103 lines
3.5 KiB
C#
103 lines
3.5 KiB
C#
using System.Runtime.CompilerServices;
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namespace PerfectNumbers.Core;
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public struct CycleRemainderStepper
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{
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private readonly ulong _cycleLength;
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private ulong _previousPrime;
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private ulong _currentRemainder;
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private bool _hasState;
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public CycleRemainderStepper(ulong cycleLength)
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{
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if (cycleLength == 0UL)
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{
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throw new ArgumentOutOfRangeException(nameof(cycleLength), "Cycle length must be non-zero.");
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}
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_cycleLength = cycleLength;
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_previousPrime = 0UL;
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_currentRemainder = 0UL;
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_hasState = false;
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}
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[MethodImpl(MethodImplOptions.AggressiveInlining)]
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public void Reset()
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{
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// TODO: Inline this reset at the call sites so the hot loops reuse struct reinitialization
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// measured fastest in MersenneDivisorCycleLengthGpuBenchmarks, avoiding the extra
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// method call when scanners need to restart stepping.
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_previousPrime = 0UL;
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_currentRemainder = 0UL;
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_hasState = false;
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}
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public ulong Initialize(ulong prime)
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{
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ulong cycleLength = _cycleLength;
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ulong remainder = prime;
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if (remainder >= cycleLength)
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{
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// TODO: Swap this `%` for the shared divisor-cycle remainder helper so initialization reuses the cached
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// cycle deltas benchmarked faster than on-the-fly modulo work, matching the
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// MersenneDivisorCycleLengthGpuBenchmarks winner for both CPU and GPU call sites.
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remainder %= cycleLength;
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}
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_previousPrime = prime;
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_currentRemainder = remainder;
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_hasState = true;
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return remainder;
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}
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public ulong ComputeNext(ulong prime)
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{
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if (!_hasState)
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{
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throw new InvalidOperationException("CycleRemainderStepper must be initialized before computing.");
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}
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if (prime <= _previousPrime)
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{
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throw new ArgumentOutOfRangeException(nameof(prime), "Primes must be processed in strictly increasing order.");
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}
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ulong cycleLength = _cycleLength;
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ulong delta = prime - _previousPrime;
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_previousPrime = prime;
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if (ulong.MaxValue - _currentRemainder < delta)
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{
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UInt128 extended = (UInt128)_currentRemainder + delta;
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// TODO: Replace this `%` with the UInt128-aware cycle reducer so large deltas use the cached subtraction
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// ladder instead of falling back to the slower modulo implementation.
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_currentRemainder = (ulong)(extended % cycleLength);
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return _currentRemainder;
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}
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_currentRemainder += delta;
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if (_currentRemainder >= cycleLength)
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{
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_currentRemainder -= cycleLength;
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if (_currentRemainder >= cycleLength)
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{
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// TODO: Route this `%` through the shared divisor-cycle helper so repeated wrap-arounds avoid
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// modulo operations and match the benchmarked fast path highlighted in
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// MersenneDivisorCycleLengthGpuBenchmarks.
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_currentRemainder %= cycleLength;
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}
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}
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return _currentRemainder;
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}
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public void CaptureState(out ulong previousPrime, out ulong previousRemainder, out bool hasState)
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{
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// TODO: Expose these fields directly once the residue scanners adopt the single-cycle helper
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// so the hot path can read them without paying for an additional wrapper call per iteration.
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previousPrime = _previousPrime;
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previousRemainder = _currentRemainder;
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hasState = _hasState;
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}
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}
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