using Friflo.Engine.ECS;
using Friflo.Engine.ECS.Systems;

namespace MrGameEng.Core;

/// <summary>
/// A scene owns its ECS world (<see cref="EntityStore"/>) and two system roots:
/// <see cref="UpdateSystems"/> for game logic and <see cref="DrawSystems"/> for rendering.
/// Override <see cref="OnLoad"/> to create entities and register systems.
/// </summary>
public abstract class Scene
{
    /// <summary>The ECS world of this scene.</summary>
    public EntityStore Store { get; } = new();

    /// <summary>Systems executed every update tick, in registration order.</summary>
    public SystemRoot UpdateSystems { get; }

    /// <summary>Systems executed every draw tick, in registration order.</summary>
    public SystemRoot DrawSystems { get; }

    /// <summary>Engine context. Valid from <see cref="OnLoad"/> until <see cref="OnUnload"/>.</summary>
    public EngineContext Context => _context ?? throw new InvalidOperationException("Scene is not loaded.");

    /// <summary>True while the scene is the active, loaded scene.</summary>
    public bool IsLoaded => _context is not null;

    private EngineContext? _context;

    /// <summary>Initializes the scene's ECS world and system roots.</summary>
    protected Scene()
    {
        UpdateSystems = new SystemRoot(Store, "Update");
        DrawSystems = new SystemRoot(Store, "Draw");
    }

    /// <summary>Called once when the scene becomes active: create entities, add systems.</summary>
    protected abstract void OnLoad();

    /// <summary>Called once when the scene is replaced or the game exits. Release scene resources here.</summary>
    protected virtual void OnUnload() { }

    /// <summary>Runs the update phase. Called by <see cref="SceneManager"/>.</summary>
    public virtual void Update(GameClock clock) =>
        UpdateSystems.Update(new UpdateTick(clock.DeltaTime, (float)clock.TotalTime));

    /// <summary>Runs the draw phase. Called by <see cref="SceneManager"/>.</summary>
    public virtual void Draw(GameClock clock) =>
        DrawSystems.Update(new UpdateTick(clock.DeltaTime, (float)clock.TotalTime));

    internal void Load(EngineContext context)
    {
        _context = context;
        OnLoad();
    }

    internal void Unload()
    {
        OnUnload();
        _context = null;
    }
}