using System.Numerics;

namespace AcDream.Core.Selection;

/// <summary>
/// Pure mouse-pixel to world-ray conversion shared by retail selection paths.
/// </summary>
public static class WorldPicker
{
    /// <summary>
    /// Unprojects a pixel coordinate to a world-space ray using System.Numerics'
    /// row-vector convention (<c>view * projection</c>).
    /// </summary>
    /// <returns>
    /// The camera viewpoint and normalized direction, or two zero vectors
    /// when the camera transform is singular.
    /// </returns>
    public static (Vector3 Origin, Vector3 Direction) BuildRay(
        float mouseX,
        float mouseY,
        float viewportW,
        float viewportH,
        Matrix4x4 view,
        Matrix4x4 projection)
    {
        float ndcX = (2f * mouseX) / viewportW - 1f;
        float ndcY = 1f - (2f * mouseY) / viewportH;

        Matrix4x4 vp = view * projection;
        if (!Matrix4x4.Invert(vp, out Matrix4x4 invVp)
            || !Matrix4x4.Invert(view, out Matrix4x4 invView))
            return (Vector3.Zero, Vector3.Zero);

        Vector4 nearClip = new(ndcX, ndcY, -1f, 1f);
        Vector4 farClip = new(ndcX, ndcY, 1f, 1f);
        Vector4 near = Vector4.Transform(nearClip, invVp);
        Vector4 far = Vector4.Transform(farClip, invVp);
        if (near.W == 0f || far.W == 0f)
            return (Vector3.Zero, Vector3.Zero);

        Vector3 nearWorld = new Vector3(near.X, near.Y, near.Z) / near.W;
        Vector3 farWorld = new Vector3(far.X, far.Y, far.Z) / far.W;
        Vector3 direction = farWorld - nearWorld;
        if (direction.LengthSquared() < 1e-10f)
            return (Vector3.Zero, Vector3.Zero);

        // Render::pick_ray @ 0x0054B610 stores a direction through the pixel;
        // GfxObjUnderSelectionRay @ 0x0054C740 pairs it with Render::viewpoint,
        // not the near-plane point. Inverse-view origin is that viewpoint in
        // System.Numerics' row-vector convention.
        Vector3 viewpoint = Vector3.Transform(Vector3.Zero, invView);
        return (viewpoint, Vector3.Normalize(direction));
    }
}