Code:
/ 4.0 / 4.0 / untmp / DEVDIV_TFS / Dev10 / Releases / RTMRel / ndp / fx / src / Core / System / Linq / Parallel / QueryOperators / Unary / DefaultIfEmptyQueryOperator.cs / 1305376 / DefaultIfEmptyQueryOperator.cs
// ==++== // // Copyright (c) Microsoft Corporation. All rights reserved. // // ==--== // =+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+ // // DefaultIfEmptyQueryOperator.cs // //[....] // // =-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=- using System.Collections.Generic; using System.Threading; using System.Diagnostics.Contracts; namespace System.Linq.Parallel { ////// This operator just exposes elements directly from the underlying data source, if /// it's not empty, or yields a single default element if the data source is empty. /// There is a minimal amount of synchronization at the beginning, until all partitions /// have registered whether their stream is empty or not. Once the 0th partition knows /// that at least one other partition is non-empty, it may proceed. Otherwise, it is /// the 0th partition which yields the default value. /// ///internal sealed class DefaultIfEmptyQueryOperator : UnaryQueryOperator { private readonly TSource m_defaultValue; // The default value to use (if empty). //---------------------------------------------------------------------------------------- // Initializes a new reverse operator. // // Arguments: // child - the child whose data we will reverse // internal DefaultIfEmptyQueryOperator(IEnumerable child, TSource defaultValue) :base(child) { Contract.Assert(child != null, "child data source cannot be null"); m_defaultValue = defaultValue; SetOrdinalIndexState(ExchangeUtilities.Worse(Child.OrdinalIndexState, OrdinalIndexState.Correct)); } //--------------------------------------------------------------------------------------- // Just opens the current operator, including opening the child and wrapping it with // partitions as needed. // internal override QueryResults Open(QuerySettings settings, bool preferStriping) { // We just open the child operator. QueryResults childQueryResults = Child.Open(settings, preferStriping); return new UnaryQueryOperatorResults(childQueryResults, this, settings, preferStriping); } internal override void WrapPartitionedStream ( PartitionedStream inputStream, IPartitionedStreamRecipient recipient, bool preferStriping, QuerySettings settings) { int partitionCount = inputStream.PartitionCount; // Generate the shared data. Shared sharedEmptyCount = new Shared (0); CountdownEvent sharedLatch = new CountdownEvent(partitionCount - 1); PartitionedStream outputStream = new PartitionedStream (partitionCount, inputStream.KeyComparer, OrdinalIndexState); for (int i = 0; i < partitionCount; i++) { outputStream[i] = new DefaultIfEmptyQueryOperatorEnumerator ( inputStream[i], m_defaultValue, i, partitionCount, sharedEmptyCount, sharedLatch, settings.CancellationState.MergedCancellationToken); } recipient.Receive(outputStream); } //--------------------------------------------------------------------------------------- // Returns an enumerable that represents the query executing sequentially. // internal override IEnumerable AsSequentialQuery(CancellationToken token) { return Child.AsSequentialQuery(token).DefaultIfEmpty(m_defaultValue); } //--------------------------------------------------------------------------------------- // Whether this operator performs a premature merge. // internal override bool LimitsParallelism { get { return false; } } //---------------------------------------------------------------------------------------- // The enumerator type responsible for executing the default-if-empty operation. // class DefaultIfEmptyQueryOperatorEnumerator : QueryOperatorEnumerator { private QueryOperatorEnumerator m_source; // The data source to enumerate. private bool m_lookedForEmpty; // Whether this partition has looked for empty yet. private int m_partitionIndex; // This enumerator's partition index. private int m_partitionCount; // The number of partitions. private TSource m_defaultValue; // The default value if the 0th partition is empty. // Data shared among partitions. private Shared m_sharedEmptyCount; // The number of empty partitions. private CountdownEvent m_sharedLatch; // Shared latch, signaled when partitions process the 1st item. private CancellationToken m_cancelToken; // Token used to cancel this operator. //--------------------------------------------------------------------------------------- // Instantiates a new select enumerator. // internal DefaultIfEmptyQueryOperatorEnumerator( QueryOperatorEnumerator source, TSource defaultValue, int partitionIndex, int partitionCount, Shared sharedEmptyCount, CountdownEvent sharedLatch, CancellationToken cancelToken) { Contract.Assert(source != null); Contract.Assert(0 <= partitionIndex && partitionIndex < partitionCount); Contract.Assert(partitionCount > 0); Contract.Assert(sharedEmptyCount != null); Contract.Assert(sharedLatch != null); m_source = source; m_defaultValue = defaultValue; m_partitionIndex = partitionIndex; m_partitionCount = partitionCount; m_sharedEmptyCount = sharedEmptyCount; m_sharedLatch = sharedLatch; m_cancelToken = cancelToken; } //---------------------------------------------------------------------------------------- // Straightforward IEnumerator methods. // internal override bool MoveNext(ref TSource currentElement, ref TKey currentKey) { Contract.Assert(m_source != null); bool moveNextResult = m_source.MoveNext(ref currentElement, ref currentKey); // There is special logic the first time this function is called. if (!m_lookedForEmpty) { // Ensure we don't enter this loop again. m_lookedForEmpty = true; if (!moveNextResult) { if (m_partitionIndex == 0) { // If this is the 0th partition, we must wait for all others. Note: we could // actually do a wait-any here: if at least one other partition finds an element, // there is strictly no need to wait. But this would require extra coordination // which may or may not be worth the trouble. m_sharedLatch.Wait(m_cancelToken); m_sharedLatch.Dispose(); // Now see if there were any other partitions with data. if (m_sharedEmptyCount.Value == m_partitionCount - 1) { // No data, we will yield the default value. currentElement = m_defaultValue; currentKey = default(TKey); return true; } else { // Another partition has data, we are done. return false; } } else { // Not the 0th partition, we will increment the shared empty counter. Interlocked.Increment(ref m_sharedEmptyCount.Value); } } // Every partition (but the 0th) will signal the latch the first time. if (m_partitionIndex != 0) { m_sharedLatch.Signal(); } } return moveNextResult; } protected override void Dispose(bool disposing) { m_source.Dispose(); } } } } // File provided for Reference Use Only by Microsoft Corporation (c) 2007.
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