Rerfactor package parsing.

Frameworks/MQTTnet.NetStandard/Serializer/MqttPacketBodyReader.cs

@@ -0,0 +1,54 @@
+using System;
+using System.Text;
+
+namespace MQTTnet.Serializer
+{
+    public class MqttPacketBodyReader
+    {
+        private readonly byte[] _buffer;
+        private int _offset;
+
+        public MqttPacketBodyReader(byte[] buffer)
+        {
+            _buffer = buffer;
+        }
+
+        public int Length => _buffer.Length - _offset;
+
+        public bool EndOfStream => _offset == _buffer.Length;
+
+        public byte ReadByte()
+        {
+            return _buffer[_offset++];
+        }
+
+        public ArraySegment<byte> ReadRemainingData()
+        {
+            return new ArraySegment<byte>(_buffer, _offset, _buffer.Length - _offset);
+        }
+
+        public ushort ReadUInt16()
+        {
+            var msb = _buffer[_offset++];
+            var lsb = _buffer[_offset++];
+            
+            return (ushort)(msb << 8 | lsb);
+        }
+
+        public ArraySegment<byte> ReadWithLengthPrefix()
+        {
+            var length = ReadUInt16();
+
+            var result = new ArraySegment<byte>(_buffer, _offset, length);
+            _offset += length;
+
+            return result;
+        }
+
+        public string ReadStringWithLengthPrefix()
+        {
+            var buffer = ReadWithLengthPrefix();
+            return Encoding.UTF8.GetString(buffer.Array, buffer.Offset, buffer.Count);
+        }
+    }
+}