#region License
// Copyright (c) 2007 James Newton-King
//
// Permission is hereby granted, free of charge, to any person
// obtaining a copy of this software and associated documentation
// files (the "Software"), to deal in the Software without
// restriction, including without limitation the rights to use,
// copy, modify, merge, publish, distribute, sublicense, and/or sell
// copies of the Software, and to permit persons to whom the
// Software is furnished to do so, subject to the following
// conditions:
//
// The above copyright notice and this permission notice shall be
// included in all copies or substantial portions of the Software.
//
// THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND,
// EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES
// OF MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND
// NONINFRINGEMENT. IN NO EVENT SHALL THE AUTHORS OR COPYRIGHT
// HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER LIABILITY,
// WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING
// FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR
// OTHER DEALINGS IN THE SOFTWARE.
#endregion

using System;
using System.Collections.Generic;
using System.Globalization;
using System.Text;
using System.IO;
using Newtonsoft.Json.Utilities;
using Newtonsoft.Json.Linq;

namespace Newtonsoft.Json.Bson
{
  /// <summary>
  /// Represents a reader that provides fast, non-cached, forward-only access to serialized Json data.
  /// </summary>
  public class BsonReader : JsonReader
  {
    private const int MaxCharBytesSize = 128;
    private static readonly byte[] SeqRange1 = new byte[] {0, 127}; // range of 1-byte sequence
    private static readonly byte[] SeqRange2 = new byte[] {194, 223}; // range of 2-byte sequence
    private static readonly byte[] SeqRange3 = new byte[] {224, 239}; // range of 3-byte sequence
    private static readonly byte[] SeqRange4 = new byte[] {240, 244}; // range of 4-byte sequence

    private readonly BinaryReader _reader;
    private readonly List<ContainerContext> _stack;

    private byte[] _byteBuffer;
    private char[] _charBuffer;

    private BsonType _currentElementType;
    private BsonReaderState _bsonReaderState;
    private ContainerContext _currentContext;

    private bool _readRootValueAsArray;
    private bool _jsonNet35BinaryCompatibility;
    private DateTimeKind _dateTimeKindHandling;

    private enum BsonReaderState
    {
      Normal,
      ReferenceStart,
      ReferenceRef,
      ReferenceId,
      CodeWScopeStart,
      CodeWScopeCode,
      CodeWScopeScope,
      CodeWScopeScopeObject,
      CodeWScopeScopeEnd
    }

    private class ContainerContext
    {
      public readonly BsonType Type;
      public int Length;
      public int Position;

      public ContainerContext(BsonType type)
      {
        Type = type;
      }
    }

    /// <summary>
    /// Gets or sets a value indicating whether binary data reading should compatible with incorrect Json.NET 3.5 written binary.
    /// </summary>
    /// <value>
    /// 	<c>true</c> if binary data reading will be compatible with incorrect Json.NET 3.5 written binary; otherwise, <c>false</c>.
    /// </value>
    public bool JsonNet35BinaryCompatibility
    {
      get { return _jsonNet35BinaryCompatibility; }
      set { _jsonNet35BinaryCompatibility = value; }
    }

    /// <summary>
    /// Gets or sets a value indicating whether the root object will be read as a JSON array.
    /// </summary>
    /// <value>
    /// 	<c>true</c> if the root object will be read as a JSON array; otherwise, <c>false</c>.
    /// </value>
    public bool ReadRootValueAsArray
    {
      get { return _readRootValueAsArray; }
      set { _readRootValueAsArray = value; }
    }

    /// <summary>
    /// Gets or sets the <see cref="DateTimeKind" /> used when reading <see cref="DateTime"/> values from BSON.
    /// </summary>
    /// <value>The <see cref="DateTimeKind" /> used when reading <see cref="DateTime"/> values from BSON.</value>
    public DateTimeKind DateTimeKindHandling
    {
      get { return _dateTimeKindHandling; }
      set { _dateTimeKindHandling = value; }
    }

    /// <summary>
    /// Initializes a new instance of the <see cref="BsonReader"/> class.
    /// </summary>
    /// <param name="stream">The stream.</param>
    public BsonReader(Stream stream)
      : this(stream, false, DateTimeKind.Local)
    {
    }

    /// <summary>
    /// Initializes a new instance of the <see cref="BsonReader"/> class.
    /// </summary>
    /// <param name="reader">The reader.</param>
    public BsonReader(BinaryReader reader)
      : this(reader, false, DateTimeKind.Local)
    {
    }

    /// <summary>
    /// Initializes a new instance of the <see cref="BsonReader"/> class.
    /// </summary>
    /// <param name="stream">The stream.</param>
    /// <param name="readRootValueAsArray">if set to <c>true</c> the root object will be read as a JSON array.</param>
    /// <param name="dateTimeKindHandling">The <see cref="DateTimeKind" /> used when reading <see cref="DateTime"/> values from BSON.</param>
    public BsonReader(Stream stream, bool readRootValueAsArray, DateTimeKind dateTimeKindHandling)
    {
      ValidationUtils.ArgumentNotNull(stream, "stream");
      _reader = new BinaryReader(stream);
      _stack = new List<ContainerContext>();
      _readRootValueAsArray = readRootValueAsArray;
      _dateTimeKindHandling = dateTimeKindHandling;
    }

    /// <summary>
    /// Initializes a new instance of the <see cref="BsonReader"/> class.
    /// </summary>
    /// <param name="reader">The reader.</param>
    /// <param name="readRootValueAsArray">if set to <c>true</c> the root object will be read as a JSON array.</param>
    /// <param name="dateTimeKindHandling">The <see cref="DateTimeKind" /> used when reading <see cref="DateTime"/> values from BSON.</param>
    public BsonReader(BinaryReader reader, bool readRootValueAsArray, DateTimeKind dateTimeKindHandling)
    {
      ValidationUtils.ArgumentNotNull(reader, "reader");
      _reader = reader;
      _stack = new List<ContainerContext>();
      _readRootValueAsArray = readRootValueAsArray;
      _dateTimeKindHandling = dateTimeKindHandling;
    }

    private string ReadElement()
    {
      _currentElementType = ReadType();
      string elementName = ReadString();
      return elementName;
    }

    /// <summary>
    /// Reads the next JSON token from the stream as a <see cref="T:Byte[]"/>.
    /// </summary>
    /// <returns>
    /// A <see cref="T:Byte[]"/> or a null reference if the next JSON token is null. This method will return <c>null</c> at the end of an array.
    /// </returns>
    public override byte[] ReadAsBytes()
    {
      Read();

      if (IsWrappedInTypeObject())
      {
        byte[] data = ReadAsBytes();
        Read();
        SetToken(JsonToken.Bytes, data);
        return data;
      }

      if (TokenType == JsonToken.Null)
        return null;
      if (TokenType == JsonToken.Bytes)
        return (byte[])Value;

      if (TokenType == JsonToken.EndArray)
        return null;

      throw CreateReaderException(this, "Error reading bytes. Expected bytes but got {0}.".FormatWith(CultureInfo.InvariantCulture, TokenType));
    }

    private bool IsWrappedInTypeObject()
    {
      if (TokenType == JsonToken.StartObject)
      {
        Read();
        if (Value.ToString() == "$type")
        {
          Read();
          if (Value != null && Value.ToString().StartsWith("System.Byte[]"))
          {
            Read();
            if (Value.ToString() == "$value")
            {
              return true;
            }
          }
        }

        throw CreateReaderException(this, "Unexpected token when reading bytes: {0}.".FormatWith(CultureInfo.InvariantCulture, JsonToken.StartObject));
      }

      return false;
    }

    /// <summary>
    /// Reads the next JSON token from the stream as a <see cref="Nullable{Decimal}"/>.
    /// </summary>
    /// <returns>A <see cref="Nullable{Decimal}"/>. This method will return <c>null</c> at the end of an array.</returns>
    public override decimal? ReadAsDecimal()
    {
      Read();

      if (TokenType == JsonToken.Integer || TokenType == JsonToken.Float)
      {
        SetToken(JsonToken.Float, Convert.ToDecimal(Value, CultureInfo.InvariantCulture));
        return (decimal)Value;
      }

      if (TokenType == JsonToken.Null)
        return null;

      decimal d;
      if (TokenType == JsonToken.String)
      {
        if (decimal.TryParse((string)Value, NumberStyles.Number, Culture, out d))
        {
          SetToken(JsonToken.Float, d);
          return d;
        }
        else
        {
          throw CreateReaderException(this, "Could not convert string to decimal: {0}.".FormatWith(CultureInfo.InvariantCulture, Value));
        }
      }

      if (TokenType == JsonToken.EndArray)
        return null;

      throw CreateReaderException(this, "Error reading decimal. Expected a number but got {0}.".FormatWith(CultureInfo.InvariantCulture, TokenType));
    }

    /// <summary>
    /// Reads the next JSON token from the stream as a <see cref="Nullable{Int32}"/>.
    /// </summary>
    /// <returns>A <see cref="Nullable{Int32}"/>. This method will return <c>null</c> at the end of an array.</returns>
    public override int? ReadAsInt32()
    {
      Read();

      if (TokenType == JsonToken.Integer || TokenType == JsonToken.Float)
      {
        SetToken(JsonToken.Float, Convert.ToInt32(Value, CultureInfo.InvariantCulture));
        return (int)Value;
      }

      if (TokenType == JsonToken.Null)
        return null;

      int i;
      if (TokenType == JsonToken.String)
      {
        if (int.TryParse((string)Value, NumberStyles.Integer, Culture, out i))
        {
          SetToken(JsonToken.Integer, i);
          return i;
        }
        else
        {
          throw CreateReaderException(this, "Could not convert string to integer: {0}.".FormatWith(CultureInfo.InvariantCulture, Value));
        }
      }

      if (TokenType == JsonToken.EndArray)
        return null;

      throw CreateReaderException(this, "Error reading integer. Expected a number but got {0}.".FormatWith(CultureInfo.InvariantCulture, TokenType));
    }

#if !NET20
    /// <summary>
    /// Reads the next JSON token from the stream as a <see cref="Nullable{DateTimeOffset}"/>.
    /// </summary>
    /// <returns>
    /// A <see cref="Nullable{DateTimeOffset}"/>. This method will return <c>null</c> at the end of an array.
    /// </returns>
    public override DateTimeOffset? ReadAsDateTimeOffset()
    {
      Read();

      if (TokenType == JsonToken.Date)
      {
        SetToken(JsonToken.Date, new DateTimeOffset((DateTime) Value));
        return (DateTimeOffset) Value;
      }

      if (TokenType == JsonToken.Null)
        return null;

      DateTimeOffset dt;
      if (TokenType == JsonToken.String)
      {
        if (DateTimeOffset.TryParse((string)Value, Culture, DateTimeStyles.None, out dt))
        {
          SetToken(JsonToken.Date, dt);
          return dt;
        }
        else
        {
          throw CreateReaderException(this, "Could not convert string to DateTimeOffset: {0}.".FormatWith(CultureInfo.InvariantCulture, Value));
        }
      }

      if (TokenType == JsonToken.EndArray)
        return null;

      throw CreateReaderException(this, "Error reading date. Expected date but got {0}.".FormatWith(CultureInfo.InvariantCulture, TokenType));
    }
#endif

    /// <summary>
    /// Reads the next JSON token from the stream.
    /// </summary>
    /// <returns>
    /// true if the next token was read successfully; false if there are no more tokens to read.
    /// </returns>
    public override bool Read()
    {
      try
      {
        bool success;

        switch (_bsonReaderState)
        {
          case BsonReaderState.Normal:
            success = ReadNormal();
            break;
          case BsonReaderState.ReferenceStart:
          case BsonReaderState.ReferenceRef:
          case BsonReaderState.ReferenceId:
            success = ReadReference();
            break;
          case BsonReaderState.CodeWScopeStart:
          case BsonReaderState.CodeWScopeCode:
          case BsonReaderState.CodeWScopeScope:
          case BsonReaderState.CodeWScopeScopeObject:
          case BsonReaderState.CodeWScopeScopeEnd:
            success = ReadCodeWScope();
            break;
          default:
            throw CreateReaderException(this, "Unexpected state: {0}".FormatWith(CultureInfo.InvariantCulture, _bsonReaderState));
        }

        if (!success)
        {
          SetToken(JsonToken.None);
          return false;
        }

        return true;
      }
      catch (EndOfStreamException)
      {
        SetToken(JsonToken.None);
        return false;
      }
    }

    /// <summary>
    /// Changes the <see cref="JsonReader.State"/> to Closed.
    /// </summary>
    public override void Close()
    {
      base.Close();

      if (CloseInput && _reader != null)
        _reader.Close();
    }

    private bool ReadCodeWScope()
    {
      switch (_bsonReaderState)
      {
        case BsonReaderState.CodeWScopeStart:
          SetToken(JsonToken.PropertyName, "$code");
          _bsonReaderState = BsonReaderState.CodeWScopeCode;
          return true;
        case BsonReaderState.CodeWScopeCode:
          // total CodeWScope size - not used
          ReadInt32();

          SetToken(JsonToken.String, ReadLengthString());
          _bsonReaderState = BsonReaderState.CodeWScopeScope;
          return true;
        case BsonReaderState.CodeWScopeScope:
          if (CurrentState == State.PostValue)
          {
            SetToken(JsonToken.PropertyName, "$scope");
            return true;
          }
          else
          {
            SetToken(JsonToken.StartObject);
            _bsonReaderState = BsonReaderState.CodeWScopeScopeObject;

            ContainerContext newContext = new ContainerContext(BsonType.Object);
            PushContext(newContext);
            newContext.Length = ReadInt32();

            return true;
          }
        case BsonReaderState.CodeWScopeScopeObject:
          bool result = ReadNormal();
          if (result && TokenType == JsonToken.EndObject)
            _bsonReaderState = BsonReaderState.CodeWScopeScopeEnd;

          return result;
        case BsonReaderState.CodeWScopeScopeEnd:
          SetToken(JsonToken.EndObject);
          _bsonReaderState = BsonReaderState.Normal;
          return true;
        default:
          throw new ArgumentOutOfRangeException();
      }
    }

    private bool ReadReference()
    {
      switch (CurrentState)
      {
        case State.ObjectStart:
          {
            SetToken(JsonToken.PropertyName, "$ref");
            _bsonReaderState = BsonReaderState.ReferenceRef;
            return true;
          }
        case State.Property:
          {
            if (_bsonReaderState == BsonReaderState.ReferenceRef)
            {
              SetToken(JsonToken.String, ReadLengthString());
              return true;
            }
            else if (_bsonReaderState == BsonReaderState.ReferenceId)
            {
              SetToken(JsonToken.Bytes, ReadBytes(12));
              return true;
            }
            else
            {
              throw CreateReaderException(this, "Unexpected state when reading BSON reference: " + _bsonReaderState);
            }
          }
        case State.PostValue:
          {
            if (_bsonReaderState == BsonReaderState.ReferenceRef)
            {
              SetToken(JsonToken.PropertyName, "$id");
              _bsonReaderState = BsonReaderState.ReferenceId;
              return true;
            }
            else if (_bsonReaderState == BsonReaderState.ReferenceId)
            {
              SetToken(JsonToken.EndObject);
              _bsonReaderState = BsonReaderState.Normal;
              return true;
            }
            else
            {
              throw CreateReaderException(this, "Unexpected state when reading BSON reference: " + _bsonReaderState);
            }
          }
        default:
          throw CreateReaderException(this, "Unexpected state when reading BSON reference: " + CurrentState);
      }
    }

    private bool ReadNormal()
    {
      switch (CurrentState)
      {
        case State.Start:
          {
            JsonToken token = (!_readRootValueAsArray) ? JsonToken.StartObject : JsonToken.StartArray;
            BsonType type = (!_readRootValueAsArray) ? BsonType.Object : BsonType.Array;

            SetToken(token);
            ContainerContext newContext = new ContainerContext(type);
            PushContext(newContext);
            newContext.Length = ReadInt32();
            return true;
          }
        case State.Complete:
        case State.Closed:
          return false;
        case State.Property:
          {
            ReadType(_currentElementType);
            return true;
          }
        case State.ObjectStart:
        case State.ArrayStart:
        case State.PostValue:
          ContainerContext context = _currentContext;
          if (context == null)
            return false;

          int lengthMinusEnd = context.Length - 1;

          if (context.Position < lengthMinusEnd)
          {
            if (context.Type == BsonType.Array)
            {
              ReadElement();
              ReadType(_currentElementType);
              return true;
            }
            else
            {
              SetToken(JsonToken.PropertyName, ReadElement());
              return true;
            }
          }
          else if (context.Position == lengthMinusEnd)
          {
            if (ReadByte() != 0)
              throw CreateReaderException(this, "Unexpected end of object byte value.");

            PopContext();
            if (_currentContext != null)
              MovePosition(context.Length);

            JsonToken endToken = (context.Type == BsonType.Object) ? JsonToken.EndObject : JsonToken.EndArray;
            SetToken(endToken);
            return true;
          }
          else
          {
            throw CreateReaderException(this, "Read past end of current container context.");
          }
        case State.ConstructorStart:
          break;
        case State.Constructor:
          break;
        case State.Error:
          break;
        case State.Finished:
          break;
        default:
          throw new ArgumentOutOfRangeException();
      }

      return false;
    }

    private void PopContext()
    {
      _stack.RemoveAt(_stack.Count - 1);
      if (_stack.Count == 0)
        _currentContext = null;
      else
        _currentContext = _stack[_stack.Count - 1];
    }

    private void PushContext(ContainerContext newContext)
    {
      _stack.Add(newContext);
      _currentContext = newContext;
    }

    private byte ReadByte()
    {
      MovePosition(1);
      return _reader.ReadByte();
    }

    private void ReadType(BsonType type)
    {
      switch (type)
      {
        case BsonType.Number:
          SetToken(JsonToken.Float, ReadDouble());
          break;
        case BsonType.String:
        case BsonType.Symbol:
          SetToken(JsonToken.String, ReadLengthString());
          break;
        case BsonType.Object:
          {
            SetToken(JsonToken.StartObject);

            ContainerContext newContext = new ContainerContext(BsonType.Object);
            PushContext(newContext);
            newContext.Length = ReadInt32();
            break;
          }
        case BsonType.Array:
          {
            SetToken(JsonToken.StartArray);

            ContainerContext newContext = new ContainerContext(BsonType.Array);
            PushContext(newContext);
            newContext.Length = ReadInt32();
            break;
          }
        case BsonType.Binary:
          SetToken(JsonToken.Bytes, ReadBinary());
          break;
        case BsonType.Undefined:
          SetToken(JsonToken.Undefined);
          break;
        case BsonType.Oid:
          byte[] oid = ReadBytes(12);
          SetToken(JsonToken.Bytes, oid);
          break;
        case BsonType.Boolean:
          bool b = Convert.ToBoolean(ReadByte());
          SetToken(JsonToken.Boolean, b);
          break;
        case BsonType.Date:
          long ticks = ReadInt64();
          DateTime utcDateTime = JsonConvert.ConvertJavaScriptTicksToDateTime(ticks);

          DateTime dateTime;
          switch (DateTimeKindHandling)
          {
            case DateTimeKind.Unspecified:
              dateTime = DateTime.SpecifyKind(utcDateTime, DateTimeKind.Unspecified);
              break;
            case DateTimeKind.Local:
              dateTime = utcDateTime.ToLocalTime();
              break;
            default:
              dateTime = utcDateTime;
              break;
          }

          SetToken(JsonToken.Date, dateTime);
          break;
        case BsonType.Null:
          SetToken(JsonToken.Null);
          break;
        case BsonType.Regex:
          string expression = ReadString();
          string modifiers = ReadString();

          string regex = @"/" + expression + @"/" + modifiers;
          SetToken(JsonToken.String, regex);
          break;
        case BsonType.Reference:
          SetToken(JsonToken.StartObject);
          _bsonReaderState = BsonReaderState.ReferenceStart;
          break;
        case BsonType.Code:
          SetToken(JsonToken.String, ReadLengthString());
          break;
        case BsonType.CodeWScope:
          SetToken(JsonToken.StartObject);
          _bsonReaderState = BsonReaderState.CodeWScopeStart;
          break;
        case BsonType.Integer:
          SetToken(JsonToken.Integer, (long) ReadInt32());
          break;
        case BsonType.TimeStamp:
        case BsonType.Long:
          SetToken(JsonToken.Integer, ReadInt64());
          break;
        default:
          throw new ArgumentOutOfRangeException("type", "Unexpected BsonType value: " + type);
      }
    }

    private byte[] ReadBinary()
    {
      int dataLength = ReadInt32();

      BsonBinaryType binaryType = (BsonBinaryType) ReadByte();

#pragma warning disable 612,618
      // the old binary type has the data length repeated in the data for some reason
      if (binaryType == BsonBinaryType.Data && !_jsonNet35BinaryCompatibility)
      {
        dataLength = ReadInt32();
      }
#pragma warning restore 612,618

      return ReadBytes(dataLength);
    }

    private string ReadString()
    {
      EnsureBuffers();

      StringBuilder builder = null;

      int totalBytesRead = 0;
      // used in case of left over multibyte characters in the buffer
      int offset = 0;
      do
      {
        int count = offset;
        byte b;
        while (count < MaxCharBytesSize && (b = _reader.ReadByte()) > 0)
        {
          _byteBuffer[count++] = b;
        }
        int byteCount = count - offset;
        totalBytesRead += byteCount;

        if (count < MaxCharBytesSize && builder == null)
        {
          // pref optimization to avoid reading into a string builder
          // if string is smaller than the buffer then return it directly
          int length = Encoding.UTF8.GetChars(_byteBuffer, 0, byteCount, _charBuffer, 0);

          MovePosition(totalBytesRead + 1);
          return new string(_charBuffer, 0, length);
        }
        else
        {
          // calculate the index of the end of the last full character in the buffer
          int lastFullCharStop = GetLastFullCharStop(count - 1);

          int charCount = Encoding.UTF8.GetChars(_byteBuffer, 0, lastFullCharStop + 1, _charBuffer, 0);

          if (builder == null)
            builder = new StringBuilder(MaxCharBytesSize*2);

          builder.Append(_charBuffer, 0, charCount);

          if (lastFullCharStop < byteCount - 1)
          {
            offset = byteCount - lastFullCharStop - 1;
            // copy left over multi byte characters to beginning of buffer for next iteration
            Array.Copy(_byteBuffer, lastFullCharStop + 1, _byteBuffer, 0, offset);
          }
          else
          {
            // reached end of string
            if (count < MaxCharBytesSize)
            {
              MovePosition(totalBytesRead + 1);
              return builder.ToString();
            }

            offset = 0;
          }
        }
      } while (true);
    }

    private string ReadLengthString()
    {
      int length = ReadInt32();

      MovePosition(length);

      string s = GetString(length - 1);
      _reader.ReadByte();

      return s;
    }

    private string GetString(int length)
    {
      if (length == 0)
        return string.Empty;

      EnsureBuffers();

      StringBuilder builder = null;

      int totalBytesRead = 0;

      // used in case of left over multibyte characters in the buffer
      int offset = 0;
      do
      {
        int count = ((length - totalBytesRead) > MaxCharBytesSize - offset)
                      ? MaxCharBytesSize - offset
                      : length - totalBytesRead;

        int byteCount = _reader.Read(_byteBuffer, offset, count);

        if (byteCount == 0)
          throw new EndOfStreamException("Unable to read beyond the end of the stream.");

        totalBytesRead += byteCount;

        // Above, byteCount is how many bytes we read this time.
        // Below, byteCount is how many bytes are in the _byteBuffer.
        byteCount += offset;

        if (byteCount == length)
        {
          // pref optimization to avoid reading into a string builder
          // first iteration and all bytes read then return string directly
          int charCount = Encoding.UTF8.GetChars(_byteBuffer, 0, byteCount, _charBuffer, 0);
          return new string(_charBuffer, 0, charCount);
        }
        else
        {
          int lastFullCharStop = GetLastFullCharStop(byteCount - 1);

          if (builder == null)
            builder = new StringBuilder(length);

          int charCount = Encoding.UTF8.GetChars(_byteBuffer, 0, lastFullCharStop + 1, _charBuffer, 0);
          builder.Append(_charBuffer, 0, charCount);

          if (lastFullCharStop < byteCount - 1)
          {
            offset = byteCount - lastFullCharStop - 1;
            // copy left over multi byte characters to beginning of buffer for next iteration
            Array.Copy(_byteBuffer, lastFullCharStop + 1, _byteBuffer, 0, offset);
          }
          else
          {
            offset = 0;
          }
        }
      } while (totalBytesRead < length);

      return builder.ToString();
    }

    private int GetLastFullCharStop(int start)
    {
      int lookbackPos = start;
      int bis = 0;
      while (lookbackPos >= 0)
      {
        bis = BytesInSequence(_byteBuffer[lookbackPos]);
        if (bis == 0)
        {
          lookbackPos--;
          continue;
        }
        else if (bis == 1)
        {
          break;
        }
        else
        {
          lookbackPos--;
          break;
        }
      }
      if (bis == start - lookbackPos)
      {
        //Full character.
        return start;
      }
      else
      {
        return lookbackPos;
      }
    }

    private int BytesInSequence(byte b)
    {
      if (b <= SeqRange1[1]) return 1;
      if (b >= SeqRange2[0] && b <= SeqRange2[1]) return 2;
      if (b >= SeqRange3[0] && b <= SeqRange3[1]) return 3;
      if (b >= SeqRange4[0] && b <= SeqRange4[1]) return 4;
      return 0;
    }

    private void EnsureBuffers()
    {
      if (_byteBuffer == null)
      {
        _byteBuffer = new byte[MaxCharBytesSize];
      }
      if (_charBuffer == null)
      {
        int charBufferSize = Encoding.UTF8.GetMaxCharCount(MaxCharBytesSize);
        _charBuffer = new char[charBufferSize];
      }
    }

    private double ReadDouble()
    {
      MovePosition(8);
      return _reader.ReadDouble();
    }

    private int ReadInt32()
    {
      MovePosition(4);
      return _reader.ReadInt32();
    }

    private long ReadInt64()
    {
      MovePosition(8);
      return _reader.ReadInt64();
    }

    private BsonType ReadType()
    {
      MovePosition(1);
      return (BsonType) _reader.ReadSByte();
    }

    private void MovePosition(int count)
    {
      _currentContext.Position += count;
    }

    private byte[] ReadBytes(int count)
    {
      MovePosition(count);
      return _reader.ReadBytes(count);
    }
  }
}