@abandonware/noble/lib/hci-socket/gatt.js

A Node.js BLE (Bluetooth Low Energy) central library.

const debug = require('debug')('att');

const events = require('events');
const util = require('util');

/* eslint-disable no-unused-vars */
const ATT_OP_ERROR = 0x01;
const ATT_OP_MTU_REQ = 0x02;
const ATT_OP_MTU_RESP = 0x03;
const ATT_OP_FIND_INFO_REQ = 0x04;
const ATT_OP_FIND_INFO_RESP = 0x05;
const ATT_OP_READ_BY_TYPE_REQ = 0x08;
const ATT_OP_READ_BY_TYPE_RESP = 0x09;
const ATT_OP_READ_REQ = 0x0a;
const ATT_OP_READ_RESP = 0x0b;
const ATT_OP_READ_BLOB_REQ = 0x0c;
const ATT_OP_READ_BLOB_RESP = 0x0d;
const ATT_OP_READ_BY_GROUP_REQ = 0x10;
const ATT_OP_READ_BY_GROUP_RESP = 0x11;
const ATT_OP_WRITE_REQ = 0x12;
const ATT_OP_WRITE_RESP = 0x13;
const ATT_OP_PREPARE_WRITE_REQ = 0x16;
const ATT_OP_PREPARE_WRITE_RESP = 0x17;
const ATT_OP_EXECUTE_WRITE_REQ = 0x18;
const ATT_OP_EXECUTE_WRITE_RESP = 0x19;
const ATT_OP_HANDLE_NOTIFY = 0x1b;
const ATT_OP_HANDLE_IND = 0x1d;
const ATT_OP_HANDLE_CNF = 0x1e;
const ATT_OP_WRITE_CMD = 0x52;

const ATT_ECODE_SUCCESS = 0x00;
const ATT_ECODE_INVALID_HANDLE = 0x01;
const ATT_ECODE_READ_NOT_PERM = 0x02;
const ATT_ECODE_WRITE_NOT_PERM = 0x03;
const ATT_ECODE_INVALID_PDU = 0x04;
const ATT_ECODE_AUTHENTICATION = 0x05;
const ATT_ECODE_REQ_NOT_SUPP = 0x06;
const ATT_ECODE_INVALID_OFFSET = 0x07;
const ATT_ECODE_AUTHORIZATION = 0x08;
const ATT_ECODE_PREP_QUEUE_FULL = 0x09;
const ATT_ECODE_ATTR_NOT_FOUND = 0x0a;
const ATT_ECODE_ATTR_NOT_LONG = 0x0b;
const ATT_ECODE_INSUFF_ENCR_KEY_SIZE = 0x0c;
const ATT_ECODE_INVAL_ATTR_VALUE_LEN = 0x0d;
const ATT_ECODE_UNLIKELY = 0x0e;
const ATT_ECODE_INSUFF_ENC = 0x0f;
const ATT_ECODE_UNSUPP_GRP_TYPE = 0x10;
const ATT_ECODE_INSUFF_RESOURCES = 0x11;

const GATT_PRIM_SVC_UUID = 0x2800;
const GATT_INCLUDE_UUID = 0x2802;
const GATT_CHARAC_UUID = 0x2803;

const GATT_CLIENT_CHARAC_CFG_UUID = 0x2902;
const GATT_SERVER_CHARAC_CFG_UUID = 0x2903;

const ATT_CID = 0x0004;
/* eslint-enable no-unused-vars */

const Gatt = function (address, aclStream) {
  this._address = address;
  this._aclStream = aclStream;

  this._services = {};
  this._characteristics = {};
  this._descriptors = {};

  this._currentCommand = null;
  this._commandQueue = [];

  this._mtu = 23;
  this._desired_mtu = 256;
  this._security = 'low';

  this.onAclStreamDataBinded = this.onAclStreamData.bind(this);
  this.onAclStreamEncryptBinded = this.onAclStreamEncrypt.bind(this);
  this.onAclStreamEncryptFailBinded = this.onAclStreamEncryptFail.bind(this);
  this.onAclStreamEndBinded = this.onAclStreamEnd.bind(this);

  this._aclStream.on('data', this.onAclStreamDataBinded);
  this._aclStream.on('encrypt', this.onAclStreamEncryptBinded);
  this._aclStream.on('encryptFail', this.onAclStreamEncryptFailBinded);
  this._aclStream.on('end', this.onAclStreamEndBinded);
};

util.inherits(Gatt, events.EventEmitter);

Gatt.prototype.onAclStreamData = function (cid, data) {
  if (cid !== ATT_CID) {
    return;
  }

  if (this._currentCommand && (data.toString('hex') === this._currentCommand.buffer.toString('hex'))) {
    debug(`${this._address}: echo ... echo ... echo ...`);
  } else if (data[0] % 2 === 0) {
    if (process.env.NOBLE_MULTI_ROLE) {
      debug(`${this._address}: multi-role flag in use, ignoring command meant for peripheral role.`);
    } else {
      const requestType = data[0];
      if (requestType === ATT_OP_MTU_REQ) {
        debug(`${this._address}: replying to MTU request`);
        this.writeAtt(this.mtuResponse(this._desired_mtu));
      } else {
        debug(`${this._address}: replying with REQ_NOT_SUPP to 0x${requestType.toString(16)}`);
        this.writeAtt(this.errorResponse(requestType, 0x0000, ATT_ECODE_REQ_NOT_SUPP));
      }
    }
  } else if (data[0] === ATT_OP_HANDLE_NOTIFY || data[0] === ATT_OP_HANDLE_IND) {
    const valueHandle = data.readUInt16LE(1);
    const valueData = data.slice(3);

    this.emit('handleNotify', this._address, valueHandle, valueData);

    if (data[0] === ATT_OP_HANDLE_IND) {
      this._queueCommand(this.handleConfirmation(), null, () => {
        this.emit('handleConfirmation', this._address, valueHandle);
      });
    }

    for (const serviceUuid in this._services) {
      for (const characteristicUuid in this._characteristics[serviceUuid]) {
        if (this._characteristics[serviceUuid][characteristicUuid].valueHandle === valueHandle) {
          this.emit('notification', this._address, serviceUuid, characteristicUuid, valueData);
        }
      }
    }
  } else if (!this._currentCommand) {
    debug(`${this._address}: uh oh, no current command`);
  } else {
    if (data[0] === ATT_OP_ERROR &&
        (data[4] === ATT_ECODE_AUTHENTICATION || data[4] === ATT_ECODE_AUTHORIZATION || data[4] === ATT_ECODE_INSUFF_ENC) &&
        this._security !== 'medium') {
      this._aclStream.encrypt();
      return;
    }

    if (data[0] === ATT_OP_ERROR && data[4] === ATT_ECODE_INVALID_PDU) {
      debug('Error: can\'t change MTU, invalid PDU');
      return;
    }

    debug(`${this._address}: read: ${data.toString('hex')}`);

    this._currentCommand.callback(data);

    this._currentCommand = null;

    while (this._commandQueue.length) {
      this._currentCommand = this._commandQueue.shift();

      this.writeAtt(this._currentCommand.buffer);

      if (this._currentCommand.callback) {
        break;
      } else if (this._currentCommand.writeCallback) {
        this._currentCommand.writeCallback();

        this._currentCommand = null;
      }
    }
  }
};

Gatt.prototype.onAclStreamEncrypt = function (encrypt) {
  if (encrypt) {
    this._security = 'medium';

    this.writeAtt(this._currentCommand.buffer);
  }
};

Gatt.prototype.onAclStreamEncryptFail = function () {

};

Gatt.prototype.onAclStreamEnd = function () {
  this._aclStream.removeListener('data', this.onAclStreamDataBinded);
  this._aclStream.removeListener('encrypt', this.onAclStreamEncryptBinded);
  this._aclStream.removeListener('encryptFail', this.onAclStreamEncryptFailBinded);
  this._aclStream.removeListener('end', this.onAclStreamEndBinded);
};

Gatt.prototype.writeAtt = function (data) {
  debug(`${this._address}: write: ${data.toString('hex')}`);

  this._aclStream.write(ATT_CID, data);
};

Gatt.prototype.errorResponse = function (opcode, handle, status) {
  const buf = Buffer.alloc(5);

  buf.writeUInt8(ATT_OP_ERROR, 0);
  buf.writeUInt8(opcode, 1);
  buf.writeUInt16LE(handle, 2);
  buf.writeUInt8(status, 4);

  return buf;
};

Gatt.prototype._queueCommand = function (buffer, callback, writeCallback) {
  this._commandQueue.push({
    buffer,
    callback,
    writeCallback
  });

  if (this._currentCommand === null) {
    while (this._commandQueue.length) {
      this._currentCommand = this._commandQueue.shift();

      this.writeAtt(this._currentCommand.buffer);

      if (this._currentCommand.callback) {
        break;
      } else if (this._currentCommand.writeCallback) {
        this._currentCommand.writeCallback();

        this._currentCommand = null;
      }
    }
  }
};

Gatt.prototype.mtuRequest = function (mtu) {
  const buf = Buffer.alloc(3);

  buf.writeUInt8(ATT_OP_MTU_REQ, 0);
  buf.writeUInt16LE(mtu, 1);

  return buf;
};

Gatt.prototype.mtuResponse = function (mtu) {
  const buf = Buffer.alloc(3);

  buf.writeUInt8(ATT_OP_MTU_RESP, 0);
  buf.writeUInt16LE(mtu, 1);

  return buf;
};

Gatt.prototype.readByGroupRequest = function (startHandle, endHandle, groupUuid) {
  const buf = Buffer.alloc(7);

  buf.writeUInt8(ATT_OP_READ_BY_GROUP_REQ, 0);
  buf.writeUInt16LE(startHandle, 1);
  buf.writeUInt16LE(endHandle, 3);
  buf.writeUInt16LE(groupUuid, 5);

  return buf;
};

Gatt.prototype.readByTypeRequest = function (startHandle, endHandle, groupUuid) {
  const buf = Buffer.alloc(7);

  buf.writeUInt8(ATT_OP_READ_BY_TYPE_REQ, 0);
  buf.writeUInt16LE(startHandle, 1);
  buf.writeUInt16LE(endHandle, 3);
  buf.writeUInt16LE(groupUuid, 5);

  return buf;
};

Gatt.prototype.readRequest = function (handle) {
  const buf = Buffer.alloc(3);

  buf.writeUInt8(ATT_OP_READ_REQ, 0);
  buf.writeUInt16LE(handle, 1);

  return buf;
};

Gatt.prototype.readBlobRequest = function (handle, offset) {
  const buf = Buffer.alloc(5);

  buf.writeUInt8(ATT_OP_READ_BLOB_REQ, 0);
  buf.writeUInt16LE(handle, 1);
  buf.writeUInt16LE(offset, 3);

  return buf;
};

Gatt.prototype.findInfoRequest = function (startHandle, endHandle) {
  const buf = Buffer.alloc(5);

  buf.writeUInt8(ATT_OP_FIND_INFO_REQ, 0);
  buf.writeUInt16LE(startHandle, 1);
  buf.writeUInt16LE(endHandle, 3);

  return buf;
};

Gatt.prototype.writeRequest = function (handle, data, withoutResponse) {
  const buf = Buffer.alloc(3 + data.length);

  buf.writeUInt8(withoutResponse ? ATT_OP_WRITE_CMD : ATT_OP_WRITE_REQ, 0);
  buf.writeUInt16LE(handle, 1);

  for (let i = 0; i < data.length; i++) {
    buf.writeUInt8(data.readUInt8(i), i + 3);
  }

  return buf;
};

Gatt.prototype.prepareWriteRequest = function (handle, offset, data) {
  const buf = Buffer.alloc(5 + data.length);

  buf.writeUInt8(ATT_OP_PREPARE_WRITE_REQ, 0);
  buf.writeUInt16LE(handle, 1);
  buf.writeUInt16LE(offset, 3);

  for (let i = 0; i < data.length; i++) {
    buf.writeUInt8(data.readUInt8(i), i + 5);
  }

  return buf;
};

Gatt.prototype.executeWriteRequest = function (handle, cancelPreparedWrites) {
  const buf = Buffer.alloc(2);

  buf.writeUInt8(ATT_OP_EXECUTE_WRITE_REQ, 0);
  buf.writeUInt8(cancelPreparedWrites ? 0 : 1, 1);

  return buf;
};

Gatt.prototype.handleConfirmation = function () {
  const buf = Buffer.alloc(1);

  buf.writeUInt8(ATT_OP_HANDLE_CNF, 0);

  return buf;
};

Gatt.prototype.exchangeMtu = function () {
  this._queueCommand(this.mtuRequest(this._desired_mtu), (data) => {
    const opcode = data[0];

    if (opcode === ATT_OP_MTU_RESP) {
      const newMtu = data.readUInt16LE(1);

      debug(`${this._address}: new MTU is ${newMtu}`);

      this._mtu = newMtu;
    }

    this.emit('mtu', this._address, this._mtu);
  });
};

Gatt.prototype.addService = function (service) {
  this._services[service.uuid] = service;
};

Gatt.prototype.discoverServices = function (uuids) {
  const services = [];

  const callback = (data) => {
    const opcode = data[0];
    let i = 0;

    if (opcode === ATT_OP_READ_BY_GROUP_RESP) {
      const type = data[1];
      const num = (data.length - 2) / type;

      for (i = 0; i < num; i++) {
        const offset = 2 + i * type;
        services.push({
          startHandle: data.readUInt16LE(offset),
          endHandle: data.readUInt16LE(offset + 2),
          uuid: (type === 6) ? data.readUInt16LE(offset + 4).toString(16) : data.slice(offset + 4).slice(0, 16).toString('hex').match(/.{1,2}/g).reverse().join('')
        });
      }
    }

    if (opcode !== ATT_OP_READ_BY_GROUP_RESP || services[services.length - 1].endHandle === 0xffff) {
      const serviceUuids = [];
      for (i = 0; i < services.length; i++) {
        const uuid = services[i].uuid.trim();
        if ((uuids.length === 0 || uuids.indexOf(uuid) !== -1) && serviceUuids.indexOf(uuid) === -1) {
          serviceUuids.push(uuid);
        }

        this._services[services[i].uuid] = services[i];
      }
      this.emit('servicesDiscovered', this._address, JSON.parse(JSON.stringify(services)) /* services */);
      this.emit('servicesDiscover', this._address, serviceUuids);
    } else {
      this._queueCommand(this.readByGroupRequest(services[services.length - 1].endHandle + 1, 0xffff, GATT_PRIM_SVC_UUID), callback);
    }
  };

  this._queueCommand(this.readByGroupRequest(0x0001, 0xffff, GATT_PRIM_SVC_UUID), callback);
};

Gatt.prototype.discoverIncludedServices = function (serviceUuid, uuids) {
  const service = this._services[serviceUuid];
  const includedServices = [];

  const callback = (data) => {
    const opcode = data[0];
    let i = 0;

    if (opcode === ATT_OP_READ_BY_TYPE_RESP) {
      const type = data[1];
      const num = (data.length - 2) / type;

      for (i = 0; i < num; i++) {
        const offset = 2 + i * type;
        includedServices.push({
          endHandle: data.readUInt16LE(offset),
          startHandle: data.readUInt16LE(offset + 2),
          uuid: (type === 8) ? data.readUInt16LE(offset + 6).toString(16) : data.slice(offset + 6).slice(0, 16).toString('hex').match(/.{1,2}/g).reverse().join('')
        });
      }
    }

    if (opcode !== ATT_OP_READ_BY_TYPE_RESP || includedServices[includedServices.length - 1].endHandle === service.endHandle) {
      const includedServiceUuids = [];

      for (i = 0; i < includedServices.length; i++) {
        if (uuids.length === 0 || uuids.indexOf(includedServices[i].uuid) !== -1) {
          includedServiceUuids.push(includedServices[i].uuid);
        }
      }

      this.emit('includedServicesDiscover', this._address, service.uuid, includedServiceUuids);
    } else {
      this._queueCommand(this.readByTypeRequest(includedServices[includedServices.length - 1].endHandle + 1, service.endHandle, GATT_INCLUDE_UUID), callback);
    }
  };

  this._queueCommand(this.readByTypeRequest(service.startHandle, service.endHandle, GATT_INCLUDE_UUID), callback);
};

Gatt.prototype.addCharacteristics = function (serviceUuid, characteristics) {
  this._characteristics[serviceUuid] = this._characteristics[serviceUuid] || {};
  this._descriptors[serviceUuid] = this._descriptors[serviceUuid] || {};

  for (let i = 0; i < characteristics.length; i++) {
    this._characteristics[serviceUuid][characteristics[i].uuid] = characteristics[i];
  }
};

Gatt.prototype.discoverCharacteristics = function (serviceUuid, characteristicUuids) {
  const service = this._services[serviceUuid];
  const characteristics = [];

  this._characteristics[serviceUuid] = this._characteristics[serviceUuid] || {};
  this._descriptors[serviceUuid] = this._descriptors[serviceUuid] || {};

  const callback = (data) => {
    const opcode = data[0];
    let i = 0;

    if (opcode === ATT_OP_READ_BY_TYPE_RESP) {
      const type = data[1];
      const num = (data.length - 2) / type;

      for (i = 0; i < num; i++) {
        const offset = 2 + i * type;
        characteristics.push({
          startHandle: data.readUInt16LE(offset),
          properties: data.readUInt8(offset + 2),
          valueHandle: data.readUInt16LE(offset + 3),
          uuid: (type === 7) ? data.readUInt16LE(offset + 5).toString(16) : data.slice(offset + 5).slice(0, 16).toString('hex').match(/.{1,2}/g).reverse().join('')
        });
      }
    }

    if (opcode !== ATT_OP_READ_BY_TYPE_RESP || characteristics[characteristics.length - 1].valueHandle === service.endHandle) {
      const characteristicsDiscovered = [];
      for (i = 0; i < characteristics.length; i++) {
        const properties = characteristics[i].properties;

        const characteristic = {
          properties: [],
          uuid: characteristics[i].uuid
        };

        // work around name-clash of numeric vs. string-array properties field:
        characteristics[i].propsDecoded = characteristic.properties;
        characteristics[i].rawProps = properties;

        if (i !== 0) {
          characteristics[i - 1].endHandle = characteristics[i].startHandle - 1;
        }

        if (i === (characteristics.length - 1)) {
          characteristics[i].endHandle = service.endHandle;
        }

        this._characteristics[serviceUuid][characteristics[i].uuid] = characteristics[i];

        if (properties & 0x01) {
          characteristic.properties.push('broadcast');
        }

        if (properties & 0x02) {
          characteristic.properties.push('read');
        }

        if (properties & 0x04) {
          characteristic.properties.push('writeWithoutResponse');
        }

        if (properties & 0x08) {
          characteristic.properties.push('write');
        }

        if (properties & 0x10) {
          characteristic.properties.push('notify');
        }

        if (properties & 0x20) {
          characteristic.properties.push('indicate');
        }

        if (properties & 0x40) {
          characteristic.properties.push('authenticatedSignedWrites');
        }

        if (properties & 0x80) {
          characteristic.properties.push('extendedProperties');
        }

        if (characteristicUuids.length === 0 || characteristicUuids.indexOf(characteristic.uuid) !== -1) {
          characteristicsDiscovered.push(characteristic);
        }
      }

      this.emit('characteristicsDiscovered', this._address, serviceUuid, characteristics);
      this.emit('characteristicsDiscover', this._address, serviceUuid, characteristicsDiscovered);
    } else {
      this._queueCommand(this.readByTypeRequest(characteristics[characteristics.length - 1].valueHandle + 1, service.endHandle, GATT_CHARAC_UUID), callback);
    }
  };

  this._queueCommand(this.readByTypeRequest(service.startHandle, service.endHandle, GATT_CHARAC_UUID), callback);
};

Gatt.prototype.read = function (serviceUuid, characteristicUuid) {
  const characteristic = this._characteristics[serviceUuid][characteristicUuid];

  let readData = Buffer.alloc(0);

  const callback = (data) => {
    const opcode = data[0];

    if (opcode === ATT_OP_READ_RESP || opcode === ATT_OP_READ_BLOB_RESP) {
      readData = Buffer.from(`${readData.toString('hex')}${data.slice(1).toString('hex')}`, 'hex');

      if (data.length === this._mtu) {
        this._queueCommand(this.readBlobRequest(characteristic.valueHandle, readData.length), callback);
      } else {
        this.emit('read', this._address, serviceUuid, characteristicUuid, readData);
      }
    } else {
      this.emit('read', this._address, serviceUuid, characteristicUuid, readData);
    }
  };

  this._queueCommand(this.readRequest(characteristic.valueHandle), callback);
};

Gatt.prototype.write = function (serviceUuid, characteristicUuid, data, withoutResponse) {
  const characteristic = this._characteristics[serviceUuid][characteristicUuid];

  if (withoutResponse) {
    this._queueCommand(this.writeRequest(characteristic.valueHandle, data, true), null, () => {
      this.emit('write', this._address, serviceUuid, characteristicUuid);
    });
  } else if (data.length + 3 > this._mtu) {
    return this.longWrite(serviceUuid, characteristicUuid, data, withoutResponse);
  } else {
    this._queueCommand(this.writeRequest(characteristic.valueHandle, data, false), data => {
      const opcode = data[0];

      if (opcode === ATT_OP_WRITE_RESP) {
        this.emit('write', this._address, serviceUuid, characteristicUuid);
      }
    });
  }
};

/* Perform a "long write" as described Bluetooth Spec section 4.9.4 "Write Long Characteristic Values" */
Gatt.prototype.longWrite = function (serviceUuid, characteristicUuid, data, withoutResponse) {
  const characteristic = this._characteristics[serviceUuid][characteristicUuid];
  const limit = this._mtu - 5;

  const prepareWriteCallback = (dataChunk) => {
    return (resp) => {
      const opcode = resp[0];

      if (opcode !== ATT_OP_PREPARE_WRITE_RESP) {
        debug(`${this._address}: unexpected reply opcode %d (expecting ATT_OP_PREPARE_WRITE_RESP)`, opcode);
      } else {
        const expectedLength = dataChunk.length + 5;

        if (resp.length !== expectedLength) {
          /* the response should contain the data packet echoed back to the caller */
          debug(`${this._address}: unexpected prepareWriteResponse length %d (expecting %d)`, resp.length, expectedLength);
        }
      }
    };
  };

  /* split into prepare-write chunks and queue them */
  let offset = 0;

  while (offset < data.length) {
    const end = offset + limit;
    const chunk = data.slice(offset, end);
    this._queueCommand(this.prepareWriteRequest(characteristic.valueHandle, offset, chunk), prepareWriteCallback(chunk));
    offset = end;
  }

  /* queue the execute command with a callback to emit the write signal when done */
  this._queueCommand(this.executeWriteRequest(characteristic.valueHandle), resp => {
    const opcode = resp[0];

    if (opcode === ATT_OP_EXECUTE_WRITE_RESP && !withoutResponse) {
      this.emit('write', this._address, serviceUuid, characteristicUuid);
    }
  });
};

Gatt.prototype.broadcast = function (serviceUuid, characteristicUuid, broadcast) {
  const characteristic = this._characteristics[serviceUuid][characteristicUuid];

  this._queueCommand(this.readByTypeRequest(characteristic.startHandle, characteristic.endHandle, GATT_SERVER_CHARAC_CFG_UUID), data => {
    const opcode = data[0];
    if (opcode === ATT_OP_READ_BY_TYPE_RESP) {
      const handle = data.readUInt16LE(2);
      let value = data.readUInt16LE(4);

      if (broadcast) {
        value |= 0x0001;
      } else {
        value &= 0xfffe;
      }

      const valueBuffer = Buffer.alloc(2);
      valueBuffer.writeUInt16LE(value, 0);

      this._queueCommand(this.writeRequest(handle, valueBuffer, false), data => {
        const opcode = data[0];

        if (opcode === ATT_OP_WRITE_RESP) {
          this.emit('broadcast', this._address, serviceUuid, characteristicUuid, broadcast);
        }
      });
    }
  });
};

Gatt.prototype.notify = function (serviceUuid, characteristicUuid, notify) {
  const characteristic = this._characteristics[serviceUuid][characteristicUuid];

  this._queueCommand(this.readByTypeRequest(characteristic.startHandle, characteristic.endHandle, GATT_CLIENT_CHARAC_CFG_UUID), data => {
    const opcode = data[0];
    if (opcode === ATT_OP_READ_BY_TYPE_RESP) {
      const handle = data.readUInt16LE(2);
      let value = data.readUInt16LE(4);

      const useNotify = characteristic.properties & 0x10;
      const useIndicate = characteristic.properties & 0x20;

      if (notify) {
        if (useNotify) {
          value |= 0x0001;
        } else if (useIndicate) {
          value |= 0x0002;
        }
      } else {
        if (useNotify) {
          value &= 0xfffe;
        } else if (useIndicate) {
          value &= 0xfffd;
        }
      }

      const valueBuffer = Buffer.alloc(2);
      valueBuffer.writeUInt16LE(value, 0);

      this._queueCommand(this.writeRequest(handle, valueBuffer, false), data => {
        const opcode = data[0];

        if (opcode === ATT_OP_WRITE_RESP) {
          this.emit('notify', this._address, serviceUuid, characteristicUuid, notify);
        }
      });
    }
  });
};

Gatt.prototype.discoverDescriptors = function (serviceUuid, characteristicUuid) {
  const characteristic = this._characteristics[serviceUuid][characteristicUuid];
  const descriptors = [];

  this._descriptors[serviceUuid][characteristicUuid] = {};

  const callback = data => {
    const opcode = data[0];
    let i = 0;

    if (opcode === ATT_OP_FIND_INFO_RESP) {
      const format = data[1];
      const elen = 2 + (format === 0x01 ? 2 : 16);
      const num = (data.length - 2) / (elen);
      for (i = 0; i < num; i++) {
        const offset = 2 + (i * elen);
        descriptors.push({
          handle: data.readUInt16LE(offset + 0),
          uuid: (format === 0x01) ? data.readUInt16LE(offset + 2).toString(16) : data.slice(offset + 2).slice(0, 16).toString('hex').match(/.{1,2}/g).reverse().join('')
        });
      }
    }

    if (opcode !== ATT_OP_FIND_INFO_RESP || descriptors[descriptors.length - 1].handle === characteristic.endHandle) {
      const descriptorUuids = [];
      for (i = 0; i < descriptors.length; i++) {
        descriptorUuids.push(descriptors[i].uuid);

        this._descriptors[serviceUuid][characteristicUuid][descriptors[i].uuid] = descriptors[i];
      }

      this.emit('descriptorsDiscover', this._address, serviceUuid, characteristicUuid, descriptorUuids);
    } else {
      this._queueCommand(this.findInfoRequest(descriptors[descriptors.length - 1].handle + 1, characteristic.endHandle), callback);
    }
  };

  this._queueCommand(this.findInfoRequest(characteristic.valueHandle + 1, characteristic.endHandle), callback);
};

Gatt.prototype.readValue = function (serviceUuid, characteristicUuid, descriptorUuid) {
  const descriptor = this._descriptors[serviceUuid][characteristicUuid][descriptorUuid];

  let readData = Buffer.alloc(0);

  const callback = (data) => {
    const opcode = data[0];
    if (opcode === ATT_OP_READ_RESP || opcode === ATT_OP_READ_BLOB_RESP) {
      readData = Buffer.from(`${readData.toString('hex')}${data.slice(1).toString('hex')}`, 'hex');
      if (data.length === this._mtu) {
        this._queueCommand(this.readBlobRequest(descriptor.handle, readData.length), callback);
      } else {
        this.emit('valueRead', this._address, serviceUuid, characteristicUuid, descriptorUuid, readData);
      }
    } else {
      this.emit('valueRead', this._address, serviceUuid, characteristicUuid, descriptorUuid, readData);
    }
  };

  this._queueCommand(this.readRequest(descriptor.handle), callback);
};

Gatt.prototype.writeValue = function (serviceUuid, characteristicUuid, descriptorUuid, data) {
  const descriptor = this._descriptors[serviceUuid][characteristicUuid][descriptorUuid];

  this._queueCommand(this.writeRequest(descriptor.handle, data, false), data => {
    const opcode = data[0];

    if (opcode === ATT_OP_WRITE_RESP) {
      this.emit('valueWrite', this._address, serviceUuid, characteristicUuid, descriptorUuid);
    }
  });
};

Gatt.prototype.readHandle = function (handle) {
  let readData = Buffer.alloc(0);

  const callback = (data) => {
    const opcode = data[0];
    if (opcode === ATT_OP_READ_RESP || opcode === ATT_OP_READ_BLOB_RESP) {
      readData = Buffer.from(`${readData.toString('hex')}${data.slice(1).toString('hex')}`, 'hex');
      if (data.length === this._mtu) {
        this._queueCommand(this.readBlobRequest(handle, readData.length), callback);
      } else {
        this.emit('handleRead', this._address, handle, readData);
      }
    } else {
      this.emit('handleRead', this._address, handle, readData);
    }
  };

  this._queueCommand(this.readRequest(handle), callback);
};

Gatt.prototype.writeHandle = function (handle, data, withoutResponse) {
  if (withoutResponse) {
    this._queueCommand(this.writeRequest(handle, data, true), null, () => {
      this.emit('handleWrite', this._address, handle);
    });
  } else {
    this._queueCommand(this.writeRequest(handle, data, false), data => {
      const opcode = data[0];

      if (opcode === ATT_OP_WRITE_RESP) {
        this.emit('handleWrite', this._address, handle);
      }
    });
  }
};

module.exports = Gatt;