mlemanczyk/even-perfect-numbers-scanner
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Replace heuristic sqrt with log2-based estimate

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Marek Lemańczyk committed 2025-10-29 01:29:12 +01:00
1 parent eeeff42be6
commit 5a9e103bd5
2 files changed
+26 -32
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PerfectNumbers.Core/HeuristicCombinedPrimeTester.cs
+13 -16
View File
@@ -311,31 +311,28 @@ Cleanup:
return n; return n;
} }
ulong bit = 1UL << 62; int log2 = BitOperations.Log2(n);
while (bit > n) ulong estimate = 1UL << ((log2 + 1) >> 1);
ulong quotient = n / estimate;
while (estimate > quotient)
{ {
bit >>= 2; estimate = (estimate + quotient) >> 1;
quotient = n / estimate;
} }
ulong result = 0UL; while (true)
ulong remainder = n;
while (bit != 0)
{ {
ulong candidate = result + bit; ulong nextEstimate = estimate + 1UL;
if (remainder >= candidate) if (nextEstimate > n / nextEstimate)
{ {
remainder -= candidate; break;
result = (result >> 1) + bit;
}
else
{
result >>= 1;
} }
bit >>= 2; estimate = nextEstimate;
} }
return result; return estimate;
} }
private static ReadOnlySpan<uint> GetCombinedDivisors(byte nMod10) private static ReadOnlySpan<uint> GetCombinedDivisors(byte nMod10)
PerfectNumbers.Core/HeuristicPrimeTester.cs
+13 -16
View File
@@ -436,31 +436,28 @@ Cleanup:
return n; return n;
} }
ulong bit = 1UL << 62; int log2 = BitOperations.Log2(n);
while (bit > n) ulong estimate = 1UL << ((log2 + 1) >> 1);
ulong quotient = n / estimate;
while (estimate > quotient)
{ {
bit >>= 2; estimate = (estimate + quotient) >> 1;
quotient = n / estimate;
} }
ulong result = 0UL; while (true)
ulong remainder = n;
while (bit != 0)
{ {
ulong candidate = result + bit; ulong nextEstimate = estimate + 1UL;
if (remainder >= candidate) if (nextEstimate > n / nextEstimate)
{ {
remainder -= candidate; break;
result = (result >> 1) + bit;
}
else
{
result >>= 1;
} }
bit >>= 2; estimate = nextEstimate;
} }
return result; return estimate;
} }
private static ReadOnlySpan<byte> GetGroupBEndingOrder(byte nMod10) => nMod10 switch private static ReadOnlySpan<byte> GetGroupBEndingOrder(byte nMod10) => nMod10 switch