using System;
using System.IO;
using System.Threading;
using System.Threading.Tasks;

namespace RdtClient.Service.Services
{
    /// <summary>
    ///     Class for streaming data with throttling support.
    ///     Taken from Downloader: https://github.com/bezzad/Downloader
    /// </summary>
    public class ThrottledStream : Stream
    {
        public Int64 Speed => (Int64)_bandwidth.AverageSpeed;

        private Bandwidth _bandwidth;
        private Int64 _bandwidthLimit;
        private readonly Stream _baseStream;

        /// <summary>
        ///     Initializes a new instance of the <see cref="T:ThrottledStream" /> class.
        /// </summary>
        /// <param name="baseStream">The base stream.</param>
        /// <param name="bandwidthLimit">The maximum bytes per second that can be transferred through the base stream.</param>
        /// <exception cref="ArgumentNullException">Thrown when <see cref="baseStream" /> is a null reference.</exception>
        /// <exception cref="ArgumentOutOfRangeException">Thrown when <see cref="BandwidthLimit" /> is a negative value.</exception>
        public ThrottledStream(Stream baseStream, Int64 bandwidthLimit)
        {
            if (bandwidthLimit < 0)
            {
                throw new ArgumentOutOfRangeException(nameof(bandwidthLimit),
                                                      bandwidthLimit,
                                                      "The maximum number of bytes per second can't be negative.");
            }

            _baseStream = baseStream ?? throw new ArgumentNullException(nameof(baseStream));
            BandwidthLimit = bandwidthLimit;
        }

        public static Int64 Infinite => Int64.MaxValue;

        /// <summary>
        ///     Bandwidth Limit (in B/s)
        /// </summary>
        /// <value>The maximum bytes per second.</value>
        public Int64 BandwidthLimit
        {
            get => _bandwidthLimit;
            set
            {
                _bandwidthLimit = value <= 0 ? Infinite : value;
                _bandwidth ??= new Bandwidth();
                _bandwidth.BandwidthLimit = _bandwidthLimit;
            }
        }

        /// <inheritdoc />
        public override Boolean CanRead => _baseStream.CanRead;

        /// <inheritdoc />
        public override Boolean CanSeek => _baseStream.CanSeek;

        /// <inheritdoc />
        public override Boolean CanWrite => _baseStream.CanWrite;

        /// <inheritdoc />
        public override Int64 Length => _baseStream.Length;

        /// <inheritdoc />
        public override Int64 Position
        {
            get => _baseStream.Position;
            set => _baseStream.Position = value;
        }
        
        /// <inheritdoc />
        public override void Flush()
        {
            _baseStream.Flush();
        }

        /// <inheritdoc />
        public override Int64 Seek(Int64 offset, SeekOrigin origin)
        {
            return _baseStream.Seek(offset, origin);
        }

        /// <inheritdoc />
        public override void SetLength(Int64 value)
        {
            _baseStream.SetLength(value);
        }

        /// <inheritdoc />
        public override Int32 Read(Byte[] buffer, Int32 offset, Int32 count)
        {
            Throttle(count).Wait();

            return _baseStream.Read(buffer, offset, count);
        }

        public override async Task<Int32> ReadAsync(Byte[] buffer,
                                                    Int32 offset,
                                                    Int32 count,
                                                    CancellationToken cancellationToken)
        {
            await Throttle(count).ConfigureAwait(false);

            return await _baseStream.ReadAsync(buffer, offset, count, cancellationToken).ConfigureAwait(false);
        }

        /// <inheritdoc />
        public override void Write(Byte[] buffer, Int32 offset, Int32 count)
        {
            Throttle(count).Wait();
            _baseStream.Write(buffer, offset, count);
        }

        /// <inheritdoc />
        public override async Task WriteAsync(Byte[] buffer, Int32 offset, Int32 count, CancellationToken cancellationToken)
        {
            await Throttle(count).ConfigureAwait(false);
            await _baseStream.WriteAsync(buffer, offset, count, cancellationToken).ConfigureAwait(false);
        }

        private async Task Throttle(Int32 transmissionVolume)
        {
            // Make sure the buffer isn't empty.
            if (BandwidthLimit > 0 && transmissionVolume > 0)
            {
                // Calculate the time to sleep.
                _bandwidth.CalculateSpeed(transmissionVolume);

                await Sleep(_bandwidth.PopSpeedRetrieveTime()).ConfigureAwait(false);
            }
        }

        private static async Task Sleep(Int32 time)
        {
            if (time > 0)
            {
                await Task.Delay(time).ConfigureAwait(false);
            }
        }

        /// <inheritdoc />
        public override String ToString()
        {
            return _baseStream.ToString();
        }
    }

    internal class Bandwidth
    {
        private const Double OneSecond = 1000; // millisecond
        private Int64 _count;
        private Int32 _lastSecondCheckpoint;
        private Int64 _lastTransferredBytesCount;
        private Int32 _speedRetrieveTime;

        public Bandwidth()
        {
            BandwidthLimit = Int64.MaxValue;
            Reset();
        }

        public Double Speed { get; private set; }
        public Double AverageSpeed { get; private set; }
        public Int64 BandwidthLimit { get; set; }

        public void CalculateSpeed(Int64 receivedBytesCount)
        {
            var elapsedTime = (Environment.TickCount - _lastSecondCheckpoint) + 1;
            receivedBytesCount = Interlocked.Add(ref _lastTransferredBytesCount, receivedBytesCount);
            var momentSpeed = (receivedBytesCount * OneSecond) / elapsedTime; // B/s

            if (OneSecond < elapsedTime)
            {
                Speed = momentSpeed;
                AverageSpeed = ((AverageSpeed * _count) + Speed) / (_count + 1);
                _count++;
                SecondCheckpoint();
            }

            if (momentSpeed >= BandwidthLimit)
            {
                var expectedTime = (receivedBytesCount * OneSecond) / BandwidthLimit;
                Interlocked.Add(ref _speedRetrieveTime, (Int32) expectedTime - elapsedTime);
            }
        }

        public Int32 PopSpeedRetrieveTime()
        {
            return Interlocked.Exchange(ref _speedRetrieveTime, 0);
        }

        public void Reset()
        {
            SecondCheckpoint();
            _count = 0;
            Speed = 0;
            AverageSpeed = 0;
        }

        private void SecondCheckpoint()
        {
            Interlocked.Exchange(ref _lastSecondCheckpoint, Environment.TickCount);
            Interlocked.Exchange(ref _lastTransferredBytesCount, 0);
        }
    }
}