using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Threading.Tasks;

namespace Firebird2D.EventPipelines
{
    /// <summary>
    /// EventBus manages event pipelines and allows registering callbacks (hooks) if a pipeline is not Build Yet, as well as creating, retrieving, and destroying pipelines.
    /// </summary>
    public class EventBus
    {
        // Dictionary that stores various pipelines for specific FirebirdEvents.
        // The key is the event, and the value is the corresponding pipeline.
        private Dictionary<FirebirdEvent, I_Pipeline> pipelines = new();

        // Dictionary that stores a list of callbacks for each FirebirdEvent.
        // These callbacks will be invoked when the event's pipeline is build.
        private Dictionary<FirebirdEvent, List<Action<I_Pipeline>>> pendingHooks = new();

        /// <summary>
        /// Registers a callback (hook) for a specific event. The callback will be invoked when the pipeline for this event is built.
        /// </summary>
        /// <param name="callback">The function to be called when the pipeline for the event is built.</param>
        /// <param name="type">The event for which the callback should be registered.</param>
        /// <exception cref="ArgumentNullException">Thrown when the callback is null.</exception>
        public void OnPipelineIsBuild(Action<I_Pipeline> callback, FirebirdEvent type)
        {
            if (callback == null) throw new ArgumentNullException(nameof(callback));

            if (!pendingHooks.TryGetValue(type, out var hookList))
            {
                hookList = new();
                pendingHooks[type] = hookList;
            }

            hookList.Add(callback);
        }

        /// <summary>
        /// Attempts to create or retrieve a pipeline for the given event.
        /// If the pipeline already exists, it is returned.
        /// Otherwise, a new pipeline is created and stored.
        /// </summary>
        /// <param name="type">The event for which the pipeline should be created or retrieved.</param>
        /// <param name="argsType">The type of the arguments to be passed to the pipeline.</param>
        /// <param name="master">The master object used when creating the pipeline.</param>
        /// <returns>The pipeline for the given event.</returns>
        public I_Pipeline GetOrBuildPipeline(FirebirdEvent type, Type argsType, object master)
        {
            if (pipelines.TryGetValue(type, out var pipeline)) return pipeline;

            var genericPipelineType = typeof(Pipeline<>).MakeGenericType(argsType);
            var pipelineInstance = (I_Pipeline)Activator.CreateInstance(genericPipelineType, master)!;
            pipelines[type] = pipelineInstance;
            return pipelineInstance;
        }

        /// <summary>
        /// Retrieves the pipeline for an event if it exists; otherwise, returns null.
        /// </summary>
        /// <param name="type">The event for which the pipeline should be retrieved.</param>
        /// <param name="argsType">The type of the arguments for the pipeline.</param>
        /// <returns>The pipeline for the event, or null if it does not exist.</returns>
        public I_Pipeline? GetPipeline(FirebirdEvent type, Type argsType)
        {
            if (!pipelines.TryGetValue(type, out var pipeline)) return null;
            return pipeline;
        }

        /// <summary>
        /// Destroys the pipeline for an event. Only the "master" of the pipeline is allowed to destroy it.
        /// </summary>
        /// <param name="type">The event for which the pipeline should be destroyed.</param>
        /// <param name="master">The master object that has permission to destroy the pipeline.</param>
        /// <exception cref="InvalidDataException">Thrown when a non-authorized user attempts to destroy the pipeline.</exception>
        public void DestroyPipeline(FirebirdEvent type, object master)
        {
            if (!pipelines.ContainsKey(type)) return;
            if (!pipelines[type].PipelineMaster.Equals(master)) throw new InvalidDataException("Only the pipeline master is allowed to delete this pipeline.");
            pipelines.Remove(type);
        }
    }
}