightChild;
/** The result of this node, if completed */
private R localResult;
/**
* Constructor for root nodes.
*
* @param helper The {@code PipelineHelper} describing the stream pipeline
* up to this operation
* @param spliterator The {@code Spliterator} describing the source for this
* pipeline
*/
protected AbstractTask(PipelineHelper<P_OUT> helper,
Spliterator<P_IN> spliterator) {
super(null);
this.helper = helper;
this.spliterator = spliterator;
this.targetSize = 0L;
}
/**
* Constructor for non-root nodes.
*
* @param parent this node's parent task
* @param spliterator {@code Spliterator} describing the subtree rooted at
* this node, obtained by splitting the parent {@code Spliterator}
*/
protected AbstractTask(K parent,
Spliterator<P_IN> spliterator) {
super(parent);
this.spliterator = spliterator;
this.helper = parent.helper;
this.targetSize = parent.targetSize;
}
/**
* Constructs a new node of type T whose parent is the receiver; must call
* the AbstractTask(T, Spliterator) constructor with the receiver and the
* provided Spliterator.
*
* @param spliterator {@code Spliterator} describing the subtree rooted at
* this node, obtained by splitting the parent {@code Spliterator}
* @return newly constructed child node
*/
protected abstract K makeChild(Spliterator<P_IN> spliterator);
/**
* Computes the result associated with a leaf node. Will be called by
* {@code compute()} and the result passed to @{code setLocalResult()}
*
* @return the computed result of a leaf node
*/
protected abstract R doLeaf();
/**
* Returns a suggested target leaf size based on the initial size estimate.
*
* @return suggested target leaf size
*/
public static long suggestTargetSize(long sizeEstimate) {
long est = sizeEstimate / LEAF_TARGET;
return est > 0L ? est : 1L;
}
/**
* Returns the targetSize, initializing it via the supplied
* size estimate if not already initialized.
*/
protected final long getTargetSize(long sizeEstimate) {
long s;
return ((s = targetSize) != 0 ? s :
(targetSize = suggestTargetSize(sizeEstimate)));
}
/**
* Returns the local result, if any. Subclasses should use
* {@link #setLocalResult(Object)} and {@link #getLocalResult()} to manage
* results. This returns the local result so that calls from within the
* fork-join framework will return the correct result.
*
* @return local result for this node previously stored with
* {@link #setLocalResult}
*/
@Override
public R getRawResult() {
return localResult;
}
/**
* Does nothing; instead, subclasses should use
* {@link #setLocalResult(Object)}} to manage results.
*
* @param result must be null, or an exception is thrown (this is a safety
* tripwire to detect when {@code setRawResult()} is being used
* instead of {@code setLocalResult()}
*/
@Override
protected void setRawResult(R result) {
if (result != null)
throw new IllegalStateException();
}
/**
* Retrieves a result previously stored with {@link #setLocalResult}
*
* @return local result for this node previously stored with
* {@link #setLocalResult}
*/
protected R getLocalResult() {
return localResult;
}
/**
* Associates the result with the task, can be retrieved with
* {@link #getLocalResult}
*
* @param localResult local result for this node
*/
protected void setLocalResult(R localResult) {
this.localResult = localResult;
}
/**
* Indicates whether this task is a leaf node. (Only valid after
* {@link #compute} has been called on this node). If the node is not a
* leaf node, then children will be non-null and numChildren will be
* positive.
*
* @return {@code true} if this task is a leaf node
*/
protected boolean isLeaf() {
return leftChild == null;
}
/**
* Indicates whether this task is the root node
*
* @return {@code true} if this task is the root node.
*/
protected boolean isRoot() {
return getParent() == null;
}
/**
* Returns the parent of this task, or null if this task is the root
*
* @return the parent of this task, or null if this task is the root
*/
@SuppressWarnings("unchecked")
protected K getParent() {
return (K) getCompleter();
}
/**
* Decides whether or not to split a task further or compute it
* directly. If computing directly, calls {@code doLeaf} and pass
* the result to {@code setRawResult}. Otherwise splits off
* subtasks, forking one and continuing as the other.
*
* <p> The method is structured to conserve resources across a
* range of uses. The loop continues with one of the child tasks
* when split, to avoid deep recursion. To cope with spliterators
* that may be systematically biased toward left-heavy or
* right-heavy splits, we alternate which child is forked versus
* continued in the loop.
*/
@Override
public void compute() {
Spliterator<P_IN> rs = spliterator, ls; // right, left spliterators
long sizeEstimate = rs.estimateSize();
long sizeThreshold = getTargetSize(sizeEstimate);
boolean forkRight = false;
@SuppressWarnings("unchecked") K task = (K) this;
while (sizeEstimate > sizeThreshold && (ls = rs.trySplit()) != null) {
K leftChild, rightChild, taskToFork;
task.leftChild = leftChild = task.makeChild(ls);
task.rightChild = rightChild = task.makeChild(rs);
task.setPendingCount(1);
if (forkRight) {
forkRight = false;
rs = ls;
task = leftChild;
taskToFork = rightChild;
}
else {
forkRight = true;
task = rightChild;
taskToFork = leftChild;
}
taskToFork.fork();
sizeEstimate = rs.estimateSize();
}
task.setLocalResult(task.doLeaf());
task.tryComplete();
}
/**
* {@inheritDoc}
*
* @implNote
* Clears spliterator and children fields. Overriders MUST call
* {@code super.onCompletion} as the last thing they do if they want these
* cleared.
*/
@Override
public void onCompletion(CountedCompleter<?> caller) {
spliterator = null;
leftChild = rightChild = null;
}
/**
* Returns whether this node is a "leftmost" node -- whether the path from
* the root to this node involves only traversing leftmost child links. For
* a leaf node, this means it is the first leaf node in the encounter order.
*
* @return {@code true} if this node is a "leftmost" node
*/
protected boolean isLeftmostNode() {
@SuppressWarnings("unchecked")
K node = (K) this;
while (node != null) {
K parent = node.getParent();
if (parent != null && parent.leftChild != node)
return false;
node = parent;
}
return true;
}
}