TakeOrSkipQueryOperator.cs source code in C# .NET

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                         / 4.0 / 4.0 / untmp / DEVDIV_TFS / Dev10 / Releases / RTMRel / ndp / fx / src / Core / System / Linq / Parallel / QueryOperators / Unary / TakeOrSkipQueryOperator.cs / 1305376 / TakeOrSkipQueryOperator.cs
                        
                        
                            // ==++== 
//
//   Copyright (c) Microsoft Corporation.  All rights reserved.
//
// ==--== 
// =+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+
// 
// TakeOrSkipQueryOperator.cs 
//
// [....] 
//
// =-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-

using System.Collections.Generic; 
using System.Threading;
using System.Diagnostics.Contracts; 
 
namespace System.Linq.Parallel
{ 
    /// 
    /// Take and Skip either take or skip a specified number of elements, captured in the
    /// count argument.  These will work a little bit like TakeWhile and SkipWhile: there
    /// are two phases, (1) Search and (2) Yield.  In the search phase, our goal is to 
    /// find the 'count'th index from the input.  We do this in parallel by sharing a count-
    /// sized array.  Each thread ----s to populate the array with indices in ascending 
    /// order.  This requires synchronization for inserts.  We use a simple heap, for decent 
    /// worst case performance.  After a thread has scanned ï¿½countï¿½ elements, or its current
    /// index is greater than or equal to the maximum index in the array (and the array is 
    /// fully populated), the thread can stop searching.  All threads issue a barrier before
    /// moving to the Yield phase.  When the Yield phase is entered, the count-1th element
    /// of the array contains: in the case of Take, the maximum index (exclusive) to be
    /// returned; or in the case of Skip, the minimum index (inclusive) to be returned.  The 
    /// Yield phase simply consists of yielding these elements as output.
    ///  
    ///  
    internal sealed class TakeOrSkipQueryOperator : UnaryQueryOperator
    { 

        private readonly int m_count; // The number of elements to take or skip.
        private readonly bool m_take; // Whether to take (true) or skip (false).
        private bool m_prematureMerge = false; // Whether to prematurely merge the input of this operator. 

        //---------------------------------------------------------------------------------------- 
        // Initializes a new take-while operator. 
        //
        // Arguments: 
        //     child  - the child data source to enumerate
        //     count  - the number of elements to take or skip
        //     take   - whether this is a Take (true) or Skip (false)
        // 

        internal TakeOrSkipQueryOperator(IEnumerable child, int count, bool take) 
            :base(child) 
        {
            Contract.Assert(child != null, "child data source cannot be null"); 

            m_count = count;
            m_take = take;
 
            SetOrdinalIndexState(OutputOrdinalIndexState());
        } 
 
        /// 
        /// Determines the order index state for the output operator 
        /// 
        private OrdinalIndexState OutputOrdinalIndexState()
        {
            OrdinalIndexState indexState = Child.OrdinalIndexState; 

            if (indexState == OrdinalIndexState.Indexible) 
            { 
                return OrdinalIndexState.Indexible;
            } 

            if (indexState.IsWorseThan(OrdinalIndexState.Increasing))
            {
                m_prematureMerge = true; 
                indexState = OrdinalIndexState.Correct;
            } 
 
            // If the operator is skip and the index was correct, now it is only increasing.
            if (!m_take && indexState == OrdinalIndexState.Correct) 
            {
                indexState = OrdinalIndexState.Increasing;
            }
 
            return indexState;
        } 
 
        internal override void WrapPartitionedStream(
            PartitionedStream inputStream, IPartitionedStreamRecipient recipient, bool preferStriping, QuerySettings settings) 
        {
            Contract.Assert(Child.OrdinalIndexState != OrdinalIndexState.Indexible, "Don't take this code path if the child is indexible.");

            PartitionedStream inputIntStream; 

            // If the index is not at least increasing, we need to reindex. 
            if (m_prematureMerge) 
            {
                ListQueryResults results = ExecuteAndCollectResults( 
                    inputStream, inputStream.PartitionCount, Child.OutputOrdered, preferStriping, settings);
                inputIntStream = results.GetPartitionedStream();
            }
            else 
            {
                Contract.Assert(typeof(TKey) == typeof(int)); 
                inputIntStream = (PartitionedStream)((object)inputStream); 
            }
 
            int partitionCount = inputStream.PartitionCount;
            FixedMaxHeap sharedIndices = new FixedMaxHeap(m_count); // an array used to track the sequence of indices leading up to the Nth index
            CountdownEvent sharredBarrier = new CountdownEvent(partitionCount); // a barrier to synchronize before yielding
 
            PartitionedStream outputStream =
                new PartitionedStream(partitionCount, Util.GetDefaultComparer(), OrdinalIndexState); 
            for (int i = 0; i < partitionCount; i++) 
            {
                outputStream[i] = new TakeOrSkipQueryOperatorEnumerator( 
                    inputIntStream[i], m_count, m_take, sharedIndices, sharredBarrier, settings.CancellationState.MergedCancellationToken);
            }

            recipient.Receive(outputStream); 
        }
 
        //--------------------------------------------------------------------------------------- 
        // 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, true);
            return TakeOrSkipQueryOperatorResults.NewResults(childQueryResults, this, settings, preferStriping); 
        } 

        //--------------------------------------------------------------------------------------- 
        // Whether this operator performs a premature merge.
        //

        internal override bool LimitsParallelism 
        {
            get { return OrdinalIndexState != OrdinalIndexState.Indexible; } 
        } 

        //--------------------------------------------------------------------------------------- 
        // The enumerator type responsible for executing the Take or Skip.
        //

        class TakeOrSkipQueryOperatorEnumerator : QueryOperatorEnumerator 
        {
            private readonly QueryOperatorEnumerator m_source; // The data source to enumerate. 
            private readonly int m_count; // The number of elements to take or skip. 
            private readonly bool m_take; // Whether to execute a Take (true) or Skip (false).
 
            // These fields are all shared among partitions.
            private readonly FixedMaxHeap m_sharedIndices; // The indices shared among partitions.
            private readonly CountdownEvent m_sharedBarrier; // To separate the search/yield phases.
            private readonly CancellationToken m_cancellationToken; // Indicates that cancellation has occurred. 

            private List> m_buffer; // Our buffer. 
            private Shared m_bufferIndex; // Our current index within the buffer. [allocate in moveNext to avoid false-sharing] 

            //---------------------------------------------------------------------------------------- 
            // Instantiates a new select enumerator.
            //

            internal TakeOrSkipQueryOperatorEnumerator( 
                QueryOperatorEnumerator source, int count, bool take,
                FixedMaxHeap sharedIndices, CountdownEvent sharedBarrier, CancellationToken cancellationToken) 
            { 
                Contract.Assert(source != null);
                Contract.Assert(sharedIndices != null); 
                Contract.Assert(sharedBarrier != null);
                Contract.Assert(sharedIndices.Size == count);

                m_source = source; 
                m_count = count;
                m_take = take; 
                m_sharedIndices = sharedIndices; 
                m_sharedBarrier = sharedBarrier;
                m_cancellationToken = cancellationToken; 
            }

            //---------------------------------------------------------------------------------------
            // Straightforward IEnumerator methods. 
            //
 
            internal override bool MoveNext(ref TResult currentElement, ref int currentKey) 
            {
                Contract.Assert(m_sharedIndices != null); 

                // If the buffer has not been created, we will populate it lazily on demand.
                if (m_buffer == null && m_count > 0)
                { 

 
                    // Create a buffer, but don't publish it yet (in case of exception). 
                    List> buffer = new List>();
 
                    // Enter the search phase. In this phase, all partitions ---- to populate
                    // the shared indices with their first 'count' contiguous elements.
                    TResult current = default(TResult);
                    int index = default(int); 
                    int i = 0; //counter to help with cancellation
                    while (buffer.Count < m_count && m_source.MoveNext(ref current, ref index)) 
                    { 
                        if ((i++ & CancellationState.POLL_INTERVAL) == 0)
                            CancellationState.ThrowIfCanceled(m_cancellationToken); 

                        // Add the current element to our buffer.
                        // @
 
                        buffer.Add(new Pair(current, index));
 
                        // Now we will try to insert our index into the shared indices list, quitting if 
                        // our index is greater than all of the indices already inside it.
                        lock (m_sharedIndices) 
                        {
                            if (!m_sharedIndices.Insert(index))
                            {
                                // We have read past the maximum index. We can move to the barrier now. 
                                break;
                            } 
                        } 
                    }
 
                    // Before exiting the search phase, we will synchronize with others. This is a barrier.
                    m_sharedBarrier.Signal();
                    m_sharedBarrier.Wait(m_cancellationToken);
 
                    // Publish the buffer and set the index to just before the 1st element.
                    m_buffer = buffer; 
                    m_bufferIndex = new Shared(-1); 
                }
 
                // Now either enter (or continue) the yielding phase. As soon as we reach this, we know the
                // index of the 'count'-th input element.
                if (m_take)
                { 
                    // In the case of a Take, we will yield each element from our buffer for which
                    // the element is lesser than the 'count'-th index found. 
                    if (m_count == 0 || m_bufferIndex.Value >= m_buffer.Count - 1) 
                    {
                        return false; 
                    }

                    // Increment the index, and remember the values.
                    ++m_bufferIndex.Value; 
                    currentElement = m_buffer[m_bufferIndex.Value].First;
                    currentKey = m_buffer[m_bufferIndex.Value].Second; 
 
                    // Only yield the element if its index is less than or equal to the max index.
                    int maxIndex = m_sharedIndices.MaxValue; 
                    return maxIndex == -1 || m_buffer[m_bufferIndex.Value].Second <= maxIndex;
                }
                else
                { 
                    int minIndex = -1;
 
                    // If the count to skip was greater than 0, look at the buffer. 
                    if (m_count > 0)
                    { 
                        // If there wasn't enough input to skip, return right away.
                        if (m_sharedIndices.Count < m_count)
                        {
                            return false; 
                        }
 
                        minIndex = m_sharedIndices.MaxValue; 

                        // In the case of a skip, we must skip over elements whose index is lesser than the 
                        // 'count'-th index found. Once we've exhausted the buffer, we must go back and continue
                        // enumerating the data source until it is empty.
                        if (m_bufferIndex.Value < m_buffer.Count - 1)
                        { 
                            for (m_bufferIndex.Value++; m_bufferIndex.Value < m_buffer.Count; m_bufferIndex.Value++)
                            { 
                                // If the current buffered element's index is greater than the 'count'-th index, 
                                // we will yield it as a result.
                                if (m_buffer[m_bufferIndex.Value].Second > minIndex) 
                                {
                                    currentElement = m_buffer[m_bufferIndex.Value].First;
                                    currentKey = m_buffer[m_bufferIndex.Value].Second;
                                    return true; 
                                }
                            } 
                        } 
                    }
 
                    // Lastly, so long as our input still has elements, they will be yieldable.
                    if (m_source.MoveNext(ref currentElement, ref currentKey))
                    {
                        Contract.Assert(currentKey > minIndex, 
                                        "expected remaining element indices to be greater than smallest");
                        return true; 
                    } 
                }
 
                return false;
            }

            protected override void Dispose(bool disposing) 
            {
                m_source.Dispose(); 
            } 
        }
 
        //----------------------------------------------------------------------------------------
        // Returns an enumerable that represents the query executing sequentially.
        //
 
        internal override IEnumerable AsSequentialQuery(CancellationToken token)
        { 
            if (m_take) 
            {
                return Child.AsSequentialQuery(token).Take(m_count); 
            }

            IEnumerable wrappedChild = CancellableEnumerable.Wrap(Child.AsSequentialQuery(token), token);
            return wrappedChild.Skip(m_count); 
        }
 
        //------------------------------------------------------------------------------------ 
        // Query results for a Take or a Skip operator. The results are indexible if the child
        // results were indexible. 
        //

        class TakeOrSkipQueryOperatorResults : UnaryQueryOperatorResults
        { 
            TakeOrSkipQueryOperator m_takeOrSkipOp; // The operator that generated the results
            int m_childCount; // The number of elements in child results 
 
            public static QueryResults NewResults(
                QueryResults childQueryResults, TakeOrSkipQueryOperator op, 
                QuerySettings settings, bool preferStriping)
            {
                if (childQueryResults.IsIndexible)
                { 
                    return new TakeOrSkipQueryOperatorResults(
                        childQueryResults, op, settings, preferStriping); 
                } 
                else
                { 
                    return new UnaryQueryOperatorResults(
                        childQueryResults, op, settings, preferStriping);
                }
            } 

            private TakeOrSkipQueryOperatorResults( 
                QueryResults childQueryResults, TakeOrSkipQueryOperator takeOrSkipOp, 
                QuerySettings settings, bool preferStriping)
                : base(childQueryResults, takeOrSkipOp, settings, preferStriping) 
            {
                m_takeOrSkipOp = takeOrSkipOp;
                Contract.Assert(m_childQueryResults.IsIndexible);
 
                m_childCount = m_childQueryResults.ElementsCount;
            } 
 
            internal override bool IsIndexible
            { 
                get { return m_childCount >= 0; }
            }

            internal override int ElementsCount 
            {
                get 
                { 
                    Contract.Assert(m_childCount >= 0);
                    if (m_takeOrSkipOp.m_take) 
                    {
                        return Math.Min(m_childCount, m_takeOrSkipOp.m_count);
                    }
                    else 
                    {
                        return Math.Max(m_childCount - m_takeOrSkipOp.m_count, 0); 
                    } 
                }
            } 

            internal override TResult GetElement(int index)
            {
                Contract.Assert(m_childCount >= 0); 
                Contract.Assert(index >= 0);
                Contract.Assert(index < ElementsCount); 
 
                if (m_takeOrSkipOp.m_take)
                { 
                    return m_childQueryResults.GetElement(index);
                }
                else
                { 
                    return m_childQueryResults.GetElement(m_takeOrSkipOp.m_count + index);
                } 
            } 
        }
    } 
}

// File provided for Reference Use Only by Microsoft Corporation (c) 2007.
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