even-perfect-numbers-scanner/PerfectNumbers.Core/MersenneNumberDivisorByDivisorTester.cs

using System;
using System.Buffers;
using System.Collections.Generic;
using System.Globalization;
using System.Threading.Tasks;
using Open.Numeric.Primes;
using System.Runtime.InteropServices;

namespace PerfectNumbers.Core;

public static class MersenneNumberDivisorByDivisorTester
{
    public static void Run(
            List<ulong> candidates,
            IMersenneNumberDivisorByDivisorTester tester,
            Dictionary<ulong, (bool DetailedCheck, bool PassedAllTests)>? previousResults,
            ulong startPrime,
            Action markComposite,
            Action clearComposite,
            Action<ulong, bool, bool, bool> printResult,
            int threadCount)
    {
        if (candidates.Count == 0)
        {
            Console.WriteLine("No candidates were provided for --mersenne=bydivisor.");
            return;
        }

        _ = threadCount;

        candidates.Sort();
        Span<ulong> candidateSpan = CollectionsMarshal.AsSpan(candidates);
        int candidateCount = candidateSpan.Length;

        bool applyStartPrime = startPrime > 0UL;
        int skippedByPreviousResults = 0;

        if (previousResults is not null && previousResults.Count > 0)
        {
            int recordedCount = previousResults.Count;
            bool preferSortedMerge = recordedCount <= 4096 || (recordedCount * 4L) <= candidateCount;
            if (preferSortedMerge)
            {
                ulong[] recordedCandidates = ArrayPool<ulong>.Shared.Rent(recordedCount);
                try
                {
                    Span<ulong> recordedSpan = recordedCandidates.AsSpan(0, recordedCount);
                    int recordedIndex = 0;
                    foreach (ulong value in previousResults.Keys)
                    {
                        recordedSpan[recordedIndex++] = value;
                    }

                    recordedSpan.Sort();

                    int writeIndex = 0;
                    int skipIndex = 0;

                    for (int readIndex = 0; readIndex < candidateCount; readIndex++)
                    {
                        ulong candidate = candidateSpan[readIndex];

                        while (skipIndex < recordedCount && recordedSpan[skipIndex] < candidate)
                        {
                            skipIndex++;
                        }

                        if (skipIndex < recordedCount && recordedSpan[skipIndex] == candidate)
                        {
                            skippedByPreviousResults++;
                            skipIndex++;
                            continue;
                        }

                        candidateSpan[writeIndex++] = candidate;
                    }

                    if (writeIndex < candidateCount)
                    {
                        candidates.RemoveRange(writeIndex, candidateCount - writeIndex);
                        candidateSpan = CollectionsMarshal.AsSpan(candidates); // candidateSpan now tracks the reduced list.
                        candidateCount = candidateSpan.Length;
                    }
                }
                finally
                {
                    ArrayPool<ulong>.Shared.Return(recordedCandidates, clearArray: true);
                }
            }
            else
            {
                int writeIndex = 0;
                for (int readIndex = 0; readIndex < candidateCount; readIndex++)
                {
                    ulong candidate = candidateSpan[readIndex];

                    if (previousResults.ContainsKey(candidate))
                    {
                        skippedByPreviousResults++;
                        continue;
                    }

                    candidateSpan[writeIndex++] = candidate;
                }

                if (writeIndex < candidateCount)
                {
                    candidates.RemoveRange(writeIndex, candidateCount - writeIndex);
                    candidateSpan = CollectionsMarshal.AsSpan(candidates); // candidateSpan now tracks the reduced list.
                    candidateCount = candidateSpan.Length;
                }
            }
        }

        if (skippedByPreviousResults > 0)
        {
            Console.WriteLine($"Skipped {skippedByPreviousResults.ToString(CultureInfo.InvariantCulture)} candidates excluded by previous results.");
        }

        int startIndex = 0;
        if (applyStartPrime)
        {
            startIndex = candidates.BinarySearch(startPrime);
            if (startIndex < 0)
            {
                startIndex = ~startIndex;
            }
            else
            {
                while (startIndex > 0 && candidates[startIndex - 1] >= startPrime)
                {
                    startIndex--;
                }
            }
        }

        List<ulong> primesToTest = candidates;
        Span<ulong> primesSpan = CollectionsMarshal.AsSpan(primesToTest);
        ulong maxPrime = 0UL;
        int primeWriteIndex = 0;

        using IEnumerator<ulong> primeEnumerator = Prime.Numbers.GetEnumerator();
        bool hasPrime = primeEnumerator.MoveNext();
        ulong currentPrime = hasPrime ? primeEnumerator.Current : 0UL;

        for (int index = startIndex; index < candidateCount; index++)
        {
            ulong candidate = primesSpan[index];

            if (candidate <= 1UL)
            {
                markComposite();
                printResult(candidate, false, false, false);
                continue;
            }

            while (hasPrime && currentPrime < candidate)
            {
                hasPrime = primeEnumerator.MoveNext();
                if (!hasPrime)
                {
                    break;
                }

                currentPrime = primeEnumerator.Current;
            }

            if (!hasPrime || currentPrime != candidate)
            {
                markComposite();
                printResult(candidate, false, false, false);
                continue;
            }

            primesSpan[primeWriteIndex++] = candidate;

            if (candidate > maxPrime)
            {
                maxPrime = candidate;
            }
        }

        if (primeWriteIndex < primesToTest.Count)
        {
            primesToTest.RemoveRange(primeWriteIndex, primesToTest.Count - primeWriteIndex);
        }

        if (primesToTest.Count == 0)
        {
            if (applyStartPrime)
            {
                Console.WriteLine($"No primes greater than or equal to {startPrime.ToString(CultureInfo.InvariantCulture)} were found for --mersenne=bydivisor.");
            }
            else
            {
                Console.WriteLine("No prime candidates remain for --mersenne=bydivisor after filtering.");
            }

            return;
        }

        if (maxPrime <= 1UL)
        {
            Console.WriteLine("The filter specified by --filter-p must contain at least one prime exponent greater than 1 for --mersenne=bydivisor.");
            return;
        }

        tester.ConfigureFromMaxPrime(maxPrime);

        int primeCount = primesToTest.Count;
        ulong[] primeBatch = ArrayPool<ulong>.Shared.Rent(primeCount);
        ulong[] allowedMaxBatch = ArrayPool<ulong>.Shared.Rent(primeCount);
        List<ulong> filteredPrimes = new(primeCount);

        try
        {
            Span<ulong> primeSpan = primeBatch.AsSpan(0, primeCount);
            Span<ulong> allowedMaxSpan = allowedMaxBatch.AsSpan(0, primeCount);

            for (int i = 0; i < primeCount; i++)
            {
                primeSpan[i] = primesToTest[i];
            }

            tester.PrepareCandidates(primeSpan, allowedMaxSpan);

            for (int i = 0; i < primeCount; i++)
            {
                ulong prime = primeSpan[i];
                ulong allowedMax = allowedMaxSpan[i];

                if (allowedMax < 3UL)
                {
                    clearComposite();
                    printResult(prime, true, true, true);
                    continue;
                }

                filteredPrimes.Add(prime);
            }
        }
        finally
        {
            ArrayPool<ulong>.Shared.Return(allowedMaxBatch, clearArray: true);
            ArrayPool<ulong>.Shared.Return(primeBatch, clearArray: true);
        }

        if (filteredPrimes.Count == 0)
        {
            if (applyStartPrime)
            {
                Console.WriteLine($"No primes greater than or equal to {startPrime.ToString(CultureInfo.InvariantCulture)} were found for --mersenne=bydivisor.");
            }

            return;
        }

        int workerCount = threadCount <= 0 ? Environment.ProcessorCount : threadCount;
        if (workerCount < 1)
        {
            workerCount = 1;
        }

        void ProcessPrime(ulong prime)
        {
            bool isPrime = tester.IsPrime(prime, out bool divisorsExhausted);

            if (!isPrime)
            {
                markComposite();
                printResult(prime, true, true, false);
                return;
            }

            clearComposite();
            printResult(prime, true, divisorsExhausted, true);
        }

        if (workerCount == 1)
        {
            foreach (ulong prime in filteredPrimes)
            {
                ProcessPrime(prime);
            }
        }
        else
        {
            ParallelOptions options = new()
            {
                MaxDegreeOfParallelism = workerCount
            };

            Parallel.ForEach(filteredPrimes, options, ProcessPrime);
        }
    }
}