using System;
using System.IO;
using System.Text;
using System.Threading.Tasks;
using MQTTnet.Core.Channel;
using MQTTnet.Core.Protocol;

namespace MQTTnet.Core.Serializer
{
    public sealed class MqttPacketWriter : IDisposable
    {
        private readonly MemoryStream _buffer = new MemoryStream(1024);

        public void InjectFixedHeader(MqttControlPacketType packetType, byte flags = 0)
        {
            var fixedHeader = (int)packetType << 4;
            fixedHeader |= flags;
            InjectFixedHeader((byte)fixedHeader);
        }

        public void Write(byte value)
        {
            _buffer.WriteByte(value);
        }

        public void Write(ushort value)
        {
            var buffer = BitConverter.GetBytes(value);
            _buffer.WriteByte(buffer[1]);
            _buffer.WriteByte(buffer[0]);
        }

        public void Write(ByteWriter value)
        {
            if (value == null) throw new ArgumentNullException(nameof(value));

            _buffer.WriteByte(value.Value);
        }

        public void Write(params byte[] value)
        {
            if (value == null) throw new ArgumentNullException(nameof(value));

            _buffer.Write(value, 0, value.Length);
        }

        public void WriteWithLengthPrefix(string value)
        {
            WriteWithLengthPrefix(Encoding.UTF8.GetBytes(value ?? string.Empty));
        }

        public void WriteWithLengthPrefix(byte[] value)
        {
            var length = (ushort)value.Length;

            Write(length);
            Write(value);
        }

        public Task WriteToAsync(IMqttCommunicationChannel destination)
        {
            if (destination == null) throw new ArgumentNullException(nameof(destination));

            return destination.WriteAsync(_buffer.ToArray());
        }

        public void Dispose()
        {
            _buffer?.Dispose();
        }

        private void InjectFixedHeader(byte fixedHeader)
        {
            if (_buffer.Length == 0)
            {
                Write(fixedHeader);
                Write(0);
                return;
            }

            var backupBuffer = _buffer.ToArray();
            var remainingLength = (int)_buffer.Length;

            _buffer.SetLength(0);

            _buffer.WriteByte(fixedHeader);

            // Alorithm taken from http://docs.oasis-open.org/mqtt/mqtt/v3.1.1/os/mqtt-v3.1.1-os.html.
            var x = remainingLength;
            do
            {
                var encodedByte = x % 128;
                x = x / 128;
                if (x > 0)
                {
                    encodedByte = encodedByte | 128;
                }

                _buffer.WriteByte((byte)encodedByte);
            } while (x > 0);

            _buffer.Write(backupBuffer, 0, backupBuffer.Length);
        }
    }
}