using System;
using System.Collections.Generic;
using System.Runtime.CompilerServices;
using System.Threading;
using System.Threading.Tasks;

namespace Capnp.Rpc
{
    /// <summary>
    /// Provides support for promise pipelining.
    /// </summary>
    public static class Impatient
    {
        static readonly ConditionalWeakTable<Task, IPromisedAnswer> _taskTable = new ConditionalWeakTable<Task, IPromisedAnswer>();
        static readonly ThreadLocal<Stack<IRpcEndpoint>> _askingEndpoint = new ThreadLocal<Stack<IRpcEndpoint>>(() => new Stack<IRpcEndpoint>());

        /// <summary>
        /// Attaches a continuation to the given promise and registers the resulting task for pipelining.
        /// </summary>
        /// <typeparam name="T">Task result type</typeparam>
        /// <param name="promise">The promise</param>
        /// <param name="then">The continuation</param>
        /// <returns>Task representing the future answer</returns>
        /// <exception cref="ArgumentNullException"><paramref name="promise"/> or <paramref name="then"/> is null.</exception>
        /// <exception cref="ArgumentException">The pomise was already registered.</exception>
        public static Task<T> MakePipelineAware<T>(IPromisedAnswer promise, Func<DeserializerState, T> then)
        {
            async Task<T> AwaitAnswer()
            {
                var result = await promise.WhenReturned;
                if (promise.IsTailCall)
                    throw new TailCallNoDataException();

                return then(result);
            }

            var rtask = AwaitAnswer();

            // Rare situation: .NET maintains a cache of some pre-computed tasks for standard results (such as (int)0, (object)null).
            // AwaitAnswer() might indeed have chosen a fast-path optimization, such that rtask is a cached object instead of a new instance.
            // Once this happens the second time, and we return the same rtask for a different promise. GetAnswer()/TryGetAnswer() may return the "wrong"
            // promise! Fortunately, this does not really matter, since the "wrong" promise is guaranteed to return exactly the same answer. :-)
            _taskTable.GetValue(rtask, _ => promise);

            return rtask;
        }

        /// <summary>
        /// Looks up the underlying promise which was previously registered for the given Task using MakePipelineAware.
        /// </summary>
        /// <param name="task"></param>
        /// <returns>The underlying promise</returns>
        /// <exception cref="ArgumentNullException"><paramref name="task"/> is null.</exception>
        /// <exception cref="ArgumentException">The task was not registered using MakePipelineAware.</exception>
        [Obsolete("Please re-generate capnp code-behind. GetAnswer(task).Access(...) was replaced by Access(task, ...)")]
        public static IPromisedAnswer GetAnswer(Task task)
        {
            if (!_taskTable.TryGetValue(task, out var answer))
            {
                throw new ArgumentException("Unknown task");
            }

            return answer;
        }

        internal static IPromisedAnswer? TryGetAnswer(Task task)
        {
            _taskTable.TryGetValue(task, out var answer);
            return answer;
        }

        /// <summary>
        /// Returns a promise-pipelined capability for a remote method invocation Task.
        /// </summary>
        /// <param name="task">remote method invocation task</param>
        /// <param name="access">path to the desired capability</param>
        /// <param name="proxyTask">task returning a proxy to the desired capability</param>
        /// <returns>Pipelined low-level capability</returns>
        public static ConsumedCapability Access(Task task, MemberAccessPath access, Task<IDisposable?> proxyTask)
        {
            var answer = TryGetAnswer(task);
            if (answer != null) return answer.Access(access, proxyTask);
            return new LazyCapability(proxyTask.AsProxyTask());
        }

        /// <summary>
        /// Returns a local "lazy" proxy for a given Task. 
        /// This is not real promise pipelining and will probably be removed.
        /// </summary>
        /// <typeparam name="TInterface">Capability interface type</typeparam>
        /// <param name="task">The task</param>
        /// <returns>A proxy for the given task.</returns>
        /// <exception cref="ArgumentNullException"><paramref name="task"/> is null.</exception>
        /// <exception cref="InvalidCapabilityInterfaceException"><typeparamref name="TInterface"/> did not
        /// quality as capability interface.</exception>
        [Obsolete("Call Eager<TInterface>(task, true) instead")]
        public static TInterface PseudoEager<TInterface>(this Task<TInterface> task)
            where TInterface : class, IDisposable
        {
            var lazyCap = new LazyCapability(task.AsProxyTask());
            return (CapabilityReflection.CreateProxy<TInterface>(lazyCap) as TInterface)!;
        }

        static readonly MemberAccessPath Path_OneAndOnly = new MemberAccessPath(0U);

        /// <summary>
        /// Returns a promise-pipelined Proxy for a remote method invocation Task.
        /// </summary>
        /// <typeparam name="TInterface">Capability interface type</typeparam>
        /// <param name="task">Task returning an interface</param>
        /// <param name="allowNoPipeliningFallback">If this flag is 'false', the <paramref name="task"/> MUST have been returned from a remote
        /// method invocation on a generated Proxy interface. Since this is the prerequisite for promise pipelining to work, the method throws an
        /// exception if the requirement is not met (i.e. the passed some Task instance was constructed "somewhere else"). Setting this flag to 'true'
        /// prevents such an exception. The method falls back to a local "lazy" proxy for the given Task. It is fully usable, but does not perform
        /// any promise pipelining (as specified for Cap'n Proto).</param>
        /// <returns>A proxy for the given future.</returns>
        /// <exception cref="ArgumentNullException"><paramref name="task"/> is null.</exception>
        /// <exception cref="InvalidCapabilityInterfaceException"><typeparamref name="TInterface"/> did not qualify as capability interface.</exception>
        /// <exception cref="ArgumentException">The task was not returned from a remote method invocation. Promise pipelining won't work.
        /// Setting <paramref name="allowNoPipeliningFallback"/>> to 'true' prevents this exception. 
        /// OR: Mismatch between generic type arguments (if capability interface is generic).</exception>
        /// <exception cref="InvalidOperationException">Mismatch between generic type arguments (if capability interface is generic).</exception>
        /// <exception cref="System.Reflection.TargetInvocationException">Problem with instatiating the Proxy (constructor threw exception).</exception>
        /// <exception cref="MemberAccessException">Caller does not have permission to invoke the Proxy constructor.</exception>
        /// <exception cref="TypeLoadException">Problem with building the Proxy type, or problem with loading some dependent class.</exception>
        public static TInterface Eager<TInterface>(this Task<TInterface> task, bool allowNoPipeliningFallback = false)
            where TInterface : class, IDisposable
        {
            var answer = TryGetAnswer(task);
            if (answer == null)
            {
                if (!allowNoPipeliningFallback)
                {
                    throw new ArgumentException("The task was not returned from a remote method invocation. See documentation for details.");
                }

                var proxyTask = task.AsProxyTask();
                if (proxyTask.ReplacementTaskIsCompletedSuccessfully())
                {
                    return proxyTask.Result.Cast<TInterface>(true);
                }
                else
                {
                    var lazyCap = new LazyCapability(proxyTask);
                    return (CapabilityReflection.CreateProxy<TInterface>(lazyCap) as TInterface)!;
                }
            }
            else
            {
                async Task<IDisposable?> AsDisposableTask()
                {
                    return await task;
                }

                return (CapabilityReflection.CreateProxy<TInterface>(answer.Access(Path_OneAndOnly, AsDisposableTask())) as TInterface)!;
            }
        }

        /// <summary>
        /// Unwraps given capability. Unwrapping walks the chain of promised capabilities and awaits their resolutions,
        /// until we get the finally resolved capability. If it is the capability, the method returns a null reference.
        /// If the capability is broken (resolved to exception, dependent answer faulted or cancelled, RPC endpoint closed),
        /// it throws an exception.
        /// </summary>
        /// <typeparam name="TInterface">Capability interface</typeparam>
        /// <param name="cap">capability to unwrap</param>
        /// <returns>Task returning the eventually resolved capability</returns>
        /// <exception cref="RpcException">Capability is broken</exception>
        public static async Task<TInterface?> Unwrap<TInterface>(this TInterface cap) where TInterface: class, IDisposable
        {
            using var proxy = cap as Proxy;

            if (proxy == null)
                return cap;

            var unwrapped = await proxy.ConsumedCap.Unwrap();
            if (unwrapped == null || unwrapped == NullCapability.Instance)
                return null;

            return ((CapabilityReflection.CreateProxy<TInterface>(unwrapped)) as TInterface)!;
        }

        internal static IRpcEndpoint? AskingEndpoint
        {
            get => _askingEndpoint.Value!.Count > 0 ? _askingEndpoint.Value.Peek() : null;
        }

        internal static void PushAskingEndpoint(IRpcEndpoint endpoint)
        {
            _askingEndpoint.Value!.Push(endpoint);
        }

        internal static void PopAskingEndpoint()
        {
            _askingEndpoint.Value!.Pop();
        }

        /// <summary>
        /// Checks whether a given task belongs to a pending RPC and requests a tail call if applicable.
        /// </summary>
        /// <typeparam name="T">Task result type</typeparam>
        /// <param name="task">Task to request</param>
        /// <param name="func">Converts the task's result to a SerializerState</param>
        /// <returns>Tail-call aware task</returns>
        public static async Task<AnswerOrCounterquestion> MaybeTailCall<T>(Task<T> task, Func<T, SerializerState> func)
        {
            if (TryGetAnswer(task) is PendingQuestion pendingQuestion &&
                pendingQuestion.RpcEndpoint == AskingEndpoint)
            {
                pendingQuestion.IsTailCall = true;
                return pendingQuestion;
            }
            else
            {
                return func(await task);
            }
        }

        /// <summary>
        /// Overload for tuple-typed tasks
        /// </summary>
        public static Task<AnswerOrCounterquestion> MaybeTailCall<T1, T2>(Task<(T1, T2)> task, Func<T1, T2, SerializerState> func)
        {
            return MaybeTailCall(task, (ValueTuple<T1, T2> t) => func(t.Item1, t.Item2));
        }

        /// <summary>
        /// Overload for tuple-typed tasks
        /// </summary>
        public static Task<AnswerOrCounterquestion> MaybeTailCall<T1, T2, T3>(Task<(T1, T2, T3)> task, Func<T1, T2, T3, SerializerState> func)
        {
            return MaybeTailCall(task, (ValueTuple<T1, T2, T3> t) => func(t.Item1, t.Item2, t.Item3));
        }

        /// <summary>
        /// Overload for tuple-typed tasks
        /// </summary>
        public static Task<AnswerOrCounterquestion> MaybeTailCall<T1, T2, T3, T4>(Task<(T1, T2, T3, T4)> task, Func<T1, T2, T3, T4, SerializerState> func)
        {
            return MaybeTailCall(task, (ValueTuple<T1, T2, T3, T4> t) => func(t.Item1, t.Item2, t.Item3, t.Item4));
        }

        /// <summary>
        /// Overload for tuple-typed tasks
        /// </summary>
        public static Task<AnswerOrCounterquestion> MaybeTailCall<T1, T2, T3, T4, T5>(Task<(T1, T2, T3, T4, T5)> task, Func<T1, T2, T3, T4, T5, SerializerState> func)
        {
            return MaybeTailCall(task, (ValueTuple<T1, T2, T3, T4, T5> t) => func(t.Item1, t.Item2, t.Item3, t.Item4, t.Item5));
        }

        /// <summary>
        /// Overload for tuple-typed tasks
        /// </summary>
        public static Task<AnswerOrCounterquestion> MaybeTailCall<T1, T2, T3, T4, T5, T6>(Task<(T1, T2, T3, T4, T5, T6)> task, Func<T1, T2, T3, T4, T5, T6, SerializerState> func)
        {
            return MaybeTailCall(task, (ValueTuple<T1, T2, T3, T4, T5, T6> t) => func(t.Item1, t.Item2, t.Item3, t.Item4, t.Item5, t.Item6));
        }

        /// <summary>
        /// Overload for tuple-typed tasks
        /// </summary>
        public static Task<AnswerOrCounterquestion> MaybeTailCall<T1, T2, T3, T4, T5, T6, T7>(Task<(T1, T2, T3, T4, T5, T6, T7)> task, Func<T1, T2, T3, T4, T5, T6, T7, SerializerState> func)
        {
            return MaybeTailCall(task, (ValueTuple<T1, T2, T3, T4, T5, T6, T7> t) => func(t.Item1, t.Item2, t.Item3, t.Item4, t.Item5, t.Item6, t.Item7));
        }
    }
}