Code:
/ 4.0 / 4.0 / untmp / DEVDIV_TFS / Dev10 / Releases / RTMRel / ndp / fx / src / Core / System / Linq / Parallel / QueryOperators / Unary / FirstQueryOperator.cs / 1305376 / FirstQueryOperator.cs
// ==++== // // Copyright (c) Microsoft Corporation. All rights reserved. // // ==--== // =+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+ // // FirstQueryOperator.cs // //[....] // // =-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=- using System.Collections.Generic; using System.Diagnostics.Contracts; using System.Threading; namespace System.Linq.Parallel { ////// First tries to discover the first element in the source, optionally matching a /// predicate. All partitions search in parallel, publish the lowest index for a /// candidate match, and reach a barrier. Only the partition that "wins" the ----, /// i.e. who found the candidate with the smallest index, will yield an element. /// ///internal sealed class FirstQueryOperator : 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 first operator. // // Arguments: // child - the child whose data we will reverse // internal FirstQueryOperator(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) { // We just open the child operator. 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) { OrdinalIndexState inputIndexState = inputStream.OrdinalIndexState; PartitionedStream intKeyStream; int partitionCount = inputStream.PartitionCount; // If the index is not at least increasing, we need to reindex. if (m_prematureMergeNeeded) { ListQueryResults listResults = ExecuteAndCollectResults(inputStream, partitionCount, Child.OutputOrdered, preferStriping, settings); intKeyStream = listResults.GetPartitionedStream(); } else { Contract.Assert(typeof(TKey) == typeof(int)); intKeyStream = (PartitionedStream )(object)inputStream; } // Generate the shared data. Shared sharedFirstCandidate = new Shared (-1); CountdownEvent sharedBarrier = new CountdownEvent(partitionCount); PartitionedStream outputStream = new PartitionedStream ( partitionCount, Util.GetDefaultComparer (), OrdinalIndexState.Shuffled); for (int i = 0; i < partitionCount; i++) { outputStream[i] = new FirstQueryOperatorEnumerator( intKeyStream[i], m_predicate, sharedFirstCandidate, 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 first operation. // class FirstQueryOperatorEnumerator : 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_sharedFirstCandidate; // The current first 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 FirstQueryOperatorEnumerator( QueryOperatorEnumerator source, Func predicate, Shared sharedFirstCandidate, CountdownEvent sharedBarrier, CancellationToken cancellationToken) { Contract.Assert(source != null); Contract.Assert(sharedFirstCandidate != null); Contract.Assert(sharedBarrier != null); m_source = source; m_predicate = predicate; m_sharedFirstCandidate = sharedFirstCandidate; m_sharedBarrier = sharedBarrier; m_cancellationToken = cancellationToken; } //---------------------------------------------------------------------------------------- // 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 lowest element. TSource candidate = default(TSource); int candidateIndex = -1; try { int key = default(int); int i = 0; while (m_source.MoveNext(ref candidate, ref key)) { if ((i++ & CancellationState.POLL_INTERVAL) == 0) CancellationState.ThrowIfCanceled(m_cancellationToken); // If the predicate is null or the current element satisfies it, we have found the // current partition's "candidate" for the first element. Note it. if (m_predicate == null || m_predicate(candidate)) { candidateIndex = key; // Try to swap our index with the shared one, so long as it's smaller. int observedSharedIndex; do { observedSharedIndex = m_sharedFirstCandidate.Value; } while ((observedSharedIndex == -1 || candidateIndex < observedSharedIndex) && Interlocked.CompareExchange(ref m_sharedFirstCandidate.Value, candidateIndex, observedSharedIndex) != observedSharedIndex); break; } else if (m_sharedFirstCandidate.Value != -1 && key > m_sharedFirstCandidate.Value) { // We've scanned past another partition's best element. Bail. break; } } } 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 might be 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_sharedFirstCandidate.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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