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// ==++==
//
// Copyright (c) Microsoft Corporation. All rights reserved.
//
// ==--==
// =+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+
//
// HashPartitionedStream.cs
//
// <OWNER>Microsoft</OWNER>
//
// =-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-
using System.Collections.Generic;
using System.Diagnostics.Contracts;
using System.Threading;
namespace System.Linq.Parallel
{
/// <summary>
/// A repartitioning stream must take input data that has already been partitioned and
/// redistribute its contents based on a new partitioning algorithm. This is accomplished
/// by making each partition p responsible for redistributing its input data to the
/// correct destination partition. Some input elements may remain in p, but many will now
/// belong to a different partition and will need to move. This requires a great deal of
/// synchronization, but allows threads to repartition data incrementally and in parallel.
/// Each partition will "pull" data on-demand instead of partitions "pushing" data, which
/// allows us to reduce some amount of synchronization overhead.
///
/// We currently only offer one form of reparitioning via hashing. This used to be an
/// abstract base class, but we have eliminated that to get rid of some virtual calls on
/// hot code paths. Uses a key selection algorithm with mod'ding to determine destination.
///
/// </summary>
/// <typeparam name="TInputOutput"></typeparam>
/// <typeparam name="THashKey"></typeparam>
/// <typeparam name="TOrderKey"></typeparam>
internal abstract class HashRepartitionStream<TInputOutput, THashKey, TOrderKey> : PartitionedStream<Pair<TInputOutput, THashKey>, TOrderKey>
{
private readonly IEqualityComparer<THashKey> m_keyComparer; // The optional key comparison routine.
private readonly IEqualityComparer<TInputOutput> m_elementComparer; // The optional element comparison routine.
private readonly int m_distributionMod; // The distribution value we'll use to scramble input.
//---------------------------------------------------------------------------------------
// Creates a new partition exchange operator.
//
internal HashRepartitionStream(
int partitionsCount, IComparer<TOrderKey> orderKeyComparer, IEqualityComparer<THashKey> hashKeyComparer,
IEqualityComparer<TInputOutput> elementComparer)
: base(partitionsCount, orderKeyComparer, OrdinalIndexState.Shuffled)
{
// elementComparer is used by operators that use elements themselves as the hash keys.
// When elements are used as keys, THashKey should be NoKeyMemoizationRequired.
m_keyComparer = hashKeyComparer;
m_elementComparer = elementComparer;
Contract.Assert(m_keyComparer == null || m_elementComparer == null);
Contract.Assert(m_elementComparer == null || typeof(THashKey) == typeof(NoKeyMemoizationRequired));
// We use the following constant when distributing hash-codes into partition streams.
// It's an (arbitrary) prime number to account for poor hashing functions, e.g. those
// that all the primitive types use (e.g. Int32 returns itself). The goal is to add some
// degree of randomization to avoid predictable distributions for certain data sequences,
// for the same reason prime numbers are frequently used in hashtable implementations.
// For instance, if all hash-codes end up as even, we would starve half of the partitions
// by just using the raw hash-code. This isn't perfect, of course, since a stream
// of integers with the same value end up in the same partition, but helps.
const int DEFAULT_HASH_MOD_DISTRIBUTION = 503;
// We need to guarantee our distribution mod is greater than the number of partitions.
m_distributionMod = DEFAULT_HASH_MOD_DISTRIBUTION;
while (m_distributionMod < partitionsCount)
{
// We use checked arithmetic here. We'll only overflow for huge numbers of partitions
// (quite unlikely), so the remote possibility of an exception is fine.
checked
{
m_distributionMod *= 2;
}
}
}
//---------------------------------------------------------------------------------------
// Manufactures a hash code for a given value or key.
//
// The hash-code used for null elements.
private const int NULL_ELEMENT_HASH_CODE = 0;
internal int GetHashCode(TInputOutput element)
{
return
(0x7fffffff &
(m_elementComparer == null ?
(element == null ? NULL_ELEMENT_HASH_CODE : element.GetHashCode()) :
m_elementComparer.GetHashCode(element)))
% m_distributionMod;
}
internal int GetHashCode(THashKey key)
{
return
(0x7fffffff &
(m_keyComparer == null ?
(key == null ? NULL_ELEMENT_HASH_CODE : key.GetHashCode()) :
m_keyComparer.GetHashCode(key))) % m_distributionMod;
}
}
}
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