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Merged
eiriktsarpalis merged 14 commits into from
Mar 1, 2021
Merged

Apply optimizations and fixes to Enumerable.Take(Range) #48634

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f38c0d6
optimize Take(Range) for countable enumerables
eiriktsarpalis Feb 22, 2021
a5339bb
Implement TakeLast over Take(Range)
eiriktsarpalis Feb 23, 2021
e1bd78c
Implement SkipLast over Take(Range)
eiriktsarpalis Feb 23, 2021
3558108
remove unneeded checked increment
eiriktsarpalis Feb 23, 2021
b89a0de
fix typo in tests
eiriktsarpalis Feb 24, 2021
faed507
remove ToLookup inputs from NonEnumeratedCount tests
eiriktsarpalis Feb 24, 2021
7dad846
remove leftover code
eiriktsarpalis Feb 24, 2021
cd1c805
Revert "remove ToLookup inputs from NonEnumeratedCount tests"
eiriktsarpalis Feb 24, 2021
9478f52
try debugging test failure
eiriktsarpalis Feb 25, 2021
101ad8e
remove ToLookup inputs from NonEnumeratedCount tests
eiriktsarpalis Feb 24, 2021
0c06999
Revert "try debugging test failure"
eiriktsarpalis Feb 25, 2021
5dbf307
Update src/libraries/System.Linq/src/System/Linq/Take.cs
eiriktsarpalis Feb 27, 2021
25e7058
eliminate redundant yield breaks
eiriktsarpalis Feb 27, 2021
554bd9d
remove accidentally reverted change
eiriktsarpalis Feb 27, 2021
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33 changes: 3 additions & 30 deletions src/libraries/System.Linq/src/System/Linq/Skip.cs
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Expand Up @@ -121,36 +121,9 @@ public static IEnumerable<TSource> SkipLast<TSource>(this IEnumerable<TSource> s

return count <= 0 ?
source.Skip(0) :
SkipLastIterator(source, count);
}

private static IEnumerable<TSource> SkipLastIterator<TSource>(IEnumerable<TSource> source, int count)
{
Debug.Assert(source != null);
Debug.Assert(count > 0);

var queue = new Queue<TSource>();

using (IEnumerator<TSource> e = source.GetEnumerator())
{
while (e.MoveNext())
{
if (queue.Count == count)
{
do
{
yield return queue.Dequeue();
queue.Enqueue(e.Current);
}
while (e.MoveNext());
break;
}
else
{
queue.Enqueue(e.Current);
}
}
}
TakeRangeFromEndIterator(source,
isStartIndexFromEnd: false, startIndex: 0,
isEndIndexFromEnd: true, endIndex: count);
}
}
}
25 changes: 25 additions & 0 deletions src/libraries/System.Linq/src/System/Linq/Take.SizeOpt.cs
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Original file line number Diff line number Diff line change
Expand Up @@ -18,5 +18,30 @@ private static IEnumerable<TSource> TakeIterator<TSource>(IEnumerable<TSource> s
if (--count == 0) break;
}
}

private static IEnumerable<TSource> TakeRangeIterator<TSource>(IEnumerable<TSource> source, int startIndex, int endIndex)
{
Debug.Assert(source != null);
Debug.Assert(startIndex >= 0 && startIndex < endIndex);

using IEnumerator<TSource> e = source.GetEnumerator();

int index = 0;
while (index < startIndex && e.MoveNext())
{
++index;
}

if (index < startIndex)
{
yield break;
}

while (index < endIndex && e.MoveNext())
{
yield return e.Current;
++index;
}
}
}
}
32 changes: 28 additions & 4 deletions src/libraries/System.Linq/src/System/Linq/Take.SpeedOpt.cs
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Expand Up @@ -2,14 +2,38 @@
// The .NET Foundation licenses this file to you under the MIT license.

using System.Collections.Generic;
using System.Diagnostics;

namespace System.Linq
{
public static partial class Enumerable
{
private static IEnumerable<TSource> TakeIterator<TSource>(IEnumerable<TSource> source, int count) =>
source is IPartition<TSource> partition ? partition.Take(count) :
source is IList<TSource> sourceList ? (IEnumerable<TSource>)new ListPartition<TSource>(sourceList, 0, count - 1) :
new EnumerablePartition<TSource>(source, 0, count - 1);
private static IEnumerable<TSource> TakeIterator<TSource>(IEnumerable<TSource> source, int count)
{
Debug.Assert(source != null);
Debug.Assert(count > 0);

return
source is IPartition<TSource> partition ? partition.Take(count) :
source is IList<TSource> sourceList ? new ListPartition<TSource>(sourceList, 0, count - 1) :
new EnumerablePartition<TSource>(source, 0, count - 1);
}

private static IEnumerable<TSource> TakeRangeIterator<TSource>(IEnumerable<TSource> source, int startIndex, int endIndex)
{
Debug.Assert(source != null);
Debug.Assert(startIndex >= 0 && startIndex < endIndex);

return
source is IPartition<TSource> partition ? TakePartitionRange(partition, startIndex, endIndex) :
source is IList<TSource> sourceList ? new ListPartition<TSource>(sourceList, startIndex, endIndex - 1) :
new EnumerablePartition<TSource>(source, startIndex, endIndex - 1);

static IPartition<TSource> TakePartitionRange(IPartition<TSource> partition, int startIndex, int endIndex)
{
partition = endIndex == 0 ? EmptyPartition<TSource>.Instance : partition.Take(endIndex);
return startIndex == 0 ? partition : partition.Skip(startIndex);
}
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}
}
}
195 changes: 81 additions & 114 deletions src/libraries/System.Linq/src/System/Linq/Take.cs
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Expand Up @@ -55,121 +55,129 @@ public static IEnumerable<TSource> Take<TSource>(this IEnumerable<TSource> sourc
{
return startIndex >= endIndex
? Empty<TSource>()
: source.Skip(startIndex).Take(endIndex - startIndex);
: TakeRangeIterator(source, startIndex, endIndex);
}

return TakeIterator(source, isStartIndexFromEnd, startIndex, isEndIndexFromEnd, endIndex);
return TakeRangeFromEndIterator(source, isStartIndexFromEnd, startIndex, isEndIndexFromEnd, endIndex);
}

private static IEnumerable<TSource> TakeIterator<TSource>(
IEnumerable<TSource> source, bool isStartIndexFromEnd, int startIndex, bool isEndIndexFromEnd, int endIndex)
private static IEnumerable<TSource> TakeRangeFromEndIterator<TSource>(IEnumerable<TSource> source, bool isStartIndexFromEnd, int startIndex, bool isEndIndexFromEnd, int endIndex)
{
Debug.Assert(source != null);
Debug.Assert(isStartIndexFromEnd || isEndIndexFromEnd);
Debug.Assert(isStartIndexFromEnd
? startIndex > 0 && (!isEndIndexFromEnd || startIndex > endIndex)
: startIndex >= 0 && (isEndIndexFromEnd || startIndex < endIndex));
Debug.Assert(endIndex >= 0);

using IEnumerator<TSource> e = source.GetEnumerator();
if (isStartIndexFromEnd)
// Attempt to extract the count of the source enumerator,
// in order to convert fromEnd indices to regular indices.
// Enumerable counts can change over time, so it is very
// important that this check happens at enumeration time;
// do not move it outside of the iterator method.
if (source.TryGetNonEnumeratedCount(out int count))
{
if (!e.MoveNext())
{
yield break;
}

int index = 0;
Queue<TSource> queue = new();
queue.Enqueue(e.Current);
startIndex = CalculateStartIndex(isStartIndexFromEnd, startIndex, count);
endIndex = CalculateEndIndex(isEndIndexFromEnd, endIndex, count);

while (e.MoveNext())
if (startIndex < endIndex)
{
checked
foreach (TSource element in TakeRangeIterator(source, startIndex, endIndex))
{
index++;
yield return element;
}
}

yield break;
}

Queue<TSource> queue;

if (queue.Count == startIndex)
if (isStartIndexFromEnd)
{
// TakeLast compat: enumerator should be disposed before yielding the first element.
using (IEnumerator<TSource> e = source.GetEnumerator())
{
if (!e.MoveNext())
{
queue.Dequeue();
yield break;
}

queue = new Queue<TSource>();
queue.Enqueue(e.Current);
}

int count = checked(index + 1);
Debug.Assert(queue.Count == Math.Min(count, startIndex));
count = 1;

startIndex = count - startIndex;
if (startIndex < 0)
{
startIndex = 0;
}
while (e.MoveNext())
{
if (count < startIndex)
{
queue.Enqueue(e.Current);
++count;
}
else
{
do
{
queue.Dequeue();
queue.Enqueue(e.Current);
checked { ++count; }
} while (e.MoveNext());
break;
}
}

if (isEndIndexFromEnd)
{
endIndex = count - endIndex;
}
else if (endIndex > count)
{
endIndex = count;
Debug.Assert(queue.Count == Math.Min(count, startIndex));
}

startIndex = CalculateStartIndex(isStartIndexFromEnd: true, startIndex, count);
endIndex = CalculateEndIndex(isEndIndexFromEnd, endIndex, count);
Debug.Assert(endIndex - startIndex <= queue.Count);

for (int rangeIndex = startIndex; rangeIndex < endIndex; rangeIndex++)
{
yield return queue.Dequeue();
}
}
else
{
int index = 0;
while (index <= startIndex)
{
if (!e.MoveNext())
{
yield break;
}
Debug.Assert(!isStartIndexFromEnd && isEndIndexFromEnd);

checked
{
index++;
}
// SkipLast compat: the enumerator should be disposed at the end of the enumeration.
using IEnumerator<TSource> e = source.GetEnumerator();

count = 0;
while (count < startIndex && e.MoveNext())
{
++count;
}

if (isEndIndexFromEnd)
if (count == startIndex)
{
if (endIndex > 0)
queue = new Queue<TSource>();
while (e.MoveNext())
{
Queue<TSource> queue = new();
do
if (queue.Count == endIndex)
{
if (queue.Count == endIndex)
do
{
queue.Enqueue(e.Current);
yield return queue.Dequeue();
}
} while (e.MoveNext());

queue.Enqueue(e.Current);
} while (e.MoveNext());
}
else
{
do
break;
}
else
{
yield return e.Current;
} while (e.MoveNext());
}
}
else
{
Debug.Assert(index < endIndex);
yield return e.Current;
while (checked(++index) < endIndex && e.MoveNext())
{
yield return e.Current;
queue.Enqueue(e.Current);
}
}
}
}

static int CalculateStartIndex(bool isStartIndexFromEnd, int startIndex, int count) =>
Math.Max(0, isStartIndexFromEnd ? count - startIndex : startIndex);

static int CalculateEndIndex(bool isEndIndexFromEnd, int endIndex, int count) =>
Math.Min(count, isEndIndexFromEnd ? count - endIndex : endIndex);
}

public static IEnumerable<TSource> TakeWhile<TSource>(this IEnumerable<TSource> source, Func<TSource, bool> predicate)
Expand Down Expand Up @@ -243,50 +251,9 @@ public static IEnumerable<TSource> TakeLast<TSource>(this IEnumerable<TSource> s

return count <= 0 ?
Empty<TSource>() :
TakeLastIterator(source, count);
}

private static IEnumerable<TSource> TakeLastIterator<TSource>(IEnumerable<TSource> source, int count)
{
Debug.Assert(source != null);
Debug.Assert(count > 0);

Queue<TSource> queue;
using (IEnumerator<TSource> e = source.GetEnumerator())
{
if (!e.MoveNext())
{
yield break;
}

queue = new Queue<TSource>();
queue.Enqueue(e.Current);

while (e.MoveNext())
{
if (queue.Count < count)
{
queue.Enqueue(e.Current);
}
else
{
do
{
queue.Dequeue();
queue.Enqueue(e.Current);
}
while (e.MoveNext());
break;
}
}
}

Debug.Assert(queue.Count <= count);
do
{
yield return queue.Dequeue();
}
while (queue.Count > 0);
TakeRangeFromEndIterator(source,
isStartIndexFromEnd: true, startIndex: count,
isEndIndexFromEnd: true, endIndex: 0);
}
}
}
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