using MessagePack;

namespace OECS;

/// <summary>
/// Saves and loads a <see cref="World"/> to/from a MessagePack stream.
///
/// Uses the compile-time generated <see cref="ComponentRegistry"/> to
/// serialize and deserialize components without reflection. All component
/// types used with the World API are discovered by the OECS.SourceGen
/// incremental generator at build time.
/// </summary>
public static class WorldSerializer
{
    /// <summary>
    /// Saves the world state to a stream.
    /// </summary>
    public static void Save(World world, Stream stream)
    {
        var entityComponents = new Dictionary<Entity, List<ComponentEntry>>();

        foreach (var desc in ComponentRegistry.Descriptors)
        {
            var set = world.Components.GetSet(desc.Type);
            if (set == null || set.Count == 0) continue;

            var entities = set.GetDenseEntities();
            for (int i = 0; i < set.Count; i++)
            {
                var entity = entities[i];
                var component = set.GetComponentAt(i);

                if (!entityComponents.TryGetValue(entity, out var list))
                {
                    list = new List<ComponentEntry>();
                    entityComponents[entity] = list;
                }

                list.Add(new ComponentEntry
                {
                    TypeName = desc.TypeName,
                    Data = desc.Serialize(component)
                });
            }
        }

        var snapshot = new WorldSnapshot
        {
            Entities = entityComponents
                .Select(kv => new EntitySnapshot
                {
                    Id = kv.Key.Id,
                    Version = kv.Key.Version,
                    Components = kv.Value.ToArray()
                })
                .ToArray()
        };

        MessagePackSerializer.Serialize(stream, snapshot);
    }

    /// <summary>
    /// Loads the world state from a stream, adding entities and components
    /// to the given world. The world should be empty or the caller is
    /// responsible for managing duplicate entities.
    /// </summary>
    public static void Load(World world, Stream stream)
    {
        var snapshot = MessagePackSerializer.Deserialize<WorldSnapshot>(stream);

        // Build a lookup by type name for O(1) descriptor resolution.
        var lookup = new Dictionary<string, ComponentDescriptor>();
        foreach (var desc in ComponentRegistry.Descriptors)
            lookup[desc.TypeName] = desc;

        // First pass: identify which entities are singleton backings.
        // Singletons are registered in ComponentRegistry with a known marker.
        var singletonTypes = new HashSet<Type>();
        foreach (var desc in ComponentRegistry.Descriptors)
        {
            if (desc.IsSingleton)
                singletonTypes.Add(desc.Type);
        }

        foreach (var es in snapshot.Entities)
        {
            var entity = world.CreateEntity(es.Entity);
            bool isSingleton = false;

            foreach (var ce in es.Components)
            {
                if (!lookup.TryGetValue(ce.TypeName, out var desc))
                    throw new InvalidOperationException(
                        $"Unknown component type '{ce.TypeName}'. " +
                        $"Ensure the component type is used with the World " +
                        $"API so the source generator can discover it.");

                if (desc.IsSingleton)
                    isSingleton = true;

                // Deserialize and add via the typed internal method.
                var component = desc.Deserialize(ce.Data);
                world.AddComponentBoxed(entity, component, desc.Type);
            }

            // If the entity has a singleton component, register it.
            if (isSingleton)
            {
                foreach (var ce in es.Components)
                {
                    if (lookup.TryGetValue(ce.TypeName, out var desc) && desc.IsSingleton)
                    {
                        world.RegisterSingletonEntity(desc.Type, entity);
                    }
                }
            }
        }
    }
}