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Restore original guard comments in pow2 helpers

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Marek Lemańczyk committed 2025-10-19 20:21:09 +02:00
1 parent da7a6cb23d
commit 79e8706644
9 files changed
+367 -158
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PerfectNumbers.Core/ULongExtensions.cs
+2 -133
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@@ -498,98 +498,8 @@ public static partial class ULongExtensions
return result;
}
[MethodImpl(MethodImplOptions.AggressiveInlining)]
internal static ulong Pow2MontgomeryModWindowedKernel(ulong exponent, in MontgomeryDivisorData divisor, bool keepMontgomery)
{
ulong modulus = divisor.Modulus;
// This should never happen in production code
// if (exponent == 0UL)
// {
// return keepMontgomery ? divisor.MontgomeryOne : 1UL % modulus;
// }
// We barely every hit it from production code. It's only 772 calls out of billions.
// if (exponent <= Pow2WindowFallbackThreshold)
// {
// return Pow2MontgomeryModSingleBit(exponent, divisor, keepMontgomery);
// }
int bitLength = GetPortableBitLength(exponent);
int windowSize = GetWindowSize(bitLength);
ulong result = divisor.MontgomeryOne;
ulong nPrime = divisor.NPrime;
int index = bitLength - 1;
while (index >= 0)
{
if (((exponent >> index) & 1UL) == 0UL)
{
result = result.MontgomeryMultiply(result, modulus, nPrime);
index--;
continue;
}
int windowStart = index - windowSize + 1;
if (windowStart < 0)
{
windowStart = 0;
}
while (((exponent >> windowStart) & 1UL) == 0UL)
{
windowStart++;
}
int windowLength = index - windowStart + 1;
for (int square = 0; square < windowLength; square++)
{
result = result.MontgomeryMultiply(result, modulus, nPrime);
}
ulong mask = (1UL << windowLength) - 1UL;
ulong windowValue = (exponent >> windowStart) & mask;
ulong multiplier = ComputeMontgomeryOddPower(windowValue, divisor, modulus, nPrime);
result = result.MontgomeryMultiply(multiplier, modulus, nPrime);
index = windowStart - 1;
}
if (!keepMontgomery)
{
result = result.MontgomeryMultiply(1UL, modulus, nPrime);
}
return result;
}
[MethodImpl(MethodImplOptions.AggressiveInlining)]
private static ulong ComputeMontgomeryOddPower(ulong exponent, in MontgomeryDivisorData divisor, ulong modulus, ulong nPrime)
{
ulong baseValue = divisor.MontgomeryTwo;
ulong power = divisor.MontgomeryOne;
ulong remaining = exponent;
while (remaining != 0UL)
{
if ((remaining & 1UL) != 0UL)
{
power = power.MontgomeryMultiply(baseValue, modulus, nPrime);
}
remaining >>= 1;
if (remaining == 0UL)
{
break;
}
baseValue = baseValue.MontgomeryMultiply(baseValue, modulus, nPrime);
}
return power;
}
[MethodImpl(MethodImplOptions.AggressiveInlining)]
private static ulong Pow2MontgomeryModSingleBit(ulong exponent, in MontgomeryDivisorData divisor, bool keepMontgomery)
[MethodImpl(MethodImplOptions.AggressiveInlining)]
private static ulong Pow2MontgomeryModSingleBit(ulong exponent, in MontgomeryDivisorData divisor, bool keepMontgomery)
{
ulong modulus = divisor.Modulus;
ulong nPrime = divisor.NPrime;
@@ -616,47 +526,6 @@ public static partial class ULongExtensions
return keepMontgomery ? result : result.MontgomeryMultiply(1UL, modulus, nPrime);
}
[MethodImpl(MethodImplOptions.AggressiveInlining)]
private static int GetPortableBitLength(ulong value)
{
// Keep this commented out. It will never happen in production code.
// if (value == 0UL)
// {
// return 0;
// }
return 64 - XMath.LeadingZeroCount(value);
}
private static int GetWindowSize(int bitLength)
{
if (bitLength <= 6)
{
return Math.Max(bitLength, 1);
}
if (bitLength <= 23)
{
return 4;
}
if (bitLength <= 79)
{
return 5;
}
if (bitLength <= 239)
{
return 6;
}
if (bitLength <= 671)
{
return 7;
}
return Pow2WindowSize;
}
[MethodImpl(MethodImplOptions.AggressiveInlining)]
public static UInt128 PowMod(this ulong exponent, UInt128 modulus)