var BIT_16 = Math.pow(2, 16);
var BIT_24 = Math.pow(2, 24);
// The maximum precision JS Numbers can hold precisely
// Don't panic: Good enough to represent byte values up to 8192 TB
var IEEE_754_BINARY_64_PRECISION = Math.pow(2, 53);
var MAX_PACKET_LENGTH            = Math.pow(2, 24) - 1;

module.exports = PacketWriter;
function PacketWriter() {
  this._buffer = new Buffer(0);
  this._offset = 0;
}

PacketWriter.prototype.toBuffer = function(parser) {
  var packets  = Math.floor(this._buffer.length / MAX_PACKET_LENGTH) + 1;
  var buffer   = this._buffer;
  this._buffer = new Buffer(this._buffer.length + packets * 4);

  for (var packet = 0; packet < packets; packet++) {
    this._offset = packet * (MAX_PACKET_LENGTH + 4);

    var isLast = (packet + 1 === packets);
    var packetLength = (isLast)
      ? buffer.length % MAX_PACKET_LENGTH
      : MAX_PACKET_LENGTH;

    var packetNumber = parser.incrementPacketNumber();

    this.writeUnsignedNumber(3, packetLength);
    this.writeUnsignedNumber(1, packetNumber);

    var start = packet * MAX_PACKET_LENGTH;
    var end   = start + packetLength;

    this.writeBuffer(buffer.slice(start, end));
  }

  return this._buffer;
};

PacketWriter.prototype.writeUnsignedNumber = function(bytes, value) {
  this._allocate(bytes);

  for (var i = 0; i < bytes; i++) {
    this._buffer[this._offset++] = (value >> (i * 8)) & 0xff;
  }
};

PacketWriter.prototype.writeFiller = function(bytes) {
  this._allocate(bytes);

  for (var i = 0; i < bytes; i++) {
    this._buffer[this._offset++] = 0x00;
  }
};

PacketWriter.prototype.writeNullTerminatedString = function(value, encoding) {
  // Typecast undefined into '' and numbers into strings
  value = value || '';
  value = value + '';

  var bytes = Buffer.byteLength(value, encoding || 'utf-8') + 1;
  this._allocate(bytes);

  this._buffer.write(value, this._offset, encoding);
  this._buffer[this._offset + bytes - 1] = 0x00;

  this._offset += bytes;
};

PacketWriter.prototype.writeString = function(value) {
  // Typecast undefined into '' and numbers into strings
  value = value || '';
  value = value + '';

  var bytes = Buffer.byteLength(value, 'utf-8');
  this._allocate(bytes);

  this._buffer.write(value, this._offset, 'utf-8');

  this._offset += bytes;
};

PacketWriter.prototype.writeBuffer = function(value) {
  var bytes = value.length;

  this._allocate(bytes);
  value.copy(this._buffer, this._offset);
  this._offset += bytes;
};

PacketWriter.prototype.writeLengthCodedNumber = function(value) {
  if (value === null) {
    this._allocate(1);
    this._buffer[this._offset++] = 251;
    return;
  }

  if (value <= 250) {
    this._allocate(1);
    this._buffer[this._offset++] = value;
    return;
  }

  if (value > IEEE_754_BINARY_64_PRECISION) {
    throw new Error(
      'writeLengthCodedNumber: JS precision range exceeded, your ' +
      'number is > 53 bit: "' + value + '"'
    );
  }

  if (value <= BIT_16) {
    this._allocate(3)
    this._buffer[this._offset++] = 252;
  } else if (value <= BIT_24) {
    this._allocate(4)
    this._buffer[this._offset++] = 253;
  } else {
    this._allocate(9);
    this._buffer[this._offset++] = 254;
  }

  // 16 Bit
  this._buffer[this._offset++] = value & 0xff;
  this._buffer[this._offset++] = (value >> 8) & 0xff;

  if (value <= BIT_16) return;

  // 24 Bit
  this._buffer[this._offset++] = (value >> 16) & 0xff;

  if (value <= BIT_24) return;

  this._buffer[this._offset++] = (value >> 24) & 0xff;

  // Hack: Get the most significant 32 bit (JS bitwise operators are 32 bit)
  value = value.toString(2);
  value = value.substr(0, value.length - 32);
  value = parseInt(value, 2);

  this._buffer[this._offset++] = value & 0xff;
  this._buffer[this._offset++] = (value >> 8) & 0xff;
  this._buffer[this._offset++] = (value >> 16) & 0xff;

  // Set last byte to 0, as we can only support 53 bits in JS (see above)
  this._buffer[this._offset++] = 0;
};

PacketWriter.prototype.writeLengthCodedBuffer = function(value) {
  var bytes = value.length;
  this.writeLengthCodedNumber(bytes);
  this.writeBuffer(value);
};

PacketWriter.prototype.writeNullTerminatedBuffer = function(value) {
  this.writeBuffer(value);
  this.writeFiller(1); // 0x00 terminator
};

PacketWriter.prototype.writeLengthCodedString = function(value) {
  if (value === null) {
    this.writeLengthCodedNumber(null);
    return;
  }

  value = (value === undefined)
    ? ''
    : String(value);

  var bytes = Buffer.byteLength(value, 'utf-8');
  this.writeLengthCodedNumber(bytes);

  if (!bytes) {
    return;
  }

  this._allocate(bytes);
  this._buffer.write(value, this._offset, 'utf-8');
  this._offset += bytes;
};

PacketWriter.prototype._allocate = function(bytes) {
  if (!this._buffer) {
    this._buffer = new Buffer(bytes);
    return;
  }

  var bytesRemaining = this._buffer.length - this._offset;
  if (bytesRemaining >= bytes) {
    return;
  }

  var oldBuffer = this._buffer;

  this._buffer = new Buffer(oldBuffer.length + bytes);
  oldBuffer.copy(this._buffer);
};