using System.Reflection;
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;

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

            // If the loaded entity is the singleton entity, ensure the
            // singleton infrastructure is initialized so that subsequent
            // SetSingleton/GetSingleton calls work correctly.
            if (entity.Id == World.SingletonEntity.Id)
            {
                world.EnsureSingleton();
            }

            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.");

                // Deserialize and fix up IRelationship.Source before adding.
                var component = desc.Deserialize(ce.Data);
                if (component is IRelationship rel)
                    FixupRelationshipSource(rel, entity);

                // Add via the typed internal method — no reflection for the add itself.
                world.AddComponentBoxed(entity, component, desc.Type);
            }
        }
    }

    /// <summary>
    /// Sets the Source field on a deserialized IRelationship to match
    /// the entity it's being restored to. Uses a small cache of PropertyInfo
    /// to avoid repeated reflection lookups.
    /// </summary>
    private static void FixupRelationshipSource(IRelationship rel, Entity entity)
    {
        var type = rel.GetType();
        if (!_sourcePropCache.TryGetValue(type, out var prop))
        {
            prop = type.GetProperty("Source");
            _sourcePropCache[type] = prop;
        }
        prop?.SetValue(rel, entity);
    }

    private static readonly Dictionary<Type, PropertyInfo?> _sourcePropCache = new();
}