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Optimize HashSet intersecting #125256

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Optimize HashSet intersecting #125256

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3ee0727
Optimize HashSet intersecting
pentp Mar 6, 2026
32b4f67
Optimize RemoveAt
pentp Mar 7, 2026
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42 changes: 41 additions & 1 deletion src/libraries/Common/src/System/Collections/Generic/BitHelper.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.Diagnostics;
using System.Numerics;

namespace System.Collections.Generic
{
Expand Down Expand Up @@ -33,6 +34,26 @@ internal void MarkBit(int bitPosition)
}
}

internal bool TryMarkBit(int bitPosition)
{
Debug.Assert(bitPosition >= 0);

uint bitArrayIndex = (uint)bitPosition / IntSize;

// Workaround for https://github.com/dotnet/runtime/issues/72004
Span<int> span = _span;
if (bitArrayIndex < (uint)span.Length)
{
int bits = span[(int)bitArrayIndex];
if ((bits & (1 << ((int)((uint)bitPosition % IntSize)))) == 0)
{
span[(int)bitArrayIndex] = bits | (1 << ((int)((uint)bitPosition % IntSize)));
return true;
}
}
return false;
}

internal bool IsMarked(int bitPosition)
{
Debug.Assert(bitPosition >= 0);
Expand All @@ -46,7 +67,26 @@ internal bool IsMarked(int bitPosition)
(span[(int)bitArrayIndex] & (1 << ((int)((uint)bitPosition % IntSize)))) != 0;
}

internal bool IsUnmarked(int bitPosition)
{
Debug.Assert(bitPosition >= 0);

uint bitArrayIndex = (uint)bitPosition / IntSize;

// Workaround for https://github.com/dotnet/runtime/issues/72004
ReadOnlySpan<int> span = _span;
return
bitArrayIndex < (uint)span.Length &&
(span[(int)bitArrayIndex] & (1 << ((int)((uint)bitPosition % IntSize)))) == 0;
}

internal int FindFirstUnmarked()
{
int i = _span.IndexOfAnyExcept(~0);
return i < 0 ? -1 : i * IntSize + BitOperations.TrailingZeroCount(~_span[i]);
}

/// <summary>How many ints must be allocated to represent n bits. Returns (n+31)/32, but avoids overflow.</summary>
internal static int ToIntArrayLength(int n) => n > 0 ? ((n - 1) / IntSize + 1) : 0;
internal static int ToIntArrayLength(int n) => (int)(((uint)n + 31) / IntSize);
}
}
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