namespace MrGameEng.Core;

/// <summary>
/// Minimal service locator used by engine modules to expose their services
/// (input, audio, assets, …) to scenes and systems without coupling modules to each other.
/// </summary>
public sealed class ServiceRegistry
{
    private readonly Dictionary<Type, object> _services = new();

    /// <summary>Registers a service instance under type <typeparamref name="T"/>. Throws if already registered.</summary>
    public void Add<T>(T service)
        where T : class
    {
        if (!_services.TryAdd(typeof(T), service))
        {
            throw new InvalidOperationException(
                $"Service of type {typeof(T)} is already registered."
            );
        }
    }

    /// <summary>Returns the registered service of type <typeparamref name="T"/>. Throws if missing.</summary>
    public T Get<T>()
        where T : class
    {
        return _services.TryGetValue(typeof(T), out var service)
            ? (T)service
            : throw new InvalidOperationException(
                $"Service of type {typeof(T)} is not registered."
            );
    }

    /// <summary>Returns the registered service of type <typeparamref name="T"/> or null.</summary>
    public T? GetOrDefault<T>()
        where T : class
    {
        return _services.TryGetValue(typeof(T), out var service) ? (T)service : null;
    }

    /// <summary>
    /// Disposes every registered <see cref="IDisposable"/> service (each instance once, even
    /// when registered under several types) and clears the registry. <paramref name="except"/>
    /// is skipped. Called on host shutdown.
    /// </summary>
    internal void DisposeServices(object? except = null)
    {
        var disposed = new HashSet<object>(ReferenceEqualityComparer.Instance);
        foreach (var service in _services.Values)
        {
            if (
                !ReferenceEquals(service, except)
                && service is IDisposable disposable
                && disposed.Add(service)
            )
            {
                disposable.Dispose();
            }
        }

        _services.Clear();
    }
}