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Avoid pre-allocating large arrays in some scenarios for Enumerable.Chunk #67210

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Merged
eiriktsarpalis merged 3 commits into from
Apr 4, 2022
Merged

Avoid pre-allocating large arrays in some scenarios for Enumerable.Chunk #67210

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820ef80
Avoid pre-allocating large arrays in some scenarios for Enumerable.Ch…
madelson Mar 27, 2022
317d0fd
Switch Chunk() to use List-based implementation.
madelson Apr 1, 2022
0624f56
Address feedback from https://github.com/dotnet/runtime/pull/67210/files
madelson Apr 2, 2022
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34 changes: 17 additions & 17 deletions src/libraries/System.Linq/src/System/Linq/Chunk.cs
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Original file line number Diff line number Diff line change
Expand Up @@ -2,6 +2,7 @@
// The .NET Foundation licenses this file to you under the MIT license.

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

namespace System.Linq
{
Expand Down Expand Up @@ -50,26 +51,25 @@ public static IEnumerable<TSource[]> Chunk<TSource>(this IEnumerable<TSource> so
private static IEnumerable<TSource[]> ChunkIterator<TSource>(IEnumerable<TSource> source, int size)
{
using IEnumerator<TSource> e = source.GetEnumerator();
while (e.MoveNext())
{
TSource[] chunk = new TSource[size];
chunk[0] = e.Current;

int i = 1;
for (; i < chunk.Length && e.MoveNext(); i++)
if (e.MoveNext())
{
List<TSource> chunkBuilder = new();
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@eiriktsarpalis eiriktsarpalis Apr 2, 2022

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I would recommend presizing the List up to a "reasonable" chunk size (e.g. new(Math.Min(chunkSize, 128))) to prevent unneeded resizes when the first chunk is filled. You could use benchmarks to determine a good cutoff number.

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@stephentoub stephentoub Apr 2, 2022

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This assumes there's going to be enough to fill a chunk. Do we know that's a good assumption? Benchmarks here can be used to show that any size is optimal, so how are you going to choose the benchmarks to use? To me, the reasonable sizes to use are either the chunk size (which the presence of this pr is dismissing) or the small, default value list uses internally so this method is out of the guessing game.

while (true)
{
chunk[i] = e.Current;
}
do
{
chunkBuilder.Add(e.Current);
}
while (chunkBuilder.Count < size && e.MoveNext());

if (i == chunk.Length)
{
yield return chunk;
}
else
{
Array.Resize(ref chunk, i);
yield return chunk;
yield break;
yield return chunkBuilder.ToArray();

if (chunkBuilder.Count < size || !e.MoveNext())
{
yield break;
}
chunkBuilder.Clear();
}
}
}
Expand Down
11 changes: 10 additions & 1 deletion src/libraries/System.Linq/tests/ChunkTests.cs
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Expand Up @@ -141,5 +141,14 @@ public void AddingToSourceBeforeIterating()

Assert.Equal(new[] {new[] {9999, 0, 888}, new[] {-1, 66, -777}, new[] {1, 2, -12345}, new[] {10}}, chunks);
}

// reproduces https://github.com/dotnet/runtime/issues/67132
[Fact]
public void DoesNotPrematurelyAllocateHugeArray()
{
int[][] chunks = Enumerable.Range(0, 10).Chunk(int.MaxValue).ToArray();

Assert.Equal(new[] { Enumerable.Range(0, 10).ToArray() }, chunks);
}
}
}
}