mlemanczyk/even-perfect-numbers-scanner
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Respect GPU configuration and reuse order factorization

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Marek Lemańczyk committed 2025-10-11 15:30:04 +02:00
1 parent 45183acd9b
commit fa74667f89
4 files changed
+258 -231
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PerfectNumbers.Core/MersenneDivisorCycles.cs
+57 -36
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@@ -273,17 +273,24 @@ public class MersenneDivisorCycles
{
cycleLength = 0UL;
if ((divisor & (divisor - 1UL)) == 0UL)
{
cycleLength = 1UL;
return true;
}
// The by-divisor scanner only supplies odd candidates of the form 2*k*exponent + 1,
// so this power-of-two fast-path never triggers on that call path.
// The historical fast-path is left here for reference but commented out because it is unreachable
// on the EvenPerfectBitScanner pipeline.
// if ((divisor & (divisor - 1UL)) == 0UL)
// {
// cycleLength = 1UL;
// return true;
// }
if (divisor <= 3UL || exponent <= 1UL)
{
cycleLength = CalculateCycleLength(divisor, divisorData);
return true;
}
// EvenPerfectBitScanner filters exponents <= 3 before invoking this helper and each
// divisor candidate is at least 2 * exponent + 1, so this guard never matches.
// The fallback calculation is therefore unreachable on the active call path.
// if (divisor <= 3UL || exponent <= 1UL)
// {
// cycleLength = CalculateCycleLength(divisor, divisorData);
// return true;
// }
ulong computedOrder = PrimeOrderCalculator.Calculate(
divisor,
@@ -298,29 +305,38 @@ public class MersenneDivisorCycles
}
ulong phi = divisor - 1UL;
if (phi == 0UL)
{
return false;
}
// Divisors reach this helper only when they exceed the cached table, which means
// divisor >= 2 * exponent + 1 and phi cannot underflow to zero here.
// The defensive underflow guard is kept for posterity but disabled for the EvenPerfectBitScanner path.
// if (phi == 0UL)
// {
// return false;
// }
int twoCount = BitOperations.TrailingZeroCount(phi);
ulong reducedPhi = phi >> twoCount;
if (reducedPhi == 0UL || (reducedPhi % exponent) != 0UL)
{
return false;
}
// Every divisor candidate generated by the by-divisor scan satisfies q ≡ 1 (mod 2 * exponent),
// ensuring reducedPhi keeps a factor of exponent and remains non-zero in this context.
// The divisibility check is therefore unreachable for the current caller and is commented out.
// if (reducedPhi == 0UL || (reducedPhi % exponent) != 0UL)
// {
// return false;
// }
ulong k = reducedPhi / exponent;
if (k == 0UL)
{
return false;
}
// reducedPhi always contains at least one factor of exponent, so dividing by exponent leaves k >= 1.
// if (k == 0UL)
// {
// return false;
// }
Dictionary<ulong, int> factorCounts = new(8);
if (twoCount > 0)
{
factorCounts[2UL] = twoCount;
}
// The divisor candidates are odd, so phi is even and this branch always records at least one factor of two.
// if (twoCount > 0)
// {
// factorCounts[2UL] = twoCount;
// }
factorCounts[2UL] = twoCount;
if (!AccumulateFactors(exponent, factorCache, factorCounts, cacheResult: true) || !AccumulateFactors(k, factorCache, factorCounts, cacheResult: false))
{
@@ -926,17 +942,22 @@ public class MersenneDivisorCycles
internal static bool TryCalculateCycleLengthHeuristic(ulong divisor, in MontgomeryDivisorData divisorData, out ulong cycleLength)
{
if ((divisor & (divisor - 1UL)) == 0UL)
{
cycleLength = 1UL;
return true;
}
// The by-divisor scanner only supplies odd candidates of the form 2*k*exponent + 1,
// so this power-of-two fast-path never triggers on that call path.
// The early exit is retained for reference but commented out because it is unreachable on the scanner path.
// if ((divisor & (divisor - 1UL)) == 0UL)
// {
// cycleLength = 1UL;
// return true;
// }
if (divisor <= 3UL)
{
cycleLength = CalculateCycleLengthFallback(divisor);
return true;
}
// By the divisor-by-divisor pipeline, candidates have the shape 2 * exponent * k + 1 with exponent >= 5,
// so the divisor never drops below 11 on this call path.
// if (divisor <= 3UL)
// {
// cycleLength = CalculateCycleLengthFallback(divisor);
// return true;
// }
if (PrimeTester.IsPrimeInternal(divisor, CancellationToken.None))
{