using System.Numerics;

namespace AcDream.Core.Selection;

/// <summary>
/// Shared screen-space projection math for the target indicator and the
/// world picker. Both call into <see cref="TryProjectSphereToScreenRect"/>
/// so the click hit-area is guaranteed to match the visible indicator
/// rect — "what you see is what you click".
///
/// <para>
/// Retail equivalent: <c>SmartBox::GetObjectBoundingBox</c> at
/// <c>0x00452e20</c>, which uses
/// <c>Render::GetViewerBBox(selection_sphere, &amp;corner1, &amp;corner2)</c>
/// to compute a camera-aligned bbox of the sphere and projects the two
/// corner points. We use the mathematical equivalent (project center,
/// compute screen radius analytically) — both produce identical pixel
/// rects for a standard right-handed perspective.
/// </para>
/// </summary>
public static class ScreenProjection
{
    /// <summary>
    /// Project a world-space sphere to a screen-space axis-aligned square
    /// rectangle.
    /// </summary>
    /// <param name="worldCenter">Sphere center in world space.</param>
    /// <param name="worldRadius">Sphere radius in world space.</param>
    /// <param name="view">View matrix (System.Numerics row-vector convention).</param>
    /// <param name="projection">Projection matrix. <c>M22 = cot(fovY/2)</c>
    /// for a standard right-handed perspective.</param>
    /// <param name="viewport">Viewport size in pixels (X = width, Y = height).</param>
    /// <param name="rectMin">Out: top-left corner of the rect in viewport pixels.</param>
    /// <param name="rectMax">Out: bottom-right corner of the rect in viewport pixels.</param>
    /// <param name="depth">Out: camera-space depth (<c>clip.W</c>) of the sphere
    /// center — use this for nearest-first sorting when multiple rects overlap.</param>
    /// <param name="minSidePixels">Minimum side length of the rect. Distant
    /// entities clamp to this so they remain pickable / visible. 12 px
    /// matches the indicator's clamp floor.</param>
    /// <returns>
    /// <c>true</c> if the sphere is in front of the camera and the rect was
    /// produced; <c>false</c> if the center is behind the camera
    /// (<c>clip.W &lt;= 0</c>) or the rect is more than a screen offset
    /// from the viewport (obviously off-screen).
    /// </returns>
    public static bool TryProjectSphereToScreenRect(
        Vector3 worldCenter, float worldRadius,
        Matrix4x4 view, Matrix4x4 projection, Vector2 viewport,
        out Vector2 rectMin, out Vector2 rectMax, out float depth,
        float minSidePixels = 12f)
    {
        rectMin = default;
        rectMax = default;
        depth   = 0f;

        var viewProj = view * projection;
        var clip = Vector4.Transform(new Vector4(worldCenter, 1f), viewProj);
        if (clip.W <= 0.001f) return false;

        depth = clip.W;

        float ndcX = clip.X / clip.W;
        float ndcY = clip.Y / clip.W;
        float screenX = (ndcX * 0.5f + 0.5f) * viewport.X;
        float screenY = (1f - (ndcY * 0.5f + 0.5f)) * viewport.Y;

        // Screen-space radius. projection.M22 = cot(fovY/2). clip.W is
        // the camera-space distance.
        float scaleY = projection.M22;
        if (scaleY <= 0f) return false;
        float screenRadius = worldRadius * scaleY * viewport.Y / (2f * clip.W);

        // Cull obviously-off-screen entities (more than a screen away).
        if (screenX + screenRadius < -viewport.X || screenX - screenRadius > 2f * viewport.X) return false;
        if (screenY + screenRadius < -viewport.Y || screenY - screenRadius > 2f * viewport.Y) return false;

        // Floor at minSidePixels so distant entities still get a visible /
        // clickable rect. The picker must apply the same floor as the
        // indicator or distant clicks won't match the visible bracket.
        if (screenRadius < minSidePixels * 0.5f) screenRadius = minSidePixels * 0.5f;

        rectMin = new Vector2(screenX - screenRadius, screenY - screenRadius);
        rectMax = new Vector2(screenX + screenRadius, screenY + screenRadius);
        return true;
    }
}