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Fix GPU-safe Montgomery window exponentiation

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Marek Lemańczyk committed 2025-10-05 06:35:18 +02:00
1 parent a98d5a7c1a
commit abab307da4
1 file changed
+15 -6
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PerfectNumbers.Core/ULongExtensions.cs
+15 -6
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@@ -9,6 +9,14 @@ public static class ULongExtensions
{ {
private const int Pow2WindowSize = 8; private const int Pow2WindowSize = 8;
private const ulong Pow2WindowFallbackThreshold = 32UL; private const ulong Pow2WindowFallbackThreshold = 32UL;
private const int Pow2WindowOddPowerCount = 1 << (Pow2WindowSize - 1);
[InlineArray(Pow2WindowOddPowerCount)]
private struct Pow2OddPowerTable
{
private ulong _element0;
}
public static ulong CalculateOrder(this ulong q) public static ulong CalculateOrder(this ulong q)
{ {
if (q <= 2UL) if (q <= 2UL)
@@ -342,8 +350,8 @@ public static class ULongExtensions
ulong result = divisor.MontgomeryOne; ulong result = divisor.MontgomeryOne;
ulong nPrime = divisor.NPrime; ulong nPrime = divisor.NPrime;
Span<ulong> oddPowers = stackalloc ulong[oddPowerCount]; Pow2OddPowerTable oddPowers = default;
InitializeMontgomeryOddPowers(divisor, modulus, nPrime, oddPowers); InitializeMontgomeryOddPowers(divisor, modulus, nPrime, ref oddPowers, oddPowerCount);
int index = bitLength - 1; int index = bitLength - 1;
while (index >= 0) while (index >= 0)
@@ -449,18 +457,19 @@ public static class ULongExtensions
} }
[MethodImpl(MethodImplOptions.AggressiveInlining)] [MethodImpl(MethodImplOptions.AggressiveInlining)]
private static void InitializeMontgomeryOddPowers(in MontgomeryDivisorData divisor, ulong modulus, ulong nPrime, Span<ulong> oddPowers) private static void InitializeMontgomeryOddPowers(in MontgomeryDivisorData divisor, ulong modulus, ulong nPrime, ref Pow2OddPowerTable oddPowers, int oddPowerCount)
{ {
oddPowers[0] = divisor.MontgomeryTwo; oddPowers[0] = divisor.MontgomeryTwo;
if (oddPowers.Length == 1) if (oddPowerCount == 1)
{ {
return; return;
} }
ulong square = divisor.MontgomeryTwoSquared; ulong square = divisor.MontgomeryTwoSquared;
for (int i = 1; i < oddPowers.Length; i++) for (int i = 1; i < oddPowerCount; i++)
{ {
oddPowers[i] = oddPowers[i - 1].MontgomeryMultiply(square, modulus, nPrime); ulong previous = oddPowers[i - 1];
oddPowers[i] = previous.MontgomeryMultiply(square, modulus, nPrime);
} }
} }