using System;
using System.Buffers;
using System.Collections.Concurrent;
using System.Collections.Generic;
using System.Numerics;
using System.Runtime.CompilerServices;
using ILGPU;
using ILGPU.Runtime;
using PerfectNumbers.Core.Gpu;

namespace PerfectNumbers.Core;

public sealed class PrimeTester
{
	public PrimeTester()
	{
	}

	[ThreadStatic]
	private static PrimeTester? _tester;

	public static PrimeTester Exclusive => _tester ??= new();

	public static bool IsPrime(ulong n, CancellationToken ct)
	{
		if (n <= 1UL)
		{
			return false;
		}

		if (n == 2UL)
		{
			throw new InvalidOperationException("PrimeTester.IsPrime encountered the sentinel input 2.");
		}

		bool isOdd = (n & 1UL) != 0UL;
		bool result = isOdd;

		bool requiresTrialDivision = result && n >= 7UL;

		if (requiresTrialDivision)
		{
			// EvenPerfectBitScanner streams exponents starting at 136,279,841, so the Mod10/GCD guard never fires on the
			// production path. Leave the logic commented out as instrumentation for diagnostic builds.
			// bool sharesMaxExponentFactor = n.Mod10() == 1UL && SharesFactorWithMaxExponent(n);
			// result &= !sharesMaxExponentFactor;

			if (result)
			{
				uint[] smallPrimeDivisors = PrimesGenerator.SmallPrimes;
				ulong[] smallPrimeDivisorsMul = PrimesGenerator.SmallPrimesPow2;

				ulong nMod10 = n.Mod10();
				switch (nMod10)
				{
					case 1UL:
						smallPrimeDivisors = PrimesGenerator.SmallPrimesLastOne;
						smallPrimeDivisorsMul = PrimesGenerator.SmallPrimesPow2LastOne;
						break;
					case 3UL:
						smallPrimeDivisors = DivisorGenerator.SmallPrimesLastThree;
						smallPrimeDivisorsMul = DivisorGenerator.SmallPrimesPow2LastThree;
						break;
					case 7UL:
						smallPrimeDivisors = PrimesGenerator.SmallPrimesLastSeven;
						smallPrimeDivisorsMul = PrimesGenerator.SmallPrimesPow2LastSeven;
						break;
					case 9UL:
						smallPrimeDivisors = DivisorGenerator.SmallPrimesLastNine;
						smallPrimeDivisorsMul = DivisorGenerator.SmallPrimesPow2LastNine;
						break;
				}

				int smallPrimeDivisorsLength = smallPrimeDivisors.Length;
				for (int i = 0; i < smallPrimeDivisorsLength; i++)
				{
					if (smallPrimeDivisorsMul[i] > n)
					{
						break;
					}

					if (n % smallPrimeDivisors[i] == 0)
					{
						result = false;
						break;
					}
				}
			}
		}

		return result;
	}

	public static bool IsPrimeGpu(ulong n)
	{
		return Exclusive.IsPrimeGpu(n, CancellationToken.None);
	}

	[MethodImpl(MethodImplOptions.AggressiveInlining)]
	public bool IsPrimeGpu(ulong n, CancellationToken ct)
        {
                bool forceCpu = GpuContextPool.ForceCpu;
                bool belowGpuRange = n < 31UL;

                if (forceCpu || belowGpuRange)
                {
                        return IsPrimeCpu(n, ct);
                }

                var limiter = GpuPrimeWorkLimiter.Acquire();
                var gpu = PrimeTesterGpuContextPool.Rent(1);
                var accelerator = gpu.Accelerator;
                var kernel = gpu.Kernel;

                ulong value = n;
                byte flag = 0;

                var input = gpu.Input;
                var output = gpu.Output;

                input.View.CopyFromCPU(ref value, 1);
                kernel(
                                1,
                                input.View,
                                gpu.DevicePrimesDefault.View,
                                gpu.DevicePrimesLastOne.View,
                                gpu.DevicePrimesLastSeven.View,
                                gpu.DevicePrimesLastThree.View,
                                gpu.DevicePrimesLastNine.View,
                                gpu.DevicePrimesPow2Default.View,
                                gpu.DevicePrimesPow2LastOne.View,
                                gpu.DevicePrimesPow2LastSeven.View,
                                gpu.DevicePrimesPow2LastThree.View,
                                gpu.DevicePrimesPow2LastNine.View,
                                output.View);
                accelerator.Synchronize();
                output.View.CopyToCPU(ref flag, 1);

                gpu.Dispose();
                limiter.Dispose();

                return flag != 0;
        }

public static bool IsPrimeCpu(ulong n, CancellationToken ct)
	{
		// Preserve the legacy entry point for callers that bypass the thread-local caches.
		return IsPrime(n, ct);
	}


	public static int GpuBatchSize { get; set; } = 262_144;

	[MethodImpl(MethodImplOptions.AggressiveInlining)]
	public static void IsPrimeBatchGpu(ReadOnlySpan<ulong> values, Span<byte> results)
        {
                var limiter = GpuPrimeWorkLimiter.Acquire();
                var gpu = PrimeTesterGpuContextPool.Rent(GpuBatchSize);
                var accelerator = gpu.Accelerator;
                var kernel = gpu.Kernel;
                int totalLength = values.Length;
                int batchSize = GpuBatchSize;

                var input = gpu.Input;
                var output = gpu.Output;
                ArrayPool<ulong> pool = ThreadStaticPools.UlongPool;
                ulong[] temp = pool.Rent(batchSize);

                int pos = 0;
                while (pos < totalLength)
                {
                        int remaining = totalLength - pos;
                        int count = remaining > batchSize ? batchSize : remaining;

                        values.Slice(pos, count).CopyTo(temp);
                        input.View.CopyFromCPU(ref temp[0], count);

                        kernel(
                                count,
                                input.View,
                                gpu.DevicePrimesDefault.View,
                                gpu.DevicePrimesLastOne.View,
                                gpu.DevicePrimesLastSeven.View,
                                gpu.DevicePrimesLastThree.View,
                                gpu.DevicePrimesLastNine.View,
                                gpu.DevicePrimesPow2Default.View,
                                gpu.DevicePrimesPow2LastOne.View,
                                gpu.DevicePrimesPow2LastSeven.View,
                                gpu.DevicePrimesPow2LastThree.View,
                                gpu.DevicePrimesPow2LastNine.View,
                                output.View);
                        accelerator.Synchronize();
                        output.View.CopyToCPU(ref results[pos], count);

                        pos += count;
                }

                pool.Return(temp, clearArray: false);

                gpu.Dispose();
                limiter.Dispose();
        }

        internal static class PrimeTesterGpuContextPool
        {
                private static readonly ConcurrentQueue<PrimeTesterGpuContextLease> Pool = new();

                internal static PrimeTesterGpuContextLease Rent(int minBufferCapacity = 1)
                {
                        if (!Pool.TryDequeue(out var lease))
                        {
                                lease = new PrimeTesterGpuContextLease(minBufferCapacity);
                        }
                        else
                        {
                                lease.EnsureCapacity(minBufferCapacity);
                        }

                        lease.ResetReturnFlag();
                        return lease;
                }

                internal static void Return(PrimeTesterGpuContextLease lease)
                {
                        lease.Accelerator.Synchronize();
                        Pool.Enqueue(lease);
                }

                internal static void DisposeAll()
                {
                        while (Pool.TryDequeue(out var lease))
                        {
                                lease.DisposeResources();
                        }
                }

                internal static void Clear(Accelerator accelerator)
                {
                        if (accelerator is null)
                        {
                                return;
                        }

                        var retained = new List<PrimeTesterGpuContextLease>();
                        while (Pool.TryDequeue(out var lease))
                        {
                                if (lease.Accelerator == accelerator)
                                {
                                        lease.DisposeResources();
                                }
                                else
                                {
                                        retained.Add(lease);
                                }
                        }

                        int retainedCount = retained.Count;
                        for (int i = 0; i < retainedCount; i++)
                        {
                                Pool.Enqueue(retained[i]);
                        }
                }

                internal sealed class PrimeTesterGpuContextLease : IDisposable
                {
                        private bool _returned;

                        internal readonly Context AcceleratorContext;
                        public readonly Accelerator Accelerator;
                        public readonly Action<Index1D, ArrayView<ulong>, ArrayView<uint>, ArrayView<uint>, ArrayView<uint>, ArrayView<uint>, ArrayView<uint>, ArrayView<ulong>, ArrayView<ulong>, ArrayView<ulong>, ArrayView<ulong>, ArrayView<ulong>, ArrayView<byte>> Kernel;
                        public readonly Action<Index1D, ArrayView<ulong>, ulong, ArrayView<byte>> HeuristicTrialDivisionKernel;
                        public readonly MemoryBuffer1D<uint, Stride1D.Dense> DevicePrimesDefault;
                        public readonly MemoryBuffer1D<uint, Stride1D.Dense> DevicePrimesLastOne;
                        public readonly MemoryBuffer1D<uint, Stride1D.Dense> DevicePrimesLastSeven;
                        public readonly MemoryBuffer1D<uint, Stride1D.Dense> DevicePrimesLastThree;
                        public readonly MemoryBuffer1D<uint, Stride1D.Dense> DevicePrimesLastNine;
                        public readonly MemoryBuffer1D<ulong, Stride1D.Dense> DevicePrimesPow2Default;
                        public readonly MemoryBuffer1D<ulong, Stride1D.Dense> DevicePrimesPow2LastOne;
                        public readonly MemoryBuffer1D<ulong, Stride1D.Dense> DevicePrimesPow2LastSeven;
                        public readonly MemoryBuffer1D<ulong, Stride1D.Dense> DevicePrimesPow2LastThree;
                        public readonly MemoryBuffer1D<ulong, Stride1D.Dense> DevicePrimesPow2LastNine;
                        public MemoryBuffer1D<ulong, Stride1D.Dense> Input;
                        public MemoryBuffer1D<byte, Stride1D.Dense> Output;
                        public int BufferCapacity;

                        internal PrimeTesterGpuContextLease(int minBufferCapacity)
                        {
                                AcceleratorContext = Context.CreateDefault();
                                Accelerator = AcceleratorContext.GetPreferredDevice(false).CreateAccelerator(AcceleratorContext);
                                Kernel = Accelerator.LoadAutoGroupedStreamKernel<Index1D, ArrayView<ulong>, ArrayView<uint>, ArrayView<uint>, ArrayView<uint>, ArrayView<uint>, ArrayView<uint>, ArrayView<ulong>, ArrayView<ulong>, ArrayView<ulong>, ArrayView<ulong>, ArrayView<ulong>, ArrayView<byte>>(PrimeTesterKernels.SmallPrimeSieveKernel);
                                HeuristicTrialDivisionKernel = Accelerator.LoadAutoGroupedStreamKernel<Index1D, ArrayView<ulong>, ulong, ArrayView<byte>>(PrimeTesterKernels.HeuristicTrialDivisionKernel);

                                var primesDefault = DivisorGenerator.SmallPrimes;
                                DevicePrimesDefault = Accelerator.Allocate1D<uint>(primesDefault.Length);
                                DevicePrimesDefault.View.CopyFromCPU(primesDefault);

                                var primesLastOne = DivisorGenerator.SmallPrimesLastOne;
                                DevicePrimesLastOne = Accelerator.Allocate1D<uint>(primesLastOne.Length);
                                DevicePrimesLastOne.View.CopyFromCPU(primesLastOne);

                                var primesLastSeven = DivisorGenerator.SmallPrimesLastSeven;
                                DevicePrimesLastSeven = Accelerator.Allocate1D<uint>(primesLastSeven.Length);
                                DevicePrimesLastSeven.View.CopyFromCPU(primesLastSeven);

                                var primesLastThree = DivisorGenerator.SmallPrimesLastThree;
                                DevicePrimesLastThree = Accelerator.Allocate1D<uint>(primesLastThree.Length);
                                DevicePrimesLastThree.View.CopyFromCPU(primesLastThree);

                                var primesLastNine = DivisorGenerator.SmallPrimesLastNine;
                                DevicePrimesLastNine = Accelerator.Allocate1D<uint>(primesLastNine.Length);
                                DevicePrimesLastNine.View.CopyFromCPU(primesLastNine);

                                var primesPow2Default = DivisorGenerator.SmallPrimesPow2;
                                DevicePrimesPow2Default = Accelerator.Allocate1D<ulong>(primesPow2Default.Length);
                                DevicePrimesPow2Default.View.CopyFromCPU(primesPow2Default);

                                var primesPow2LastOne = DivisorGenerator.SmallPrimesPow2LastOne;
                                DevicePrimesPow2LastOne = Accelerator.Allocate1D<ulong>(primesPow2LastOne.Length);
                                DevicePrimesPow2LastOne.View.CopyFromCPU(primesPow2LastOne);

                                var primesPow2LastSeven = DivisorGenerator.SmallPrimesPow2LastSeven;
                                DevicePrimesPow2LastSeven = Accelerator.Allocate1D<ulong>(primesPow2LastSeven.Length);
                                DevicePrimesPow2LastSeven.View.CopyFromCPU(primesPow2LastSeven);

                                var primesPow2LastThree = DivisorGenerator.SmallPrimesPow2LastThree;
                                DevicePrimesPow2LastThree = Accelerator.Allocate1D<ulong>(primesPow2LastThree.Length);
                                DevicePrimesPow2LastThree.View.CopyFromCPU(primesPow2LastThree);

                                var primesPow2LastNine = DivisorGenerator.SmallPrimesPow2LastNine;
                                DevicePrimesPow2LastNine = Accelerator.Allocate1D<ulong>(primesPow2LastNine.Length);
                                DevicePrimesPow2LastNine.View.CopyFromCPU(primesPow2LastNine);

                                BufferCapacity = minBufferCapacity < 1 ? 1 : minBufferCapacity;
                                Input = Accelerator.Allocate1D<ulong>(BufferCapacity);
                                Output = Accelerator.Allocate1D<byte>(BufferCapacity);
                                _returned = false;
                        }

                        public void EnsureCapacity(int minCapacity)
                        {
                                int required = minCapacity < 1 ? 1 : minCapacity;
                                if (required <= BufferCapacity)
                                {
                                        return;
                                }

                                Input.Dispose();
                                Output.Dispose();
                                Input = Accelerator.Allocate1D<ulong>(required);
                                Output = Accelerator.Allocate1D<byte>(required);
                                BufferCapacity = required;
                        }

                        internal void ResetReturnFlag()
                        {
                                _returned = false;
                        }

                        public void Dispose()
                        {
                                if (_returned)
                                {
                                        return;
                                }

                                _returned = true;
                                PrimeTesterGpuContextPool.Return(this);
                        }

                        internal void DisposeResources()
                        {
                                Input.Dispose();
                                Output.Dispose();
                                DevicePrimesDefault.Dispose();
                                DevicePrimesLastOne.Dispose();
                                DevicePrimesLastSeven.Dispose();
                                DevicePrimesLastThree.Dispose();
                                DevicePrimesLastNine.Dispose();
                                DevicePrimesPow2Default.Dispose();
                                DevicePrimesPow2LastOne.Dispose();
                                DevicePrimesPow2LastSeven.Dispose();
                                DevicePrimesPow2LastThree.Dispose();
                                DevicePrimesPow2LastNine.Dispose();
                                Accelerator.Dispose();
                                AcceleratorContext.Dispose();
                        }
                }
        }
        // Expose cache clearing for accelerator disposal coordination
	public static void ClearGpuCaches(Accelerator accelerator)
	{
		PrimeTesterGpuContextPool.Clear(accelerator);
	}

	internal static void DisposeGpuContexts()
	{
		PrimeTesterGpuContextPool.DisposeAll();
	}

	[MethodImpl(MethodImplOptions.AggressiveInlining)]
	internal static bool SharesFactorWithMaxExponent(ulong n)
	{
		// TODO: Replace this on-the-fly GCD probe with the cached factor table derived from
		// ResidueComputationBenchmarks so divisor-cycle metadata can short-circuit the test
		// instead of recomputing binary GCD for every candidate.
		ulong m = (ulong)BitOperations.Log2(n);
		return BinaryGcd(n, m) != 1UL;
	}

	internal static void SharesFactorWithMaxExponentBatch(ReadOnlySpan<ulong> values, Span<byte> results)
	{
		// TODO: Route this batch helper through the shared GPU kernel pool from
		// GpuUInt128BinaryGcdBenchmarks so we reuse cached kernels, pinned host buffers,
		// and divisor-cycle staging instead of allocating new device buffers per call.
		var gpu = PrimeTesterGpuContextPool.Rent();
		var accelerator = gpu.Accelerator;
		var kernel = accelerator.LoadAutoGroupedStreamKernel<Index1D, ArrayView<ulong>, ArrayView<byte>>(PrimeTesterKernels.SharesFactorKernel);

		int length = values.Length;
		var inputBuffer = accelerator.Allocate1D<ulong>(length);
		var resultBuffer = accelerator.Allocate1D<byte>(length);

		ArrayPool<ulong> pool = ThreadStaticPools.UlongPool;
		ulong[] temp = pool.Rent(length);
		values.CopyTo(temp);
		inputBuffer.View.CopyFromCPU(ref temp[0], length);
		kernel(length, inputBuffer.View, resultBuffer.View);
		accelerator.Synchronize();
		resultBuffer.View.CopyToCPU(ref results[0], length);
		pool.Return(temp, clearArray: false);
		resultBuffer.Dispose();
		inputBuffer.Dispose();
		gpu.Dispose();
	}

	[MethodImpl(MethodImplOptions.AggressiveInlining)]
	private static ulong BinaryGcd(ulong u, ulong v)
	{
		// TODO: Swap this handwritten binary GCD for the optimized helper measured in
		// GpuUInt128BinaryGcdBenchmarks so CPU callers share the faster subtract-less
		// ladder once the common implementation is promoted into PerfectNumbers.Core.
		if (u == 0UL)
		{
			return v;
		}

		if (v == 0UL)
		{
			return u;
		}

		int shift = BitOperations.TrailingZeroCount(u | v);
		u >>= BitOperations.TrailingZeroCount(u);

		do
		{
			v >>= BitOperations.TrailingZeroCount(v);
			if (u > v)
			{
				(u, v) = (v, u);
			}

			v -= u;
		}
		while (v != 0UL);

		return u << shift;
	}
}