using System;
using System.Runtime.InteropServices;
using AcDream.Core.Lighting;
using Silk.NET.OpenGL;

namespace AcDream.App.Rendering;

/// <summary>
/// GL wrapper that owns the SceneLighting UBO buffer, updates its
/// contents each frame, and keeps it bound at binding=1 so every
/// shader sampling <c>uLights[]</c> / <c>uFogColor</c> / etc reads
/// consistent data without per-shader re-upload.
///
/// <para>
/// Usage (r12 §13.2 + r13 §12.3):
/// <list type="number">
///   <item><description>Instantiate once at startup, after the GL context exists.</description></item>
///   <item><description>Each frame, after <see cref="LightManager.Tick"/>, call <see cref="Upload"/> with a freshly-built <see cref="SceneLightingUbo"/>.</description></item>
///   <item><description>Shader programs that declare <c>layout(std140, binding = 1) uniform SceneLighting { ... }</c> automatically pick up the data.</description></item>
/// </list>
/// </para>
/// </summary>
public sealed unsafe class SceneLightingUboBinding : IDisposable
{
    private readonly GL _gl;
    private readonly uint _ubo;
    private bool _disposed;

    public SceneLightingUboBinding(GL gl)
    {
        _gl = gl ?? throw new ArgumentNullException(nameof(gl));
        _ubo = _gl.GenBuffer();
        _gl.BindBuffer(BufferTargetARB.UniformBuffer, _ubo);
        // Pre-allocate with the final size; BufferSubData each frame.
        _gl.BufferData(
            BufferTargetARB.UniformBuffer,
            (nuint)SceneLightingUbo.SizeInBytes,
            (void*)0,
            BufferUsageARB.DynamicDraw);
        _gl.BindBuffer(BufferTargetARB.UniformBuffer, 0);

        // Bind the buffer to the chosen binding point exactly once — shaders
        // that declare this binding in their layout block will read from it
        // on every draw without further intervention.
        _gl.BindBufferBase(BufferTargetARB.UniformBuffer,
            SceneLightingUbo.BindingPoint, _ubo);
    }

    /// <summary>
    /// Push the current frame's UBO contents to the GPU. Cheap (576 bytes)
    /// so fine to call unconditionally every frame.
    /// </summary>
    public void Upload(SceneLightingUbo data)
    {
        _gl.BindBuffer(BufferTargetARB.UniformBuffer, _ubo);
        _gl.BufferSubData(BufferTargetARB.UniformBuffer,
            (nint)0, (nuint)SceneLightingUbo.SizeInBytes, &data);
        _gl.BindBuffer(BufferTargetARB.UniformBuffer, 0);
    }

    public void Dispose()
    {
        if (_disposed) return;
        _gl.DeleteBuffer(_ubo);
        _disposed = true;
    }
}