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Simplify Random for ulong #87219

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Merged
stephentoub merged 1 commit into from
Jun 8, 2023
Merged

Simplify Random for ulong #87219

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9 changes: 4 additions & 5 deletions src/libraries/System.Private.CoreLib/src/System/Random.ImplBase.cs
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Original file line number Diff line number Diff line change
@@ -1,6 +1,7 @@
// Licensed to the .NET Foundation under one or more agreements.
// The .NET Foundation licenses this file to you under the MIT license.

using System;
using System.Runtime.CompilerServices;

namespace System
Expand Down Expand Up @@ -57,21 +58,19 @@ internal static uint NextUInt32(uint maxValue, XoshiroImpl xoshiro)
[MethodImpl(MethodImplOptions.AggressiveInlining)]
internal static ulong NextUInt64(ulong maxValue, XoshiroImpl xoshiro)
{
UInt128 randomProduct = (UInt128)maxValue * xoshiro.NextUInt64();
ulong lowPart = (ulong)randomProduct;
ulong randomProduct = Math.BigMul(maxValue, xoshiro.NextUInt64(), out ulong lowPart);

if (lowPart < maxValue)
{
ulong remainder = (0ul - maxValue) % maxValue;

while (lowPart < remainder)
{
randomProduct = (UInt128)maxValue * xoshiro.NextUInt64();
lowPart = (ulong)randomProduct;
randomProduct = Math.BigMul(maxValue, xoshiro.NextUInt64(), out lowPart);
}
}

return (ulong)(randomProduct >> 64);
return randomProduct;
}
}
}
Expand Down