using System;
using System.Collections.Generic;
using System.Linq;
using gaemstone.ECS;
using gaemstone.Utility;
using static gaemstone.Flecs.Core;
using BindingFlags = System.Reflection.BindingFlags;

namespace gaemstone;

public class ModuleManager
{
	private readonly Dictionary<Entity, ModuleInfo> _modules = new();

	public Universe Universe { get; }
	public ModuleManager(Universe universe)
		=> Universe = universe;

	internal ModuleInfo? Lookup(Entity entity)
		=> _modules.GetValueOrDefault(entity);

	public void RegisterAllFrom(System.Reflection.Assembly assembly)
		=> RegisterAll(assembly.GetTypes());
	public void RegisterAll(IEnumerable<Type> types)
	{
		foreach (var type in types)
			if (type.Has<ModuleAttribute>())
				Register(type);
	}

	public EntityRef Register<T>()
		where T : class => Register(typeof(T));
	public EntityRef Register(Type moduleType)
	{
		if (!moduleType.IsClass || moduleType.IsGenericType || moduleType.IsGenericTypeDefinition) throw new Exception(
			$"Module {moduleType} must be a non-generic class");
		if (moduleType.Get<ModuleAttribute>() is not ModuleAttribute moduleAttr) throw new Exception(
			$"Module {moduleType} must be marked with ModuleAttribute");

		// Check if module type is static.
		if (moduleType.IsAbstract && moduleType.IsSealed) {

			// Static modules represent existing modules, as such they don't
			// create entities, only look up existing ones to add type lookups
			// for use with the Lookup(Type) method.

			if (moduleType.Get<PathAttribute>()?.Value is not string modulePathStr) throw new Exception(
				$"Existing module {moduleType} must have {nameof(PathAttribute)} set");
			var modulePath   = EntityPath.Parse(modulePathStr);
			var moduleEntity = Universe.LookupByPath(modulePath) ?? throw new Exception(
				$"Existing module {moduleType} with name '{modulePath}' not found");

			// This implementation is pretty naive. It simply gets all nested
			// types which are tagged with an ICreateEntityAttribute base
			// attribute and creates a lookup mapping. No sanity checking.

			foreach (var type in moduleType.GetNestedTypes()) {
				if (!type.GetCustomAttributes(true).OfType<ICreateEntityAttribute>().Any()) continue;

				var attr   = type.Get<PathAttribute>();
				var path   = EntityPath.Parse(attr?.Value ?? type.Name);
				var entity = Universe.LookupByPathOrThrow(moduleEntity, path);
				entity.CreateLookup(type);

				if (type.Has<RelationAttribute>()) entity.Add<Doc.Relation>();
			}

			return moduleEntity;

		} else {

			var path   = GetModulePath(moduleType);
			var module = new ModuleInfo(Universe, moduleType, path);
			_modules.Add(module.Entity, module);
			TryEnableModule(module);
			return module.Entity;

		}
	}

	private void TryEnableModule(ModuleInfo module)
	{
		if (module.UnmetDependencies.Count > 0) return;

		Console.WriteLine($"Enabling module {module.Path} ...");
		module.Enable();

		// Find other modules that might be missing this module as a dependency.
		foreach (var other in _modules.Values) {
			if (other.IsActive) continue;
			if (!other.UnmetDependencies.Contains(module.Entity)) continue;

			// Move the just enabled module from unmet to met depedencies.
			other.UnmetDependencies.Remove(module.Entity);
			other.MetDependencies.Add(module);

			TryEnableModule(other);
		}
	}

	public static EntityPath GetModulePath(Type type)
	{
		var attr = type.Get<ModuleAttribute>();
		if (attr == null) throw new ArgumentException(
			$"Module {type} must be marked with ModuleAttribute", nameof(type));

		var path = EntityPath.Parse(
			(type.Get<PathAttribute>() is PathAttribute pathAttr)
				? pathAttr.Value : type.Name);

		// If specified path is absolute, return it now.
		if (path.IsAbsolute) return path;

		// Otherwise, create it based on the type's assembly, namespace and name.
		var assemblyName = type.Assembly.GetName().Name!;

		if (!type.FullName!.StartsWith(assemblyName + '.')) throw new InvalidOperationException(
			$"Module {type} must be defined under namespace {assemblyName}");
		var fullNameWithoutAssembly = type.FullName![(assemblyName.Length + 1)..];

		var parts = fullNameWithoutAssembly.Split('.')[..^1];
		return new(true, parts.Prepend(assemblyName).Concat(path.GetParts()).ToArray());
	}

	internal class ModuleInfo
	{
		public Universe Universe { get; }
		public Type Type { get; }
		public EntityPath Path { get; }

		public EntityRef Entity { get; }
		public object? Instance { get; internal set; }
		public bool IsActive => Instance != null;

		public HashSet<ModuleInfo> MetDependencies { get; } = new();
		public HashSet<Entity> UnmetDependencies { get; } = new();

		public ModuleInfo(Universe universe, Type type, EntityPath path)
		{
			Universe = universe;
			Type     = type;
			Path     = path;

			if (Type.IsAbstract || Type.IsSealed) throw new Exception(
				$"Module {Type} must not be abstract, sealed or static");
			if (Type.GetConstructor(Type.EmptyTypes) == null) throw new Exception(
				$"Module {Type} must define public parameterless constructor");

			var module = Universe.New(Path).Add<Module>();

			// Add module dependencies from [DependsOn<>] attributes.
			foreach (var dependsAttr in Type.GetMultiple<AddRelationAttribute>().Where(attr =>
					attr.GetType().GetGenericTypeDefinition() == typeof(DependsOnAttribute<>))) {
				var dependsPath = GetModulePath(dependsAttr.Target);
				var dependency = Universe.LookupByPath(dependsPath) ??
					Universe.New(dependsPath).Add<Module>().Disable().Build();

				var depModule = Universe.Modules.Lookup(dependency);
				if (depModule?.IsActive == true) MetDependencies.Add(depModule);
				else { UnmetDependencies.Add(dependency); module.Disable(); }

				module.Add<DependsOn>(dependency);
			}

			Entity = module.Build().CreateLookup(Type);

			// Ensure all parent entities have Module set.
			for (var p = Entity.Parent; p != null; p = p.Parent)
				p.Add<Module>();
		}

		public void Enable()
		{
			Entity.Enable();
			Instance = Activator.CreateInstance(Type)!;
			foreach (var type in Type.GetNestedTypes(BindingFlags.Public | BindingFlags.NonPublic))
				RegisterNestedType(type);
			(Instance as IModuleInitializer)?.Initialize(Entity);
			RegisterMethods(Instance);
		}

		private void RegisterNestedType(Type type)
		{
			if (!type.GetCustomAttributes(true).OfType<ICreateEntityAttribute>().Any()) return;

			// If proxied type is specified, use it instead of the marked type.
			// Attributes are still read from the original type.
			var proxyType = type.Get<ProxyAttribute>()?.Type ?? type;

			if (!type.Has<ComponentAttribute>() && (!proxyType.IsValueType || (proxyType.GetFields().Length > 0))) {
				var typeHint = (proxyType != type) ? $"{proxyType.Name} (proxied by {type})" : type.ToString();
				throw new Exception($"Type {typeHint} must be an empty, used-defined struct.");
			}

			var path = EntityPath.Parse(type.Get<PathAttribute>()?.Value ?? proxyType.Name);
			var builder = path.IsAbsolute ? Universe.New(path) : Entity.NewChild(path);

			if (type.Get<SymbolAttribute>() is SymbolAttribute symbolAttr)
				builder.Symbol(symbolAttr.Value ?? path.Name);

			var entity = builder.Build();

			EntityRef Lookup(Type toLookup)
				=> (type != toLookup) ? Universe.LookupByTypeOrThrow(toLookup) : entity;
			foreach (var attr in type.GetMultiple<AddEntityAttribute>())
				entity.Add(Lookup(attr.Entity));
			foreach (var attr in type.GetMultiple<AddRelationAttribute>())
				entity.Add(Lookup(attr.Relation), Lookup(attr.Target));

			if (type.Get<SingletonAttribute>()?.AutoAdd == true) entity.Add(entity);
			if (type.Has<ComponentAttribute>()) entity.InitComponent(proxyType);
			else                                entity.CreateLookup(proxyType);
			if (type.Has<RelationAttribute>()) entity.Add<Doc.Relation>();
			if (type.Has<TagAttribute>())      entity.Add<Tag>();
		}

		private void RegisterMethods(object? instance)
		{
			foreach (var method in Type.GetMethods(
				BindingFlags.Public | BindingFlags.NonPublic |
				BindingFlags.Static | BindingFlags.Instance
			)) {
				if (method.Has<SystemAttribute>())
					Universe.InitSystem(instance, method).ChildOf(Entity);
				if (method.Has<ObserverAttribute>())
					Universe.InitObserver(instance, method).ChildOf(Entity);
			}
		}
	}
}