From 5d92368e5b10fb2d01ae25a2730e15cd05a84da6 Mon Sep 17 00:00:00 2001
From: Ben West <bewest@gmail.com>
Date: Sat, 24 May 2014 16:42:13 -0700
Subject: prep for installable module

---
 dexcom_reader/__init__.py         |   0
 dexcom_reader/constants.py        |  94 ++++++++++++++
 dexcom_reader/crc16.py            |  37 ++++++
 dexcom_reader/database_records.py | 189 ++++++++++++++++++++++++++++
 dexcom_reader/packetwriter.py     |  49 ++++++++
 dexcom_reader/readdata.py         | 259 ++++++++++++++++++++++++++++++++++++++
 dexcom_reader/util.py             |  82 ++++++++++++
 7 files changed, 710 insertions(+)
 create mode 100644 dexcom_reader/__init__.py
 create mode 100644 dexcom_reader/constants.py
 create mode 100644 dexcom_reader/crc16.py
 create mode 100644 dexcom_reader/database_records.py
 create mode 100644 dexcom_reader/packetwriter.py
 create mode 100644 dexcom_reader/readdata.py
 create mode 100644 dexcom_reader/util.py

(limited to 'dexcom_reader')

diff --git a/dexcom_reader/__init__.py b/dexcom_reader/__init__.py
new file mode 100644
index 0000000..e69de29
diff --git a/dexcom_reader/constants.py b/dexcom_reader/constants.py
new file mode 100644
index 0000000..2f60020
--- /dev/null
+++ b/dexcom_reader/constants.py
@@ -0,0 +1,94 @@
+import datetime
+
+
+class Error(Exception):
+  """Base error for dexcom reader."""
+
+
+class CrcError(Error):
+  """Failed to CRC properly."""
+
+
+DEXCOM_G4_USB_VENDOR = 0x22a3
+DEXCOM_G4_USB_PRODUCT = 0x0047
+
+BASE_TIME = datetime.datetime(2009, 1, 1)
+
+NULL = 0
+ACK = 1
+NAK = 2
+INVALID_COMMAND = 3
+INVALID_PARAM = 4
+INCOMPLETE_PACKET_RECEIVED = 5
+RECEIVER_ERROR = 6
+INVALID_MODE = 7
+PING = 10
+READ_FIRMWARE_HEADER = 11
+READ_DATABASE_PARTITION_INFO = 15
+READ_DATABASE_PAGE_RANGE = 16
+READ_DATABASE_PAGES = 17
+READ_DATABASE_PAGE_HEADER = 18
+READ_TRANSMITTER_ID = 25
+WRITE_TRANSMITTER_ID = 26
+READ_LANGUAGE = 27
+WRITE_LANGUAGE = 28
+READ_DISPLAY_TIME_OFFSET = 29
+WRITE_DISPLAY_TIME_OFFSET = 30
+READ_RTC = 31
+RESET_RECEIVER = 32
+READ_BATTERY_LEVEL = 33
+READ_SYSTEM_TIME = 34
+READ_SYSTEM_TIME_OFFSET = 35
+WRITE_SYSTEM_TIME = 36
+READ_GLUCOSE_UNIT = 37
+WRITE_GLUCOSE_UNIT = 38
+READ_BLINDED_MODE = 39
+WRITE_BLINDED_MODE = 40
+READ_CLOCK_MODE = 41
+WRITE_CLOCK_MODE = 42
+READ_DEVICE_MODE = 43
+ERASE_DATABASE = 45
+SHUTDOWN_RECEIVER = 46
+WRITE_PC_PARAMETERS = 47
+READ_BATTERY_STATE = 48
+READ_HARDWARE_BOARD_ID = 49
+READ_FIRMWARE_SETTINGS = 54
+READ_ENABLE_SETUP_WIZARD_FLAG = 55
+READ_SETUP_WIZARD_STATE = 57
+MAX_COMMAND = 59
+MAX_POSSIBLE_COMMAND = 255
+
+EGV_VALUE_MASK = 1023
+EGV_DISPLAY_ONLY_MASK = 32768
+EGV_TREND_ARROW_MASK = 15
+
+BATTERY_STATES = [None, 'CHARGING', 'NOT_CHARGING', 'NTC_FAULT', 'BAD_BATTERY']
+
+RECORD_TYPES = [
+  'MANUFACTURING_DATA', 'FIRMWARE_PARAMETER_DATA', 'PC_SOFTWARE_PARAMETER',
+  'SENSOR_DATA', 'EGV_DATA', 'CAL_SET', 'DEVIATION', 'INSERTION_TIME',
+  'RECEIVER_LOG_DATA', 'RECEIVER_ERROR_DATA', 'METER_DATA', 'USER_EVENT_DATA',
+  'USER_SETTING_DATA', 'MAX_VALUE',
+]
+
+TREND_ARROW_VALUES = [None, 'DOUBLE_UP', 'SINGLE_UP', '45_UP', 'FLAT',
+                      '45_DOWN', 'SINGLE_DOWN', 'DOUBLE_DOWN', 'NOT_COMPUTABLE',
+                      'OUT_OF_RANGE']
+
+SPECIAL_GLUCOSE_VALUES = {0: None,
+                          1: 'SENSOR_NOT_ACTIVE',
+                          2: 'MINIMAL_DEVIATION',
+                          3: 'NO_ANTENNA',
+                          5: 'SENSOR_NOT_CALIBRATED',
+                          6: 'COUNTS_DEVIATION',
+                          9: 'ABSOLUTE_DEVIATION',
+                          10: 'POWER_DEVIATION',
+                          12: 'BAD_RF'}
+
+
+LANGUAGES = {
+  0: None,
+  1033: 'ENGLISH',
+}
+
+
diff --git a/dexcom_reader/crc16.py b/dexcom_reader/crc16.py
new file mode 100644
index 0000000..0a55332
--- /dev/null
+++ b/dexcom_reader/crc16.py
@@ -0,0 +1,37 @@
+TABLE = [
+  0, 4129, 8258, 12387, 16516, 20645, 24774, 28903, 33032, 37161, 41290, 
+  45419, 49548, 53677, 57806, 61935, 4657, 528, 12915, 8786, 21173, 17044, 
+  29431, 25302, 37689, 33560, 45947, 41818, 54205, 50076, 62463, 58334, 9314, 
+  13379, 1056, 5121, 25830, 29895, 17572, 21637, 42346, 46411, 34088, 38153, 
+  58862, 62927, 50604, 54669, 13907, 9842, 5649, 1584, 30423, 26358, 22165, 
+  18100, 46939, 42874, 38681, 34616, 63455, 59390, 55197, 51132, 18628, 22757, 
+  26758, 30887, 2112, 6241, 10242, 14371, 51660, 55789, 59790, 63919, 35144, 
+  39273, 43274, 47403, 23285, 19156, 31415, 27286, 6769, 2640, 14899, 10770, 
+  56317, 52188, 64447, 60318, 39801, 35672, 47931, 43802, 27814, 31879, 19684, 
+  23749, 11298, 15363, 3168, 7233, 60846, 64911, 52716, 56781, 44330, 48395, 
+  36200, 40265, 32407, 28342, 24277, 20212, 15891, 11826, 7761, 3696, 65439, 
+  61374, 57309, 53244, 48923, 44858, 40793, 36728, 37256, 33193, 45514, 41451, 
+  53516, 49453, 61774, 57711, 4224, 161, 12482, 8419, 20484, 16421, 28742, 
+  24679, 33721, 37784, 41979, 46042, 49981, 54044, 58239, 62302, 689, 4752, 
+  8947, 13010, 16949, 21012, 25207, 29270, 46570, 42443, 38312, 34185, 62830, 
+  58703, 54572, 50445, 13538, 9411, 5280, 1153, 29798, 25671, 21540, 17413, 
+  42971, 47098, 34713, 38840, 59231, 63358, 50973, 55100, 9939, 14066, 1681, 
+  5808, 26199, 30326, 17941, 22068, 55628, 51565, 63758, 59695, 39368, 35305, 
+  47498, 43435, 22596, 18533, 30726, 26663, 6336, 2273, 14466, 10403, 52093, 
+  56156, 60223, 64286, 35833, 39896, 43963, 48026, 19061, 23124, 27191, 31254, 
+  2801, 6864, 10931, 14994, 64814, 60687, 56684, 52557, 48554, 44427, 40424, 
+  36297, 31782, 27655, 23652, 19525, 15522, 11395, 7392, 3265, 61215, 65342, 
+  53085, 57212, 44955, 49082, 36825, 40952, 28183, 32310, 20053, 24180, 11923, 
+  16050, 3793, 7920
+]
+
+
+def crc16(buf, start=None, end=None):
+  if start is None:
+    start = 0
+  if end is None:
+    end = len(buf)
+  num = 0
+  for i in range(start, end):
+    num = ((num<<8)&0xff00) ^ TABLE[((num>>8)&0xff)^ord(buf[i])]
+  return num & 0xffff
diff --git a/dexcom_reader/database_records.py b/dexcom_reader/database_records.py
new file mode 100644
index 0000000..bde0c67
--- /dev/null
+++ b/dexcom_reader/database_records.py
@@ -0,0 +1,189 @@
+import crc16
+import constants
+import struct
+import util
+
+
+class BaseDatabaseRecord(object):
+  FORMAT = None
+
+  @classmethod
+  def _CheckFormat(cls):
+    if cls.FORMAT is None or not cls.FORMAT:
+      raise NotImplementedError("Subclasses of %s need to define FORMAT"
+                                % cls.__name__)
+
+  @classmethod
+  def _ClassFormat(cls):
+    cls._CheckFormat()
+    return struct.Struct(cls.FORMAT)
+
+  @classmethod
+  def _ClassSize(cls):
+    return cls._ClassFormat().size
+
+  @property
+  def FMT(self):
+    self._CheckFormat()
+    return _ClassFormat()
+
+  @property
+  def SIZE(self):
+    return self._ClassSize()
+
+  @property
+  def crc(self):
+    return self.data[-1]
+
+  def __init__(self, data, raw_data):
+    self.raw_data = raw_data
+    self.data = data
+    self.check_crc()
+
+  def check_crc(self):
+    local_crc = self.calculate_crc()
+    if local_crc != self.crc:
+      raise constants.CrcError('Could not parse %s' % self.__class__.__name__)
+
+  def dump(self):
+    return ''.join('\\x%02x' % ord(c) for c in self.raw_data)
+
+  def calculate_crc(self):
+    return crc16.crc16(self.raw_data[:-2])
+
+  @classmethod
+  def Create(cls, data, record_counter):
+    offset = record_counter * cls._ClassSize()
+    raw_data = data[offset:offset + cls._ClassSize()]
+    unpacked_data = cls._ClassFormat().unpack(raw_data)
+    return cls(unpacked_data, raw_data)
+
+
+class GenericTimestampedRecord(BaseDatabaseRecord):
+  @property
+  def system_time(self):
+    return util.ReceiverTimeToTime(self.data[0])
+
+  @property
+  def display_time(self):
+    return util.ReceiverTimeToTime(self.data[1])
+
+
+class GenericXMLRecord(GenericTimestampedRecord):
+  FORMAT = '<II490sH'
+
+  @property
+  def xmldata(self):
+    data = self.data[2].replace("\x00", "")
+    return data
+
+
+class InsertionRecord(GenericTimestampedRecord):
+  FORMAT = '<3IcH'
+
+  @property
+  def insertion_time(self):
+    return util.ReceiverTimeToTime(self.data[2])
+
+  @property
+  def session_state(self):
+    states = [None, 'REMOVED', 'EXPIRED', 'RESIDUAL_DEVIATION',
+              'COUNTS_DEVIATION', 'SECOND_SESSION', 'OFF_TIME_LOSS',
+              'STARTED', 'BAD_TRANSMITTER', 'MANUFACTURING_MODE']
+    return states[ord(self.data[3])]
+
+  def __repr__(self):
+    return '%s:  state=%s' % (self.display_time, self.session_state)
+
+
+class MeterRecord(GenericTimestampedRecord):
+  FORMAT = '<2IHIH'
+
+  @property
+  def meter_glucose(self):
+    return self.data[2]
+
+  @property
+  def meter_time(self):
+    return util.ReceiverTimeToTime(self.data[3])
+
+  def __repr__(self):
+    return '%s: Meter BG:%s' % (self.display_time, self.meter_glucose)
+
+
+class EventRecord(GenericTimestampedRecord):
+  # sys_time,display_time,glucose,meter_time,crc
+  FORMAT = '<2I2c2IH'
+
+  @property
+  def event_type(self):
+    event_types = [None, 'CARBS', 'INSULIN', 'HEALTH', 'EXCERCISE',
+                    'MAX_VALUE']
+    return event_types[ord(self.data[2])]
+
+  @property
+  def event_sub_type(self):
+    subtypes = {'HEALTH': [None, 'ILLNESS', 'STRESS', 'HIGH_SYMPTOMS',
+                            'LOW_SYMTOMS', 'CYCLE', 'ALCOHOL'],
+                'EXCERCISE': [None, 'LIGHT', 'MEDIUM', 'HEAVY',
+                              'MAX_VALUE']}
+    if self.event_type in subtypes:
+      return subtypes[self.event_type][ord(self.data[3])]
+
+  @property
+  def display_time(self):
+    return util.ReceiverTimeToTime(self.data[4])
+
+  @property
+  def event_value(self):
+    value = self.data[5]
+    if self.event_type == 'INSULIN':
+      value = value / 100.0
+    return value
+
+  def __repr__(self):
+    return '%s:  event_type=%s sub_type=%s value=%s' % (self.display_time, self.event_type,
+                                    self.event_sub_type, self.event_value)
+
+
+class EGVRecord(GenericTimestampedRecord):
+  # uint, uint, ushort, byte, ushort
+  # (system_seconds, display_seconds, glucose, trend_arrow, crc)
+  FORMAT = '<2IHcH'
+
+  @property
+  def full_glucose(self):
+    return self.data[2]
+
+  @property
+  def full_trend(self):
+    return self.data[3]
+
+  @property
+  def display_only(self):
+    return bool(self.full_glucose & constants.EGV_DISPLAY_ONLY_MASK)
+
+  @property
+  def glucose(self):
+    return self.full_glucose & constants.EGV_VALUE_MASK
+
+  @property
+  def glucose_special_meaning(self):
+    if self.glucose in constants.SPECIAL_GLUCOSE_VALUES:
+      return constants.SPECIAL_GLUCOSE_VALUES[self.glucose]
+
+  @property
+  def is_special(self):
+    return self.glucose_special_meaning is not None
+
+  @property
+  def trend_arrow(self):
+    arrow_value = ord(self.full_trend) & constants.EGV_TREND_ARROW_MASK
+    return constants.TREND_ARROW_VALUES[arrow_value]
+
+  def __repr__(self):
+    if self.is_special:
+      return '%s: %s' % (self.display_time, self.glucose_special_meaning)
+    else:
+      return '%s: CGM BG:%s (%s) DO:%s' % (self.display_time, self.glucose,
+                                           self.trend_arrow, self.display_only)
diff --git a/dexcom_reader/packetwriter.py b/dexcom_reader/packetwriter.py
new file mode 100644
index 0000000..6e03342
--- /dev/null
+++ b/dexcom_reader/packetwriter.py
@@ -0,0 +1,49 @@
+import crc16
+import struct
+
+class PacketWriter(object):
+  MAX_PAYLOAD = 1584
+  MIN_LEN = 6
+  MAX_LEN = 1590
+  SOF = 0x01
+  OFFSET_SOF = 0
+  OFFSET_LENGTH = 1
+  OFFSET_CMD = 3
+  OFFSET_PAYLOAD = 4
+
+  def __init__(self):
+    self._packet = None
+
+  def Clear(self):
+    self._packet = None
+
+  def NewSOF(self, v):
+    self._packet[0] = chr(v)
+
+  def PacketString(self):
+    return ''.join(self._packet)
+ 
+  def AppendCrc(self):
+    self.SetLength()
+    ps = self.PacketString()
+    crc = crc16.crc16(ps, 0, len(ps))
+    for x in struct.pack('H', crc):
+      self._packet.append(x)
+
+  def SetLength(self):
+    self._packet[1] = chr(len(self._packet) + 2)
+
+  def _Add(self, x):
+    try:
+      len(x)
+      for y in x:
+        self._Add(y)
+    except:
+      self._packet.append(x)
+
+  def ComposePacket(self, command, payload=None):
+    assert self._packet is None
+    self._packet = ["\x01", None, "\x00", chr(command)]
+    if payload:
+      self._Add(payload)
+    self.AppendCrc()
diff --git a/dexcom_reader/readdata.py b/dexcom_reader/readdata.py
new file mode 100644
index 0000000..f398407
--- /dev/null
+++ b/dexcom_reader/readdata.py
@@ -0,0 +1,259 @@
+import crc16
+import constants
+import database_records
+import datetime
+import serial
+import sys
+import time
+import packetwriter
+import struct
+import re
+import util
+import xml.etree.ElementTree as ET
+import numpy
+import platform
+
+
+class ReadPacket(object):
+  def __init__(self, command, data):
+    self._command = command
+    self._data = data
+
+  @property
+  def command(self):
+    return self._command
+
+  @property
+  def data(self):
+    return self._data
+
+
+class Dexcom(object):
+  @staticmethod
+  def FindDevice():
+    return util.find_usbserial(constants.DEXCOM_G4_USB_VENDOR,
+                               constants.DEXCOM_G4_USB_PRODUCT)
+
+  @classmethod
+  def LocateAndDownload(cls):
+    device = cls.FindDevice()
+    if not device:
+      sys.stderr.write('Could not find Dexcom G4 Receiver!\n')
+      sys.exit(1)
+    else:
+      dex = cls(device)
+      print ('Found %s S/N: %s'
+             % (dex.GetFirmwareHeader().get('ProductName'),
+                dex.ReadManufacturingData().get('SerialNumber')))
+      print 'Transmitter paired: %s' % dex.ReadTransmitterId()
+      print 'Battery Status: %s (%d%%)' % (dex.ReadBatteryState(),
+                                           dex.ReadBatteryLevel())
+      print 'Record count:'
+      print '- Meter records: %d' % (len(dex.ReadRecords('METER_DATA')))
+      print '- CGM records: %d' % (len(dex.ReadRecords('EGV_DATA')))
+      print ('- CGM commitable records: %d'
+             % (len([not x.display_only for x in dex.ReadRecords('EGV_DATA')])))
+      print '- Event records: %d' % (len(dex.ReadRecords('USER_EVENT_DATA')))
+      print '- Insertion records: %d' % (len(dex.ReadRecords('INSERTION_TIME')))
+
+  def __init__(self, port):
+    self._port_name = port
+    self._port = None
+
+  def Connect(self):
+    if self._port is None:
+      self._port = serial.Serial(port=self._port_name, baudrate=115200)
+
+  def Disconnect(self):
+    if self._port is not None:
+      self._port.close()
+
+  @property
+  def port(self):
+    if self._port is None:
+      self.Connect()
+    return self._port
+
+  def write(self, *args, **kwargs):
+    return self.port.write(*args, **kwargs)
+
+  def read(self, *args, **kwargs):
+    return self.port.read(*args, **kwargs)
+
+  def readpacket(self, timeout=None):
+    total_read = 4
+    initial_read = self.read(total_read)
+    all_data = initial_read
+    if ord(initial_read[0]) == 1:
+      command = initial_read[3]
+      data_number = struct.unpack('<H', initial_read[1:3])[0]
+      if data_number > 6:
+        toread = abs(data_number-6)
+        second_read = self.read(toread)
+        all_data += second_read
+        total_read += toread
+        out = second_read
+      else:
+        out =  ''
+      suffix = self.read(2)
+      sent_crc = struct.unpack('<H', suffix)[0]
+      local_crc = crc16.crc16(all_data, 0, total_read)
+      if sent_crc != local_crc:
+        raise constants.CrcError("readpacket Failed CRC check")
+      num1 = total_read + 2
+      return ReadPacket(command, out)
+    else:
+      raise constants.Error('Error reading packet header!')
+
+  def Ping(self):
+    self.WriteCommand(constants.PING)
+    packet = self.readpacket()
+    return ord(packet.command) == constants.ACK
+
+  def WritePacket(self, packet):
+    if not packet:
+      raise constants.Error('Need a packet to send')
+    packetlen = len(packet)
+    if packetlen < 6 or packetlen > 1590:
+      raise constants.Error('Invalid packet length')
+    self.flush()
+    self.write(packet)
+
+  def WriteCommand(self, command_id, *args, **kwargs):
+    p = packetwriter.PacketWriter()
+    p.ComposePacket(command_id, *args, **kwargs)
+    self.WritePacket(p.PacketString())
+
+  def GenericReadCommand(self, command_id):
+    self.WriteCommand(command_id)
+    return self.readpacket()
+
+  def ReadTransmitterId(self):
+    return self.GenericReadCommand(constants.READ_TRANSMITTER_ID).data
+
+  def ReadLanguage(self):
+    lang = self.GenericReadCommand(constants.READ_LANGUAGE).data
+    return constants.LANGUAGES[struct.unpack('H', lang)[0]]
+
+  def ReadBatteryLevel(self):
+    level = self.GenericReadCommand(constants.READ_BATTERY_LEVEL).data
+    return struct.unpack('I', level)[0]
+
+  def ReadBatteryState(self):
+    state = self.GenericReadCommand(constants.READ_BATTERY_STATE).data
+    return constants.BATTERY_STATES[ord(state)]
+
+  def ReadRTC(self):
+    rtc = self.GenericReadCommand(constants.READ_RTC).data
+    return util.ReceiverTimeToTime(struct.unpack('I', rtc)[0])
+
+  def ReadSystemTime(self):
+    rtc = self.GenericReadCommand(constants.READ_SYSTEM_TIME).data
+    return util.ReceiverTimeToTime(struct.unpack('I', rtc)[0])
+
+  def ReadSystemTimeOffset(self):
+    rtc = self.GenericReadCommand(constants.READ_SYSTEM_TIME_OFFSET).data
+    return datetime.timedelta(seconds=struct.unpack('i', rtc)[0])
+
+  def ReadDisplayTimeOffset(self):
+    rtc = self.GenericReadCommand(constants.READ_DISPLAY_TIME_OFFSET).data
+    return datetime.timedelta(seconds=struct.unpack('i', rtc)[0])
+
+  def ReadDisplayTime(self):
+    return self.ReadSystemTime() + self.ReadDisplayTimeOffset()
+
+  def ReadGlucoseUnit(self):
+    UNIT_TYPE = (None, 'mg/dL', 'mmol/L')
+    gu = self.GenericReadCommand(constants.READ_GLUCOSE_UNIT).data
+    return UNIT_TYPE[ord(gu[0])]
+
+  def ReadClockMode(self):
+    CLOCK_MODE = (24, 12)
+    cm = self.GenericReadCommand(constants.READ_CLOCK_MODE).data
+    return CLOCK_MODE[ord(cm[0])]
+
+  def ReadDeviceMode(self):
+    return self.GenericReadCommand(constants.READ_DEVICE_MODE).data
+
+  def ReadManufacturingData(self):
+    data = self.ReadRecords('MANUFACTURING_DATA')[0].xmldata
+    return ET.fromstring(data)
+
+  def flush(self):
+    self.port.flush()
+
+  def clear(self):
+    self.port.flushInput()
+    self.port.flushOutput()
+
+  def GetFirmwareHeader(self):
+    i = self.GenericReadCommand(constants.READ_FIRMWARE_HEADER)
+    return ET.fromstring(i.data)
+
+  def GetFirmwareSettings(self):
+    i = self.GenericReadCommand(constants.READ_FIRMWARE_SETTINGS)
+    return ET.fromstring(i.data)
+
+  def DataPartitions(self):
+    i = self.GenericReadCommand(constants.READ_DATABASE_PARTITION_INFO)
+    return ET.fromstring(i.data)
+
+  def ReadDatabasePageRange(self, record_type):
+    record_type_index = constants.RECORD_TYPES.index(record_type)
+    self.WriteCommand(constants.READ_DATABASE_PAGE_RANGE,
+                      chr(record_type_index))
+    packet = self.readpacket()
+    return struct.unpack('II', packet.data)
+
+  def ReadDatabasePage(self, record_type, page):
+    record_type_index = constants.RECORD_TYPES.index(record_type)
+    self.WriteCommand(constants.READ_DATABASE_PAGES,
+                      (chr(record_type_index), struct.pack('I', page), chr(1)))
+    packet = self.readpacket()
+    assert ord(packet.command) == 1
+    # first index (uint), numrec (uint), record_type (byte), revision (byte),
+    # page# (uint), r1 (uint), r2 (uint), r3 (uint), ushort (Crc)
+    header_format = '<2I2c4IH'
+    header_data_len = struct.calcsize(header_format)
+    header = struct.unpack_from(header_format, packet.data)
+    header_crc = crc16.crc16(packet.data[:header_data_len-2])
+    assert header_crc == header[-1]
+    assert ord(header[2]) == record_type_index
+    assert header[4] == page
+    packet_data = packet.data[header_data_len:]
+
+    return self.ParsePage(header, packet_data)
+
+  def GenericRecordYielder(self, header, data, record_type):
+    for x in range(header[1]):
+      yield record_type.Create(data, x)
+
+  def ParsePage(self, header, data):
+    record_type = constants.RECORD_TYPES[ord(header[2])]
+    generic_parser_map = {
+      'USER_EVENT_DATA': database_records.EventRecord,
+      'METER_DATA': database_records.MeterRecord,
+      'INSERTION_TIME': database_records.InsertionRecord,
+      'EGV_DATA': database_records.EGVRecord,
+    }
+    xml_parsed = ['PC_SOFTWARE_PARAMETER', 'MANUFACTURING_DATA']
+    if record_type in generic_parser_map:
+      return self.GenericRecordYielder(header, data,
+                                       generic_parser_map[record_type])
+    elif record_type in xml_parsed:
+      return [database_records.GenericXMLRecord.Create(data, 0)]
+    else:
+      raise NotImplementedError('Parsing of %s has not yet been implemented'
+                                % record_type)
+
+  def ReadRecords(self, record_type):
+    records = []
+    assert record_type in constants.RECORD_TYPES
+    page_range = self.ReadDatabasePageRange(record_type)
+    for x in range(page_range[0], page_range[1] or 1):
+      records.extend(self.ReadDatabasePage(record_type, x))
+    return records
+
+
+if __name__ == '__main__':
+  Dexcom.LocateAndDownload()
diff --git a/dexcom_reader/util.py b/dexcom_reader/util.py
new file mode 100644
index 0000000..c719092
--- /dev/null
+++ b/dexcom_reader/util.py
@@ -0,0 +1,82 @@
+import constants
+import datetime
+import os
+import platform
+import plistlib
+import re
+import subprocess
+
+
+def ReceiverTimeToTime(rtime):
+  return constants.BASE_TIME + datetime.timedelta(seconds=rtime)
+
+
+def linux_find_usbserial(vendor, product):
+  DEV_REGEX = re.compile('^tty(USB|ACM)[0-9]+$')
+  for usb_dev_root in os.listdir('/sys/bus/usb/devices'):
+    device_name = os.path.join('/sys/bus/usb/devices', usb_dev_root)
+    if not os.path.exists(os.path.join(device_name, 'idVendor')):
+      continue
+    idv = open(os.path.join(device_name, 'idVendor')).read().strip()
+    if idv != vendor:
+      continue
+    idp = open(os.path.join(device_name, 'idProduct')).read().strip()
+    if idp != product:
+      continue
+    for root, dirs, files in os.walk(device_name):
+      for option in dirs + files:
+        if DEV_REGEX.match(option):
+          return os.path.join('/dev', option)
+
+
+def osx_find_usbserial(vendor, product):
+  def recur(v):
+    if hasattr(v, '__iter__') and 'idVendor' in v and 'idProduct' in v:
+      if v['idVendor'] == vendor and v['idProduct'] == product:
+        tmp = v
+        while True:
+          if 'IODialinDevice' not in tmp and 'IORegistryEntryChildren' in tmp:
+            tmp = tmp['IORegistryEntryChildren']
+          elif 'IODialinDevice' in tmp:
+            return tmp['IODialinDevice']
+          else:
+            break
+
+    if type(v) == list:
+      for x in v:
+        out = recur(x)
+        if out is not None:
+          return out
+    elif type(v) == dict or issubclass(type(v), dict):
+      for x in v.values():
+        out = recur(x)
+        if out is not None:
+          return out
+
+  sp = subprocess.Popen(['/usr/sbin/ioreg', '-k', 'IODialinDevice',
+                         '-r', '-t', '-l', '-a', '-x'],
+                        stdout=subprocess.PIPE,
+                        stdin=subprocess.PIPE, stderr=subprocess.PIPE)
+  stdout, _ = sp.communicate()
+  plist = plistlib.readPlistFromString(stdout)
+  return recur(plist)
+
+
+def find_usbserial(vendor, product):
+  """Find the tty device for a given usbserial devices identifiers.
+
+  Args:
+     vendor: (int) something like 0x0000
+     product: (int) something like 0x0000
+
+  Returns:
+     String, like /dev/ttyACM0 or /dev/tty.usb...
+  """
+  if platform.system() == 'Linux':
+    vendor, product = [('%04x' % (x)).strip() for x in (vendor, product)]
+    return linux_find_usbserial(vendor, product)
+  elif platform.system() == 'Darwin':
+    return osx_find_usbserial(vendor, product)
+  else:
+    raise NotImplementedError('Cannot find serial ports on %s'
+                              % platform.system())
-- 
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