namespace AcDream.App.Rendering.Gpu.Gl;

/// <summary>
/// Pure bump-plus-free-list allocator for the GL texture table (the storage
/// buffer of bindless handles at <see cref="GpuBindingModel.StorageTextureTable"/>).
///
/// GL-free by design: it knows nothing about bindless handles, retirement
/// queues, or the SSBO itself. <see cref="GlGpuDevice.RegisterTexture"/> calls
/// <see cref="Allocate"/> to get a slot to write a handle into;
/// <see cref="GlGpuDevice.ReleaseTextureSlot"/> defers the call to
/// <see cref="Release"/> through the device's <c>IGpuResourceRetirementQueue</c>
/// so a freed slot is never handed back out while a submitted frame could
/// still be reading the old handle at that index.
/// </summary>
internal sealed class GlTextureSlotAllocator
{
    private readonly uint _capacity;
    private readonly Stack<uint> _freeList = new();
    private uint _nextBumpSlot;

    public GlTextureSlotAllocator(uint capacity)
    {
        ArgumentOutOfRangeException.ThrowIfZero(capacity);
        _capacity = capacity;
    }

    /// <summary>Slots currently handed out (bumped or reused) and not yet released.</summary>
    public int LiveCount => (int)_nextBumpSlot - _freeList.Count;

    public uint Allocate()
    {
        if (_freeList.Count > 0)
            return _freeList.Pop();

        if (_nextBumpSlot >= _capacity)
        {
            throw new InvalidOperationException(
                $"The GL texture table is exhausted: {_capacity} slots are all live. " +
                "Every texture cache/atlas must release slots it no longer needs before " +
                "registering new ones.");
        }

        return _nextBumpSlot++;
    }

    /// <summary>
    /// Returns a slot to the free list. Callers must only call this once the
    /// retirement queue confirms no live frame could still reference the slot.
    /// </summary>
    public void Release(uint slot)
    {
        if (slot >= _nextBumpSlot)
        {
            throw new ArgumentOutOfRangeException(
                nameof(slot),
                slot,
                "Cannot release a slot that was never allocated.");
        }

        _freeList.Push(slot);
    }
}