using System;

namespace UniRx.Operators
{
    internal class SkipObservable<T> : OperatorObservableBase<T>
    {
        private readonly int count;
        private readonly TimeSpan duration;
        internal readonly IScheduler scheduler; // public for optimization check
        private readonly IObservable<T> source;

        public SkipObservable(IObservable<T> source, int count)
            : base(source.IsRequiredSubscribeOnCurrentThread())
        {
            this.source = source;
            this.count = count;
        }

        public SkipObservable(IObservable<T> source, TimeSpan duration, IScheduler scheduler)
            : base(scheduler == Scheduler.CurrentThread || source.IsRequiredSubscribeOnCurrentThread())
        {
            this.source = source;
            this.duration = duration;
            this.scheduler = scheduler;
        }

        // optimize combiner

        public IObservable<T> Combine(int count)
        {
            // use sum
            // xs = 6
            // xs.Skip(2) = 4
            // xs.Skip(2).Skip(3) = 1

            return new SkipObservable<T>(source, this.count + count);
        }

        public IObservable<T> Combine(TimeSpan duration)
        {
            // use max
            // xs = 6s
            // xs.Skip(2s) = 2s
            // xs.Skip(2s).Skip(3s) = 3s

            return duration <= this.duration
                ? this
                : new SkipObservable<T>(source, duration, scheduler);
        }

        protected override IDisposable SubscribeCore(IObserver<T> observer, IDisposable cancel)
        {
            if (scheduler == null) return source.Subscribe(new Skip(this, observer, cancel));

            return new Skip_(this, observer, cancel).Run();
        }

        private class Skip : OperatorObserverBase<T, T>
        {
            private int remaining;

            public Skip(SkipObservable<T> parent, IObserver<T> observer, IDisposable cancel) : base(observer, cancel)
            {
                remaining = parent.count;
            }

            public override void OnNext(T value)
            {
                if (remaining <= 0)
                    observer.OnNext(value);
                else
                    remaining--;
            }

            public override void OnError(Exception error)
            {
                try
                {
                    observer.OnError(error);
                }
                finally
                {
                    Dispose();
                }
            }

            public override void OnCompleted()
            {
                try
                {
                    observer.OnCompleted();
                }
                finally
                {
                    Dispose();
                }
            }
        }

        private class Skip_ : OperatorObserverBase<T, T>
        {
            private readonly SkipObservable<T> parent;
            private volatile bool open;

            public Skip_(SkipObservable<T> parent, IObserver<T> observer, IDisposable cancel) : base(observer, cancel)
            {
                this.parent = parent;
            }

            public IDisposable Run()
            {
                var d1 = parent.scheduler.Schedule(parent.duration, Tick);
                var d2 = parent.source.Subscribe(this);

                return StableCompositeDisposable.Create(d1, d2);
            }

            private void Tick()
            {
                open = true;
            }

            public override void OnNext(T value)
            {
                if (open) observer.OnNext(value);
            }

            public override void OnError(Exception error)
            {
                try
                {
                    observer.OnError(error);
                }
                finally
                {
                    Dispose();
                }

                ;
            }

            public override void OnCompleted()
            {
                try
                {
                    observer.OnCompleted();
                }
                finally
                {
                    Dispose();
                }

                ;
            }
        }
    }
}