using System;
using System.Collections.Concurrent;
using ILGPU;
using ILGPU.Runtime;

namespace PerfectNumbers.Core.Gpu;

public static class GpuContextPool
{
	// The pool intentionally skips pre-loading ProcessEightBitWindows kernels so accelerator initialization
	// stays stable; GpuKernelPool JIT-compiles them on first use alongside the small-cycle uploads.
	// Default device preference for generic GPU kernels (prime scans, NTT, etc.)
	internal sealed class PooledContext
	{
		public Context Context { get; }
		public Accelerator Accelerator { get; }

		public PooledContext()
		{
			Context = Context.CreateDefault();
			Accelerator = Context.GetPreferredDevice(false).CreateAccelerator(Context);
			DivisorGeneratorGpuCache.WarmUpByDivisorTables(Accelerator);
			// NOTE: Avoid loading/compiling any kernel here to prevent implicit
			// CL stream/queue creation during accelerator construction.
			// Some OpenCL drivers are fragile when a queue is created immediately
			// after device init. Real kernels will be JIT-loaded at first use.
		}

		public void Dispose()
		{
			// Ensure all cached GPU buffers for this accelerator are released.
			NttGpuMath.ClearCaches(Accelerator);
			// Release PrimeTester GPU state for this accelerator (kernel/device primes)
			// Clear any per-accelerator cached resources to avoid releasing
			// after the accelerator is destroyed.
			PrimeTester.ClearGpuCaches(Accelerator);

			Accelerator.Dispose();
			Context.Dispose();
		}
	}

	private static readonly ConcurrentQueue<PooledContext> GpuPool = new();
	private static int WarmedGpuContextCount;

	// Allows callers to choose CPU/GPU per use-case, decoupled from ForceCpu.
	public static GpuContextLease Rent()
	{
		if (GpuPool.TryDequeue(out var gpu))
		{
			return new GpuContextLease(gpu);
		}

		// Create a new accelerator when the pool does not have one available.
		return new GpuContextLease(new PooledContext());
	}

	public static void WarmUpPool(int threadCount)
	{
		if (threadCount <= WarmedGpuContextCount)
		{
			return;
		}

		int toCreate = threadCount - WarmedGpuContextCount;
		var contexts = new PooledContext[toCreate];
		for (int i = 0; i < toCreate; i++)
		{
			contexts[i] = new PooledContext();
		}

		for (int i = 0; i < toCreate; i++)
		{
			GpuPool.Enqueue(contexts[i]);
		}

		WarmedGpuContextCount = threadCount;
	}

	public static void DisposeAll()
	{
		while (GpuPool.TryDequeue(out var gpu))
		{
			gpu.Dispose();
		}

		PrimeTester.DisposeGpuContexts();
		WarmedGpuContextCount = 0;
	}

	private static void Return(PooledContext ctx)
	{
		ctx.Accelerator.Synchronize();
		GpuPool.Enqueue(ctx);
	}

	public readonly struct GpuContextLease
	{
		private readonly PooledContext _ctx;

		internal GpuContextLease(PooledContext ctx)
		{
			_ctx = ctx;
		}

		public readonly Context Context => _ctx.Context;
		public readonly Accelerator Accelerator => _ctx.Accelerator;

		public readonly void Dispose()
		{
			Return(_ctx);
		}
	}
}