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[release/9.0] Fix LINQ Last() in ConcatNIterator to also check base case (Concat2Iterator) #108664

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merged 3 commits into from
Oct 8, 2024
Merged

[release/9.0] Fix LINQ Last() in ConcatNIterator to also check base case (Concat2Iterator) #108664

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06c4a4e
Fix LINQ Last() in ConcatNIterator to also check base case (Concat2It…
andrewjsaid Oct 2, 2024
39522e1
Add a few more test cases of empty concats
andrewjsaid Oct 2, 2024
9849515
Simplify solution as suggested in PR Review
andrewjsaid Oct 3, 2024
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12 changes: 7 additions & 5 deletions src/libraries/System.Linq/src/System/Linq/Concat.SpeedOpt.cs
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Original file line number Diff line number Diff line change
Expand Up @@ -326,19 +326,21 @@ private TSource[] PreallocatingToArray()

public override TSource? TryGetLast(out bool found)
{
ConcatNIterator<TSource>? node = this;
ConcatNIterator<TSource>? node, previousN = this;
TSource? result;
do
{
TSource? result = node._head.TryGetLast(out found);
node = previousN;
result = node._head.TryGetLast(out found);
if (found)
{
return result;
}
}
while ((node = node!.PreviousN) is not null);
while ((previousN = node.PreviousN) is not null);

found = false;
return default;
Debug.Assert(node._tail is Concat2Iterator<TSource>);
return node._tail.TryGetLast(out found);
}
}

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27 changes: 27 additions & 0 deletions src/libraries/System.Linq/tests/ConcatTests.cs
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Original file line number Diff line number Diff line change
Expand Up @@ -93,6 +93,7 @@ public void VerifyEquals(IEnumerable<int> expected, IEnumerable<int> actual)
[MemberData(nameof(ConcatWithSelfData))]
[MemberData(nameof(ChainedCollectionConcatData))]
[MemberData(nameof(AppendedPrependedConcatAlternationsData))]
[MemberData(nameof(ConcatWithEmptyEnumerableData))]
public void First_Last_ElementAt(IEnumerable<int> _, IEnumerable<int> actual)
{
int count = actual.Count();
Expand Down Expand Up @@ -232,6 +233,32 @@ public static IEnumerable<object[]> AppendedPrependedConcatAlternationsData()
}
}

public static IEnumerable<object[]> ConcatWithEmptyEnumerableData()
{
List<int> baseList = [0, 1, 2, 3, 4];

yield return new object[]
{
Enumerable.Range(0, 5),
Enumerable.Concat(Enumerable.Concat(new List<int>(), new List<int>()), baseList)
};
yield return new object[]
{
Enumerable.Range(0, 5),
Enumerable.Concat(new List<int>(), baseList)
};
yield return new object[]
{
Enumerable.Range(0, 5),
Enumerable.Concat(Enumerable.Concat(baseList, new List<int>()), new List<int>())
};
yield return new object[]
{
Enumerable.Range(0, 5),
Enumerable.Concat(baseList, new List<int>())
};
}

private static IEnumerable<object[]> GenerateSourcesData(
Func<IEnumerable<int>, IEnumerable<int>> outerTransform = null,
Func<IEnumerable<int>, IEnumerable<int>> innerTransform = null)
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