Code:
/ 4.0 / 4.0 / untmp / DEVDIV_TFS / Dev10 / Releases / RTMRel / ndp / fx / src / Core / System / Linq / Parallel / QueryOperators / Unary / LastQueryOperator.cs / 1305376 / LastQueryOperator.cs
// ==++== // // Copyright (c) Microsoft Corporation. All rights reserved. // // ==--== // =+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+ // // LastQueryOperator.cs // //[....] // // =-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=- using System.Collections.Generic; using System.Threading; using System.Diagnostics.Contracts; namespace System.Linq.Parallel { ////// Last tries to discover the last element in the source, optionally matching a /// predicate. All partitions search in parallel, publish the greatest index for a /// candidate match, and reach a barrier. Only the partition that "wins" the ----, /// i.e. who found the candidate with the largest index, will yield an element. /// /// @ internal sealed class LastQueryOperator : UnaryQueryOperator { private readonly Func m_predicate; // The optional predicate used during the search. private readonly bool m_prematureMergeNeeded; // Whether to prematurely merge the input of this operator. //---------------------------------------------------------------------------------------- // Initializes a new last operator. // // Arguments: // child - the child whose data we will reverse // internal LastQueryOperator(IEnumerable child, Func predicate) :base(child) { Contract.Assert(child != null, "child data source cannot be null"); m_predicate = predicate; m_prematureMergeNeeded = Child.OrdinalIndexState.IsWorseThan(OrdinalIndexState.Increasing); } //--------------------------------------------------------------------------------------- // Just opens the current operator, including opening the child and wrapping it with // partitions as needed. // internal override QueryResults Open(QuerySettings settings, bool preferStriping) { QueryResults childQueryResults = Child.Open(settings, false); return new UnaryQueryOperatorResults(childQueryResults, this, settings, preferStriping); } internal override void WrapPartitionedStream ( PartitionedStream inputStream, IPartitionedStreamRecipient recipient, bool preferStriping, QuerySettings settings) { PartitionedStream intKeyStream; int partitionCount = inputStream.PartitionCount; // If the index is not at least increasing, we need to reindex. if (m_prematureMergeNeeded) { intKeyStream = ExecuteAndCollectResults(inputStream, partitionCount, Child.OutputOrdered, preferStriping, settings).GetPartitionedStream(); } else { Contract.Assert(inputStream is PartitionedStream ); intKeyStream = (PartitionedStream )(object)inputStream; } // Generate the shared data. Shared sharedLastCandidate = new Shared (-1); CountdownEvent sharedBarrier = new CountdownEvent(partitionCount); PartitionedStream outputStream = new PartitionedStream (partitionCount, intKeyStream.KeyComparer, OrdinalIndexState.Shuffled); for (int i = 0; i < partitionCount; i++) { outputStream[i] = new LastQueryOperatorEnumerator ( intKeyStream[i], m_predicate, sharedLastCandidate, sharedBarrier, settings.CancellationState.MergedCancellationToken); } recipient.Receive(outputStream); } //--------------------------------------------------------------------------------------- // Returns an enumerable that represents the query executing sequentially. // internal override IEnumerable AsSequentialQuery(CancellationToken token) { Contract.Assert(false, "This method should never be called as fallback to sequential is handled in ParallelEnumerable.First()."); throw new NotSupportedException(); } //--------------------------------------------------------------------------------------- // Whether this operator performs a premature merge. // internal override bool LimitsParallelism { get { return m_prematureMergeNeeded; } } //---------------------------------------------------------------------------------------- // The enumerator type responsible for executing the last operation. // class LastQueryOperatorEnumerator : QueryOperatorEnumerator { private QueryOperatorEnumerator m_source; // The data source to enumerate. private Func m_predicate; // The optional predicate used during the search. private bool m_alreadySearched; // Set once the enumerator has performed the search. // Data shared among partitions. private Shared m_sharedLastCandidate; // The current last candidate. private CountdownEvent m_sharedBarrier; // Shared barrier, signaled when partitions find their 1st element. private CancellationToken m_cancellationToken; // Token used to cancel this operator. //--------------------------------------------------------------------------------------- // Instantiates a new enumerator. // internal LastQueryOperatorEnumerator( QueryOperatorEnumerator source, Func predicate, Shared sharedLastCandidate, CountdownEvent sharedBarrier, CancellationToken cancelToken) { Contract.Assert(source != null); Contract.Assert(sharedLastCandidate != null); Contract.Assert(sharedBarrier != null); m_source = source; m_predicate = predicate; m_sharedLastCandidate = sharedLastCandidate; m_sharedBarrier = sharedBarrier; m_cancellationToken = cancelToken; } //---------------------------------------------------------------------------------------- // Straightforward IEnumerator methods. // internal override bool MoveNext(ref TSource currentElement, ref int currentKey) { Contract.Assert(m_source != null); if (m_alreadySearched) { return false; } // Look for the greatest element. TSource candidate = default(TSource); int candidateIndex = -1; try { TSource current = default(TSource); int key = default(int); int loopCount = 0; //counter to help with cancellation while (m_source.MoveNext(ref current, ref key)) { if ((loopCount & CancellationState.POLL_INTERVAL) == 0) CancellationState.ThrowIfCanceled(m_cancellationToken); // If the predicate is null or the current element satisfies it, we will remember // it as the current partition's candidate for the last element, and move on. if (m_predicate == null || m_predicate(current)) { candidate = current; candidateIndex = key; } loopCount++; } // If we found a candidate element, try to publish it, so long as it's greater. if (candidateIndex != -1) { int observedSharedIndex; do { observedSharedIndex = m_sharedLastCandidate.Value; } while ((observedSharedIndex == -1 || candidateIndex > observedSharedIndex) && Interlocked.CompareExchange(ref m_sharedLastCandidate.Value, candidateIndex, observedSharedIndex) != observedSharedIndex); } } finally { // No matter whether we exit due to an exception or normal completion, we must ensure // that we signal other partitions that we have completed. Otherwise, we can cause deadlocks. m_sharedBarrier.Signal(); } m_alreadySearched = true; // Only if we have a candidate do we wait. if (candidateIndex != -1) { m_sharedBarrier.Wait(m_cancellationToken); // Now re-read the shared index. If it's the same as ours, we won and return true. if (m_sharedLastCandidate.Value == candidateIndex) { currentElement = candidate; currentKey = 0; // 1st (and only) element, so we hardcode the output index to 0. return true; } } // If we got here, we didn't win. Return false. return false; } protected override void Dispose(bool disposing) { m_source.Dispose(); } } } } // File provided for Reference Use Only by Microsoft Corporation (c) 2007.
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