using Microsoft.Xna.Framework;
using Microsoft.Xna.Framework.Graphics;

namespace MrGameEng.Host;

/// <summary>
/// Minimal overlay renderer handed to <see cref="OverlayTransition.Draw"/>: fills rectangles
/// in normalized screen coordinates (0..1 on both axes) over the rendered scene.
/// </summary>
public sealed class TransitionRenderer : IDisposable
{
    private readonly GraphicsDevice _device;
    private readonly BasicEffect _effect;
    private readonly VertexPositionColor[] _vertices = new VertexPositionColor[6];

    /// <summary>Disposes the GPU effect. Called by <see cref="GameHost"/> on shutdown.</summary>
    public void Dispose() => _effect.Dispose();

    internal TransitionRenderer(GraphicsDevice device)
    {
        _device = device;
        _effect = new BasicEffect(device)
        {
            VertexColorEnabled = true,
            TextureEnabled = false,
            World = Matrix.Identity,
            View = Matrix.Identity,
            Projection = Matrix.CreateOrthographicOffCenter(0f, 1f, 1f, 0f, 0f, 1f),
        };
    }

    /// <summary>
    /// Fills a rectangle given in normalized screen coordinates with <paramref name="color"/>
    /// at the given <paramref name="opacity"/> (0 = invisible, 1 = solid).
    /// </summary>
    public void Fill(float x, float y, float width, float height, Color color, float opacity = 1f)
    {
        var alpha = (byte)(Math.Clamp(opacity, 0f, 1f) * color.A);
        var premultiplied = Color.FromNonPremultiplied(color.R, color.G, color.B, alpha);

        var topLeft = new Vector3(x, y, 0f);
        var topRight = new Vector3(x + width, y, 0f);
        var bottomLeft = new Vector3(x, y + height, 0f);
        var bottomRight = new Vector3(x + width, y + height, 0f);

        _vertices[0] = new VertexPositionColor(topLeft, premultiplied);
        _vertices[1] = new VertexPositionColor(topRight, premultiplied);
        _vertices[2] = new VertexPositionColor(bottomLeft, premultiplied);
        _vertices[3] = new VertexPositionColor(bottomLeft, premultiplied);
        _vertices[4] = new VertexPositionColor(topRight, premultiplied);
        _vertices[5] = new VertexPositionColor(bottomRight, premultiplied);

        _device.BlendState = BlendState.AlphaBlend;
        _device.DepthStencilState = DepthStencilState.None;
        _device.RasterizerState = RasterizerState.CullNone;

        foreach (var pass in _effect.CurrentTechnique.Passes)
        {
            pass.Apply();
            _device.DrawUserPrimitives(PrimitiveType.TriangleList, _vertices, 0, 2);
        }
    }
}