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Use double.ConvertToIntegerNative where safe to do in System.Random #112046

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merged 1 commit into from
Feb 1, 2025
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Use double.ConvertToIntegerNative where safe to do in System.Random #112046

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27 changes: 21 additions & 6 deletions src/libraries/System.Private.CoreLib/src/System/Random.CompatImpl.cs
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Original file line number Diff line number Diff line change
Expand Up @@ -25,14 +25,23 @@ public CompatSeedImpl(int seed) =>

public override int Next() => _prng.InternalSample();

public override int Next(int maxValue) => (int)(_prng.Sample() * maxValue);
public override int Next(int maxValue)
{
// We can use ConvertToIntegerNative here since we know the result of
// scaling the sample is in the range [0, int.MaxValue) and therefore
// the integer portion is exactly representable since it's < 2^52.
return double.ConvertToIntegerNative<int>(_prng.Sample() * maxValue);
}

public override int Next(int minValue, int maxValue)
{
// We can use ConvertToIntegerNative here since we know the result of
// scaling the sample is in the range [0, uint.MaxValue) and therefore
// the integer portion is exactly representable since it's < 2^52.
long range = (long)maxValue - minValue;
return range <= int.MaxValue ?
(int)(_prng.Sample() * range) + minValue :
(int)((long)(_prng.GetSampleForLargeRange() * range) + minValue);
double.ConvertToIntegerNative<int>(_prng.Sample() * range) + minValue :
(int)(double.ConvertToIntegerNative<long>(_prng.GetSampleForLargeRange() * range) + minValue);
}

public override long NextInt64()
Expand Down Expand Up @@ -136,17 +145,23 @@ public override int Next()

public override int Next(int maxValue)
{
// We can use ConvertToIntegerNative here since we know the result of
// scaling the sample is in the range [0, int.MaxValue) and therefore
// the integer portion is exactly representable since it's < 2^52.
_prng.EnsureInitialized(_seed);
return (int)(_parent.Sample() * maxValue);
return double.ConvertToIntegerNative<int>(_parent.Sample() * maxValue);
}

public override int Next(int minValue, int maxValue)
{
// We can use ConvertToIntegerNative here since we know the result of
// scaling the sample is in the range [0, uint.MaxValue) and therefore
// the integer portion is exactly representable since it's < 2^52.
_prng.EnsureInitialized(_seed);
long range = (long)maxValue - minValue;
return range <= int.MaxValue ?
(int)(_parent.Sample() * range) + minValue :
(int)((long)(_prng.GetSampleForLargeRange() * range) + minValue);
double.ConvertToIntegerNative<int>(_parent.Sample() * range) + minValue :
(int)(double.ConvertToIntegerNative<long>(_prng.GetSampleForLargeRange() * range) + minValue);
}

public override long NextInt64()
Expand Down
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