using System;
using System.Linq.Expressions;
using System.Reflection;
using System.Threading;
using System.Threading.Tasks;
using Svrnty.CQRS.Abstractions.Attributes;

namespace Svrnty.CQRS.Abstractions.Discovery;

public sealed class CommandMeta : ICommandMeta
{
    private readonly string _name;
    private readonly string _lowerCamelCaseName;

    public CommandMeta(Type commandType, Type serviceType, Type commandResultType)
    {
        CommandType = commandType;
        ServiceType = serviceType;
        CommandResultType = commandResultType;

        // Cache reflection and computed values once in constructor
        var nameAttribute = commandType.GetCustomAttribute<CommandNameAttribute>();
        _name = nameAttribute?.Name ?? commandType.Name.Replace("Command", string.Empty);
        _lowerCamelCaseName = ComputeLowerCamelCaseName(_name);

        // Build compiled delegate for handler invocation
        CompiledInvoker = BuildCompiledInvoker(serviceType, commandType, commandResultType);
    }

    public CommandMeta(Type commandType, Type serviceType)
    {
        CommandType = commandType;
        ServiceType = serviceType;

        // Cache reflection and computed values once in constructor
        var nameAttribute = commandType.GetCustomAttribute<CommandNameAttribute>();
        _name = nameAttribute?.Name ?? commandType.Name.Replace("Command", string.Empty);
        _lowerCamelCaseName = ComputeLowerCamelCaseName(_name);

        // Build compiled delegate for handler invocation
        CompiledInvoker = BuildCompiledInvoker(serviceType, commandType, null);
    }

    private static string ComputeLowerCamelCaseName(string name)
    {
        if (string.IsNullOrEmpty(name))
            return name;

        var firstLetter = char.ToLowerInvariant(name[0]);
        return $"{firstLetter}{name.Substring(1)}";
    }

    private static Func<object, object, CancellationToken, Task<object?>> BuildCompiledInvoker(
        Type serviceType,
        Type commandType,
        Type? resultType)
    {
        // Parameters: (object handler, object command, CancellationToken ct)
        var handlerParam = Expression.Parameter(typeof(object), "handler");
        var commandParam = Expression.Parameter(typeof(object), "command");
        var ctParam = Expression.Parameter(typeof(CancellationToken), "ct");

        // Cast handler to actual handler type
        var handlerCast = Expression.Convert(handlerParam, serviceType);

        // Cast command to actual command type
        var commandCast = Expression.Convert(commandParam, commandType);

        // Get HandleAsync method with correct return type
        var expectedReturnType = resultType == null ? typeof(Task) : typeof(Task<>).MakeGenericType(resultType);
        var handleMethod = serviceType.GetMethod("HandleAsync",
            BindingFlags.Public | BindingFlags.Instance,
            null,
            new[] { commandType, typeof(CancellationToken) },
            null);

        if (handleMethod == null || handleMethod.ReturnType != expectedReturnType)
            throw new InvalidOperationException($"HandleAsync method with return type {expectedReturnType} not found on {serviceType}");

        // Call handler.HandleAsync(command, ct)
        var methodCall = Expression.Call(handlerCast, handleMethod, commandCast, ctParam);

        Expression invokeCall;
        if (resultType == null)
        {
            // Command without result: Task HandleAsync(...)
            var asyncHelperMethod = typeof(CommandMeta).GetMethod(nameof(InvokeAndReturnNull), System.Reflection.BindingFlags.Static | System.Reflection.BindingFlags.NonPublic)!;
            invokeCall = Expression.Call(asyncHelperMethod, methodCall);
        }
        else
        {
            // Command with result: Task<TResult> HandleAsync(...)
            var asyncHelperMethod = typeof(CommandMeta).GetMethod(nameof(InvokeAndBox), System.Reflection.BindingFlags.Static | System.Reflection.BindingFlags.NonPublic)!
                .MakeGenericMethod(resultType);
            invokeCall = Expression.Call(asyncHelperMethod, methodCall);
        }

        // Compile to delegate
        var lambda = Expression.Lambda<Func<object, object, CancellationToken, Task<object?>>>(
            invokeCall,
            handlerParam,
            commandParam,
            ctParam);

        return lambda.Compile();
    }

    private static async Task<object?> InvokeAndReturnNull(Task task)
    {
        await task;
        return null;
    }

    private static async Task<object?> InvokeAndBox<TResult>(Task<TResult> task)
    {
        var result = await task;
        return result;
    }

    public string Name => _name;
    public Type CommandType { get; }
    public Type ServiceType { get; }
    public Type CommandResultType { get; }
    public string LowerCamelCaseName => _lowerCamelCaseName;

    /// <summary>
    /// Compiled delegate for invoking the handler without reflection.
    /// Signature: (object handler, object command, CancellationToken ct) => Task<object?>
    /// </summary>
    public Func<object, object, CancellationToken, Task<object?>> CompiledInvoker { get; }
}