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

namespace StackExchange.Redis
{
    internal abstract partial class RedisBase : IRedis
    {
        internal static readonly DateTime UnixEpoch = new DateTime(1970, 1, 1, 0, 0, 0, DateTimeKind.Utc);
        internal readonly ConnectionMultiplexer multiplexer;
        protected readonly object asyncState;

        internal RedisBase(ConnectionMultiplexer multiplexer, object asyncState)
        {
            this.multiplexer = multiplexer;
            this.asyncState = asyncState;
        }

        IConnectionMultiplexer IRedisAsync.Multiplexer => multiplexer;

        public virtual TimeSpan Ping(CommandFlags flags = CommandFlags.None)
        {
            var msg = GetTimerMessage(flags);
            return ExecuteSync(msg, ResultProcessor.ResponseTimer);
        }

        public virtual Task<TimeSpan> PingAsync(CommandFlags flags = CommandFlags.None)
        {
            var msg = GetTimerMessage(flags);
            return ExecuteAsync(msg, ResultProcessor.ResponseTimer);
        }

        public override string ToString() => multiplexer.ToString();

        public bool TryWait(Task task) => task.Wait(multiplexer.TimeoutMilliseconds);

        public void Wait(Task task) => multiplexer.Wait(task);

        public T Wait<T>(Task<T> task) => multiplexer.Wait(task);

        public void WaitAll(params Task[] tasks) => multiplexer.WaitAll(tasks);

        internal virtual Task<T> ExecuteAsync<T>(Message message, ResultProcessor<T> processor, ServerEndPoint server = null)
        {
            if (message == null) return CompletedTask<T>.Default(asyncState);
            multiplexer.CheckMessage(message);
            return multiplexer.ExecuteAsyncImpl<T>(message, processor, asyncState, server);
        }

        internal virtual T ExecuteSync<T>(Message message, ResultProcessor<T> processor, ServerEndPoint server = null)
        {
            if (message == null) return default(T); // no-op
            multiplexer.CheckMessage(message);
            return multiplexer.ExecuteSyncImpl<T>(message, processor, server);
        }

        internal virtual RedisFeatures GetFeatures(RedisKey key, CommandFlags flags, out ServerEndPoint server)
        {
            server = multiplexer.SelectServer(RedisCommand.PING, flags, key);
            var version = server == null ? multiplexer.RawConfig.DefaultVersion : server.Version;
            return new RedisFeatures(version);
        }

        protected void WhenAlwaysOrExists(When when)
        {
            switch (when)
            {
                case When.Always:
                case When.Exists:
                    break;
                default:
                    throw new ArgumentException(when + " is not valid in this context; the permitted values are: Always, Exists");
            }
        }

        protected void WhenAlwaysOrExistsOrNotExists(When when)
        {
            switch (when)
            {
                case When.Always:
                case When.Exists:
                case When.NotExists:
                    break;
                default:
                    throw new ArgumentException(when + " is not valid in this context; the permitted values are: Always, Exists, NotExists");
            }
        }

        protected void WhenAlwaysOrNotExists(When when)
        {
            switch (when)
            {
                case When.Always:
                case When.NotExists:
                    break;
                default:
                    throw new ArgumentException(when + " is not valid in this context; the permitted values are: Always, NotExists");
            }
        }

        private ResultProcessor.TimingProcessor.TimerMessage GetTimerMessage(CommandFlags flags)
        {
            // do the best we can with available commands
            var map = multiplexer.CommandMap;
            if(map.IsAvailable(RedisCommand.PING))
                return ResultProcessor.TimingProcessor.CreateMessage(-1, flags, RedisCommand.PING);
            if(map.IsAvailable(RedisCommand.TIME))
                return ResultProcessor.TimingProcessor.CreateMessage(-1, flags, RedisCommand.TIME);
            if (map.IsAvailable(RedisCommand.ECHO))
                return ResultProcessor.TimingProcessor.CreateMessage(-1, flags, RedisCommand.ECHO, RedisLiterals.PING);
            // as our fallback, we'll do something odd... we'll treat a key like a value, out of sheer desperation
            // note: this usually means: twemproxy - in which case we're fine anyway, since the proxy does the routing
            return ResultProcessor.TimingProcessor.CreateMessage(0, flags, RedisCommand.EXISTS, (RedisValue)multiplexer.UniqueId);
        }

        internal static class CursorUtils
        {
            internal const int Origin = 0, DefaultPageSize = 10;
            internal static bool IsNil(RedisValue pattern)
            {
                if (pattern.IsNullOrEmpty) return true;
                if (pattern.IsInteger) return false;
                byte[] rawValue = pattern;
                return rawValue.Length == 1 && rawValue[0] == '*';
            }
        }

        internal abstract class CursorEnumerable<T> : IEnumerable<T>, IScanningCursor
        {
            private readonly RedisBase redis;
            private readonly ServerEndPoint server;
            protected readonly int db;
            protected readonly CommandFlags flags;
            protected readonly int pageSize, initialOffset;
            protected readonly long initialCursor;
            private volatile IScanningCursor activeCursor;

            protected CursorEnumerable(RedisBase redis, ServerEndPoint server, int db, int pageSize, long cursor, int pageOffset, CommandFlags flags)
            {
                if (pageOffset < 0) throw new ArgumentOutOfRangeException(nameof(pageOffset));
                this.redis = redis;
                this.server = server;
                this.db = db;
                this.pageSize = pageSize;
                this.flags = flags;
                initialCursor = cursor;
                initialOffset = pageOffset;
            }

            public IEnumerator<T> GetEnumerator() => new CursorEnumerator(this);

            System.Collections.IEnumerator System.Collections.IEnumerable.GetEnumerator() => GetEnumerator();

            internal readonly struct ScanResult
            {
                public readonly long Cursor;
                public readonly T[] Values;
                public ScanResult(long cursor, T[] values)
                {
                    Cursor = cursor;
                    Values = values;
                }
            }

            protected abstract Message CreateMessage(long cursor);

            protected abstract ResultProcessor<ScanResult> Processor { get; }

            protected ScanResult GetNextPageSync(IScanningCursor obj, long cursor)
            {
                activeCursor = obj;
                return redis.ExecuteSync(CreateMessage(cursor), Processor, server);
            }

            protected Task<ScanResult> GetNextPageAsync(IScanningCursor obj, long cursor, out Message message)
            {
                activeCursor = obj;
                message = CreateMessage(cursor);
                return redis.ExecuteAsync(message, Processor, server);
            }

            protected bool TryWait(Task<ScanResult> pending) => redis.TryWait(pending);

            private class CursorEnumerator : IEnumerator<T>, IScanningCursor
            {
                private CursorEnumerable<T> parent;
                public CursorEnumerator(CursorEnumerable<T> parent)
                {
                    this.parent = parent ?? throw new ArgumentNullException(nameof(parent));
                    Reset();
                }

                public T Current => page[pageIndex];

                void IDisposable.Dispose() { parent = null; state = State.Disposed; }

                object System.Collections.IEnumerator.Current => page[pageIndex];

                private void LoadNextPageAsync()
                {
                    if (pending == null && nextCursor != 0)
                    {
                        pending = parent.GetNextPageAsync(this, nextCursor, out var message);
                        pendingMessage = message;
                    }
                }

                private bool SimpleNext()
                {
                    if (page != null && ++pageIndex < page.Length)
                    {
                        // first of a new page? cool; start a new background op, because we're about to exit the iterator
                        if (pageIndex == 0) LoadNextPageAsync();
                        return true;
                    }
                    return false;
                }

                private T[] page;
                private Task<ScanResult> pending;
                private Message pendingMessage;
                private int pageIndex;
                private long currentCursor, nextCursor;

                private State state;
                private enum State : byte
                {
                    Initial,
                    Running,
                    Complete,
                    Disposed,
                }

                private void ProcessReply(ScanResult result)
                {
                    currentCursor = nextCursor;
                    nextCursor = result.Cursor;
                    pageIndex = -1;
                    page = result.Values;
                    pending = null;
                    pendingMessage = null;
                }

                public bool MoveNext()
                {
                    switch(state)
                    {
                        case State.Complete:
                            return false;
                        case State.Initial:
                            ProcessReply(parent.GetNextPageSync(this, nextCursor));
                            pageIndex = parent.initialOffset - 1; // will be incremented in a moment
                            state = State.Running;
                            LoadNextPageAsync();
                            goto case State.Running;
                        case State.Running:
                            // are we working through the current buffer?
                            if (SimpleNext()) return true;

                            // do we have an outstanding operation? wait for the background task to finish
                            while (pending != null)
                            {
                                if (parent.TryWait(pending))
                                {
                                    ProcessReply(pending.Result);
                                }
                                else
                                {
                                    throw ExceptionFactory.Timeout(parent.redis.multiplexer, null, pendingMessage, parent.server);
                                }

                                if (SimpleNext()) return true;
                            }

                            // nothing outstanding? wait synchronously
                            while(nextCursor != 0)
                            {
                                ProcessReply(parent.GetNextPageSync(this, nextCursor));
                                if (SimpleNext()) return true;
                            }

                            // we're exhausted
                            state = State.Complete;
                            return false;
                        case State.Disposed:
                        default:
                            throw new ObjectDisposedException(GetType().Name);
                    }
                }

                public void Reset()
                {
                    if(state == State.Disposed) throw new ObjectDisposedException(GetType().Name);
                    nextCursor = currentCursor = parent.initialCursor;
                    pageIndex = parent.initialOffset; // don't -1 here; this makes it look "right" before incremented
                    state = State.Initial;
                    page = null;
                    pending = null;
                    pendingMessage = null;
                }

                long IScanningCursor.Cursor => currentCursor;

                int IScanningCursor.PageSize => parent.pageSize;

                int IScanningCursor.PageOffset => pageIndex;
            }

            long IScanningCursor.Cursor
            {
                get { var tmp = activeCursor; return tmp?.Cursor ?? initialCursor; }
            }

            int IScanningCursor.PageSize => pageSize;

            int IScanningCursor.PageOffset
            {
                get { var tmp = activeCursor; return tmp?.PageOffset ?? initialOffset; }
            }
        }
    }
}
