namespace AcDream.App.Rendering;

/// <summary>
/// Campaign V slice V4a bridge: memoizes a <see cref="GpuTextureSlot"/> for the
/// most recent raw GL colour-texture name a still-unmigrated private-viewport
/// renderer (<see cref="PrivateEntityViewportRenderer"/>,
/// <see cref="PaperdollViewportRenderer"/>) produced, so
/// <c>UiViewport.TextureHandle</c> — now a <see cref="GpuTextureSlot"/> — has
/// something to draw. Those renderers still allocate their FBO colour
/// attachment directly on <c>GL</c> (V4g's scope: "PrivateEntityViewportRenderer
/// → IGpuRenderTarget"), so this bridge — not their own campaign slice — is
/// what lets the RETAINED UI side of the seam move onto the RHI now.
///
/// Registers lazily and only re-registers when the producer hands back a
/// DIFFERENT raw name (the FBO's colour texture is stable across frames at a
/// fixed viewport size and only regenerates on resize) — a naive
/// register-every-frame would exhaust the 16384-slot table in seconds.
/// Deleted at slice V4g once those renderers publish a real
/// <see cref="GpuTextureSlot"/> directly.
/// </summary>
internal sealed class ExternalViewportTextureBridge
{
    private readonly IGpuDevice _device;
    private readonly IGpuSampler _sampler;
    private uint _lastRawName;
    private GpuTextureSlot _lastSlot = GpuTextureSlot.Unassigned;

    public ExternalViewportTextureBridge(IGpuDevice device)
    {
        _device = device ?? throw new ArgumentNullException(nameof(device));
        // Matches the FBO colour attachment's own fixed GL_LINEAR / CLAMP_TO_EDGE
        // parameters (PrivateEntityViewportRenderer.EnsureFramebuffer,
        // PaperdollViewportRenderer's equivalent): a bindless handle's
        // filtering comes from the bound SAMPLER object, not the texture's
        // own — now irrelevant — TEXTURE_MIN_FILTER/TEXTURE_MAG_FILTER.
        _sampler = device.CreateSampler(GpuSamplerDescription.WorldClamp);
    }

    /// <summary>
    /// Resolves <paramref name="rawGlColorTextureName"/> (0 = nothing rendered
    /// this call, matching the producers' existing "0 = no texture" return)
    /// to a texture-table slot, registering it once per distinct name.
    /// </summary>
    public GpuTextureSlot Resolve(uint rawGlColorTextureName)
    {
        if (rawGlColorTextureName == 0)
        {
            Release();
            return GpuTextureSlot.Unassigned;
        }
        if (rawGlColorTextureName == _lastRawName && _lastSlot.IsAssigned)
            return _lastSlot;

        Release();
        if (_device is not Gpu.Gl.GlGpuDevice glDevice)
        {
            throw new NotSupportedException(
                "ExternalViewportTextureBridge only supports the GL backend. " +
                "Slice V4g removes this bridge before any other backend ships.");
        }
        _lastSlot = glDevice.RegisterExternalColorTexture(rawGlColorTextureName, _sampler);
        _lastRawName = rawGlColorTextureName;
        return _lastSlot;
    }

    private void Release()
    {
        if (_lastSlot.IsAssigned)
            _device.ReleaseTextureSlot(_lastSlot);
        _lastSlot = GpuTextureSlot.Unassigned;
        _lastRawName = 0;
    }
}