namespace PerfectNumbers.Core.Gpu;

public static class GpuWorkLimiter
{
    private static SemaphoreSlim _semaphore = new(1, int.MaxValue);
    private static readonly object _lock = new();
    private static int _currentLimit = 1;

    public static Lease Acquire()
    {
        // TODO: Replace the per-call Lease allocation with the pooled struct-based guard from the
        // GpuLimiterThroughputBenchmarks so limiter acquisition does not allocate when we enter the
        // GPU scanning hot path.
        var sem = _semaphore;
        sem.Wait();
        return new Lease(sem);
    }

    public static void SetLimit(int value)
    {
        if (value < 1)
        {
            value = 1;
        }

        lock (_lock)
        {
            if (value == _currentLimit)
            {
                return;
            }

            // Swap to a new semaphore for the new limit. We intentionally
            // do not dispose the old semaphore to avoid releasing handles
            // that may still be in use by existing Lease instances.
            // TODO: Consolidate with GpuPrimeWorkLimiter so both limiters share a pooled SemaphoreSlim and avoid rebuilding the
            // limiter state whenever limits are adjusted from the CLI.
            _semaphore = new SemaphoreSlim(value, value);
            _currentLimit = value;
        }
    }

    public sealed class Lease
    {
        private readonly SemaphoreSlim _sem;

        internal Lease(SemaphoreSlim sem)
        {
            _sem = sem;
        }

        public void Dispose()
        {
            _sem.Release();
        }
    }
}