using System;
using System.Collections.Concurrent;
using System.Collections.Generic;
using System.Collections.Immutable;
using System.Linq;
using System.Reflection;
using System.Threading;
using System.Threading.Tasks;
using Microsoft.Extensions.DependencyInjection;
using Discord.Commands.Builders;
using Discord.Logging;

namespace Discord.Commands
{
    public class CommandService
    {
        public event Func<LogMessage, Task> Log { add { _logEvent.Add(value); } remove { _logEvent.Remove(value); } }
        internal readonly AsyncEvent<Func<LogMessage, Task>> _logEvent = new AsyncEvent<Func<LogMessage, Task>>();

        public event Func<CommandInfo, ICommandContext, IResult, Task> CommandExecuted { add { _commandExecutedEvent.Add(value); } remove { _commandExecutedEvent.Remove(value); } }
        internal readonly AsyncEvent<Func<CommandInfo, ICommandContext, IResult, Task>> _commandExecutedEvent = new AsyncEvent<Func<CommandInfo, ICommandContext, IResult, Task>>();

        private readonly SemaphoreSlim _moduleLock;
        private readonly ConcurrentDictionary<Type, ModuleInfo> _typedModuleDefs;
        private readonly ConcurrentDictionary<Type, ConcurrentDictionary<Type, TypeReader>> _typeReaders;
        private readonly ConcurrentDictionary<Type, TypeReader> _defaultTypeReaders;
        private readonly ImmutableList<Tuple<Type, Type>> _entityTypeReaders; //TODO: Candidate for C#7 Tuple
        private readonly HashSet<ModuleInfo> _moduleDefs;
        private readonly CommandMap _map;

        internal readonly bool _caseSensitive, _throwOnError, _ignoreExtraArgs;
        internal readonly char _separatorChar;
        internal readonly RunMode _defaultRunMode;
        internal readonly Logger _cmdLogger;
        internal readonly LogManager _logManager;

        public IEnumerable<ModuleInfo> Modules => _moduleDefs.Select(x => x);
        public IEnumerable<CommandInfo> Commands => _moduleDefs.SelectMany(x => x.Commands);
        public ILookup<Type, TypeReader> TypeReaders => _typeReaders.SelectMany(x => x.Value.Select(y => new { y.Key, y.Value })).ToLookup(x => x.Key, x => x.Value);

        public CommandService() : this(new CommandServiceConfig()) { }
        public CommandService(CommandServiceConfig config)
        {
            _caseSensitive = config.CaseSensitiveCommands;
            _throwOnError = config.ThrowOnError;
            _ignoreExtraArgs = config.IgnoreExtraArgs;
            _separatorChar = config.SeparatorChar;
            _defaultRunMode = config.DefaultRunMode;
            if (_defaultRunMode == RunMode.Default)
                throw new InvalidOperationException("The default run mode cannot be set to Default.");

            _logManager = new LogManager(config.LogLevel);
            _logManager.Message += async msg => await _logEvent.InvokeAsync(msg).ConfigureAwait(false);
            _cmdLogger = _logManager.CreateLogger("Command");

            _moduleLock = new SemaphoreSlim(1, 1);
            _typedModuleDefs = new ConcurrentDictionary<Type, ModuleInfo>();
            _moduleDefs = new HashSet<ModuleInfo>();
            _map = new CommandMap(this);
            _typeReaders = new ConcurrentDictionary<Type, ConcurrentDictionary<Type, TypeReader>>();

            _defaultTypeReaders = new ConcurrentDictionary<Type, TypeReader>();
            foreach (var type in PrimitiveParsers.SupportedTypes)
            {
                _defaultTypeReaders[type] = PrimitiveTypeReader.Create(type);
                _defaultTypeReaders[typeof(Nullable<>).MakeGenericType(type)] = NullableTypeReader.Create(type, _defaultTypeReaders[type]);
            }

            _defaultTypeReaders[typeof(string)] =
                new PrimitiveTypeReader<string>((string x, out string y) => { y = x; return true; }, 0);

            var entityTypeReaders = ImmutableList.CreateBuilder<Tuple<Type, Type>>();
            entityTypeReaders.Add(new Tuple<Type, Type>(typeof(IMessage), typeof(MessageTypeReader<>)));
            entityTypeReaders.Add(new Tuple<Type, Type>(typeof(IChannel), typeof(ChannelTypeReader<>)));
            entityTypeReaders.Add(new Tuple<Type, Type>(typeof(IRole), typeof(RoleTypeReader<>)));
            entityTypeReaders.Add(new Tuple<Type, Type>(typeof(IUser), typeof(UserTypeReader<>)));
            _entityTypeReaders = entityTypeReaders.ToImmutable();
        }

        //Modules
        public async Task<ModuleInfo> CreateModuleAsync(string primaryAlias, Action<ModuleBuilder> buildFunc)
        {
            await _moduleLock.WaitAsync().ConfigureAwait(false);
            try
            {
                var builder = new ModuleBuilder(this, null, primaryAlias);
                buildFunc(builder);

                var module = builder.Build(this, null);

                return LoadModuleInternal(module);
            }
            finally
            {
                _moduleLock.Release();
            }
        }

        /// <summary>
        /// Add a command module from a type
        /// </summary>
        /// <typeparam name="T">The type of module</typeparam>
        /// <param name="services">An IServiceProvider for your dependency injection solution, if using one - otherwise, pass null</param>
        /// <returns>A built module</returns>
        public Task<ModuleInfo> AddModuleAsync<T>(IServiceProvider services) => AddModuleAsync(typeof(T), services);
        public async Task<ModuleInfo> AddModuleAsync(Type type, IServiceProvider services)
        {
            services = services ?? EmptyServiceProvider.Instance;

            await _moduleLock.WaitAsync().ConfigureAwait(false);
            try
            {
                var typeInfo = type.GetTypeInfo();

                if (_typedModuleDefs.ContainsKey(type))
                    throw new ArgumentException($"This module has already been added.");

                var module = (await ModuleClassBuilder.BuildAsync(this, services, typeInfo).ConfigureAwait(false)).FirstOrDefault();

                if (module.Value == default(ModuleInfo))
                    throw new InvalidOperationException($"Could not build the module {type.FullName}, did you pass an invalid type?");

                _typedModuleDefs[module.Key] = module.Value;

                return LoadModuleInternal(module.Value);
            }
            finally
            {
                _moduleLock.Release();
            }
        }
        /// <summary>
        /// Add command modules from an assembly
        /// </summary>
        /// <param name="assembly">The assembly containing command modules</param>
        /// <param name="services">An IServiceProvider for your dependency injection solution, if using one - otherwise, pass null</param>
        /// <returns>A collection of built modules</returns>
        public async Task<IEnumerable<ModuleInfo>> AddModulesAsync(Assembly assembly, IServiceProvider services)
        {
            services = services ?? EmptyServiceProvider.Instance;

            await _moduleLock.WaitAsync().ConfigureAwait(false);
            try
            {
                var types = await ModuleClassBuilder.SearchAsync(assembly, this).ConfigureAwait(false);
                var moduleDefs = await ModuleClassBuilder.BuildAsync(types, this, services).ConfigureAwait(false);

                foreach (var info in moduleDefs)
                {
                    _typedModuleDefs[info.Key] = info.Value;
                    LoadModuleInternal(info.Value);
                }

                return moduleDefs.Select(x => x.Value).ToImmutableArray();
            }
            finally
            {
                _moduleLock.Release();
            }
        }
        private ModuleInfo LoadModuleInternal(ModuleInfo module)
        {
            _moduleDefs.Add(module);

            foreach (var command in module.Commands)
                _map.AddCommand(command);

            foreach (var submodule in module.Submodules)
                LoadModuleInternal(submodule);

            return module;
        }

        public async Task<bool> RemoveModuleAsync(ModuleInfo module)
        {
            await _moduleLock.WaitAsync().ConfigureAwait(false);
            try
            {
                return RemoveModuleInternal(module);
            }
            finally
            {
                _moduleLock.Release();
            }
        }
        public Task<bool> RemoveModuleAsync<T>() => RemoveModuleAsync(typeof(T));
        public async Task<bool> RemoveModuleAsync(Type type)
        {
            await _moduleLock.WaitAsync().ConfigureAwait(false);
            try
            {
                if (!_typedModuleDefs.TryRemove(type, out var module))
                    return false;

                return RemoveModuleInternal(module);
            }
            finally
            {
                _moduleLock.Release();
            }
        }
        private bool RemoveModuleInternal(ModuleInfo module)
        {
            if (!_moduleDefs.Remove(module))
                return false;

            foreach (var cmd in module.Commands)
                _map.RemoveCommand(cmd);

            foreach (var submodule in module.Submodules)
            {
                RemoveModuleInternal(submodule);
            }

            return true;
        }

        //Type Readers
        /// <summary>
        /// Adds a custom <see cref="TypeReader"/> to this <see cref="CommandService"/> for the supplied object type. 
        /// If <typeparamref name="T"/> is a <see cref="ValueType"/>, a <see cref="NullableTypeReader{T}"/> will also be added.
        /// If a default <see cref="TypeReader"/> exists for <typeparamref name="T"/>, a warning will be logged and the default <see cref="TypeReader"/> will be replaced. 
        /// </summary>
        /// <typeparam name="T">The object type to be read by the <see cref="TypeReader"/>.</typeparam>
        /// <param name="reader">An instance of the <see cref="TypeReader"/> to be added.</param>
        public void AddTypeReader<T>(TypeReader reader)
            => AddTypeReader(typeof(T), reader);
        /// <summary>
        /// Adds a custom <see cref="TypeReader"/> to this <see cref="CommandService"/> for the supplied object type. 
        /// If <paramref name="type"/> is a <see cref="ValueType"/>, a <see cref="NullableTypeReader{T}"/> for the value type will also be added.
        /// If a default <see cref="TypeReader"/> exists for <paramref name="type"/>, a warning will be logged and the default <see cref="TypeReader"/> will be replaced.
        /// </summary>
        /// <param name="type">A <see cref="Type"/> instance for the type to be read.</param>
        /// <param name="reader">An instance of the <see cref="TypeReader"/> to be added.</param>
        public void AddTypeReader(Type type, TypeReader reader)
        {
            if (_defaultTypeReaders.ContainsKey(type))
                _ = _cmdLogger.WarningAsync($"The default TypeReader for {type.FullName} was replaced by {reader.GetType().FullName}." +
                    $"To suppress this message, use AddTypeReader<T>(reader, true).");
            AddTypeReader(type, reader, true);
        }
        /// <summary>
        /// Adds a custom <see cref="TypeReader"/> to this <see cref="CommandService"/> for the supplied object type. 
        /// If <typeparamref name="T"/> is a <see cref="ValueType"/>, a <see cref="NullableTypeReader{T}"/> will also be added.
        /// </summary>
        /// <typeparam name="T">The object type to be read by the <see cref="TypeReader"/>.</typeparam>
        /// <param name="reader">An instance of the <see cref="TypeReader"/> to be added.</param>
        /// <param name="replaceDefault">If <paramref name="reader"/> should replace the default <see cref="TypeReader"/> for <typeparamref name="T"/> if one exists.</param>
        public void AddTypeReader<T>(TypeReader reader, bool replaceDefault)
            => AddTypeReader(typeof(T), reader, replaceDefault);
        /// <summary>
        /// Adds a custom <see cref="TypeReader"/> to this <see cref="CommandService"/> for the supplied object type. 
        /// If <paramref name="type"/> is a <see cref="ValueType"/>, a <see cref="NullableTypeReader{T}"/> for the value type will also be added. 
        /// </summary>
        /// <param name="type">A <see cref="Type"/> instance for the type to be read.</param>
        /// <param name="reader">An instance of the <see cref="TypeReader"/> to be added.</param>
        /// <param name="replaceDefault">If <paramref name="reader"/> should replace the default <see cref="TypeReader"/> for <paramref name="type"/> if one exists.</param>
        public void AddTypeReader(Type type, TypeReader reader, bool replaceDefault)
        {
            if (replaceDefault && _defaultTypeReaders.ContainsKey(type))
            {
                _defaultTypeReaders.AddOrUpdate(type, reader, (k, v) => reader);
                if (type.GetTypeInfo().IsValueType)
                {
                    var nullableType = typeof(Nullable<>).MakeGenericType(type);
                    var nullableReader = NullableTypeReader.Create(type, reader);
                    _defaultTypeReaders.AddOrUpdate(nullableType, nullableReader, (k, v) => nullableReader);
                }
            }
            else
            {
                var readers = _typeReaders.GetOrAdd(type, x => new ConcurrentDictionary<Type, TypeReader>());
                readers[reader.GetType()] = reader;

                if (type.GetTypeInfo().IsValueType)
                    AddNullableTypeReader(type, reader);
            }
        }
        internal void AddNullableTypeReader(Type valueType, TypeReader valueTypeReader)
        {
            var readers = _typeReaders.GetOrAdd(typeof(Nullable<>).MakeGenericType(valueType), x => new ConcurrentDictionary<Type, TypeReader>());
            var nullableReader = NullableTypeReader.Create(valueType, valueTypeReader);
            readers[nullableReader.GetType()] = nullableReader;
        }
        internal IDictionary<Type, TypeReader> GetTypeReaders(Type type)
        {
            if (_typeReaders.TryGetValue(type, out var definedTypeReaders))
                return definedTypeReaders;
            return null;
        }
        internal TypeReader GetDefaultTypeReader(Type type)
        {
            if (_defaultTypeReaders.TryGetValue(type, out var reader))
                return reader;
            var typeInfo = type.GetTypeInfo();

            //Is this an enum?
            if (typeInfo.IsEnum)
            {
                reader = EnumTypeReader.GetReader(type);
                _defaultTypeReaders[type] = reader;
                return reader;
            }

            //Is this an entity?
            for (int i = 0; i < _entityTypeReaders.Count; i++)
            {
                if (type == _entityTypeReaders[i].Item1 || typeInfo.ImplementedInterfaces.Contains(_entityTypeReaders[i].Item1))
                {
                    reader = Activator.CreateInstance(_entityTypeReaders[i].Item2.MakeGenericType(type)) as TypeReader;
                    _defaultTypeReaders[type] = reader;
                    return reader;
                }
            }
            return null;
        }

        //Execution
        public SearchResult Search(ICommandContext context, int argPos)
            => Search(context, context.Message.Content.Substring(argPos));
        public SearchResult Search(ICommandContext context, string input)
        {
            string searchInput = _caseSensitive ? input : input.ToLowerInvariant();
            var matches = _map.GetCommands(searchInput).OrderByDescending(x => x.Command.Priority).ToImmutableArray();

            if (matches.Length > 0)
                return SearchResult.FromSuccess(input, matches);
            else
                return SearchResult.FromError(CommandError.UnknownCommand, "Unknown command.");
        }

        public Task<IResult> ExecuteAsync(ICommandContext context, int argPos, IServiceProvider services, MultiMatchHandling multiMatchHandling = MultiMatchHandling.Exception)
            => ExecuteAsync(context, context.Message.Content.Substring(argPos), services, multiMatchHandling);
        public async Task<IResult> ExecuteAsync(ICommandContext context, string input, IServiceProvider services, MultiMatchHandling multiMatchHandling = MultiMatchHandling.Exception)
        {
            services = services ?? EmptyServiceProvider.Instance;

            var searchResult = Search(context, input);
            if (!searchResult.IsSuccess)
                return searchResult;

            var commands = searchResult.Commands;
            var preconditionResults = new Dictionary<CommandMatch, PreconditionResult>();

            foreach (var match in commands)
            {
                preconditionResults[match] = await match.Command.CheckPreconditionsAsync(context, services).ConfigureAwait(false);
            }

            var successfulPreconditions = preconditionResults
                .Where(x => x.Value.IsSuccess)
                .ToArray();

            if (successfulPreconditions.Length == 0)
            {
                //All preconditions failed, return the one from the highest priority command
                var bestCandidate = preconditionResults
                    .OrderByDescending(x => x.Key.Command.Priority)
                    .FirstOrDefault(x => !x.Value.IsSuccess);
                return bestCandidate.Value;
            }

            //If we get this far, at least one precondition was successful.

            var parseResultsDict = new Dictionary<CommandMatch, ParseResult>();
            foreach (var pair in successfulPreconditions)
            {
                var parseResult = await pair.Key.ParseAsync(context, searchResult, pair.Value, services).ConfigureAwait(false);

                if (parseResult.Error == CommandError.MultipleMatches)
                {
                    IReadOnlyList<TypeReaderValue> argList, paramList;
                    switch (multiMatchHandling)
                    {
                        case MultiMatchHandling.Best:
                            argList = parseResult.ArgValues.Select(x => x.Values.OrderByDescending(y => y.Score).First()).ToImmutableArray();
                            paramList = parseResult.ParamValues.Select(x => x.Values.OrderByDescending(y => y.Score).First()).ToImmutableArray();
                            parseResult = ParseResult.FromSuccess(argList, paramList);
                            break;
                    }
                }

                parseResultsDict[pair.Key] = parseResult;
            }

            // Calculates the 'score' of a command given a parse result
            float CalculateScore(CommandMatch match, ParseResult parseResult)
            {
                float argValuesScore = 0, paramValuesScore = 0;

                if (match.Command.Parameters.Count > 0)
                {
                    var argValuesSum = parseResult.ArgValues?.Sum(x => x.Values.OrderByDescending(y => y.Score).FirstOrDefault().Score) ?? 0;
                    var paramValuesSum = parseResult.ParamValues?.Sum(x => x.Values.OrderByDescending(y => y.Score).FirstOrDefault().Score) ?? 0;

                    argValuesScore = argValuesSum / match.Command.Parameters.Count;
                    paramValuesScore = paramValuesSum / match.Command.Parameters.Count;
                }

                var totalArgsScore = (argValuesScore + paramValuesScore) / 2;
                return match.Command.Priority + totalArgsScore * 0.99f;
            }

            //Order the parse results by their score so that we choose the most likely result to execute
            var parseResults = parseResultsDict
                .OrderByDescending(x => CalculateScore(x.Key, x.Value));

            var successfulParses = parseResults
                .Where(x => x.Value.IsSuccess)
                .ToArray();

            if (successfulParses.Length == 0)
            {
                //All parses failed, return the one from the highest priority command, using score as a tie breaker
                var bestMatch = parseResults
                    .FirstOrDefault(x => !x.Value.IsSuccess);
                return bestMatch.Value;
            }

            //If we get this far, at least one parse was successful. Execute the most likely overload.
            var chosenOverload = successfulParses[0];
            return await chosenOverload.Key.ExecuteAsync(context, chosenOverload.Value, services).ConfigureAwait(false);
        }
    }
}