3 files changed, 30 insertions, 26 deletions

diff --git a/README b/README
index a070a05..1f42487 100644
--- a/README
+++ b/README
@@ -32,21 +32,21 @@ $ . glucometerutils-venv/bin/activate
 Please see the following table for the driver for each device that is known and
 supported.
 
-| Manufacturer | Model Name              | Driver             | Dependencies  |
-| ---          | ---                     | ---                | ---           |
-| LifeScan     | OneTouch Ultra 2        | `otultra2`         | [pyserial]    |
-| LifeScan     | OneTouch Ultra Easy     | `otultraeasy`      | [pyserial]    |
-| LifeScan     | OneTouch Ultra Mini     | `otultraeasy`      | [pyserial]    |
-| LifeScan     | OneTouch Verio (USB)    | `otverio2015`      | [python-scsi] |
-| LifeScan     | OneTouch Select Plus    | `otverio2015`      | [python-scsi] |
-| Abbott       | FreeStyle InsuLinx†     | `fsinsulinx`       | [hidapi]‡     |
-| Abbott       | FreeStyle Libre         | `fslibre`          | [hidapi]‡     |
-| Abbott       | FreeStyle Optium        | `fsoptium`         | [pyserial]    |
-| Abbott       | FreeStyle Precision Neo | `fsprecisionneo`   | [hidapi]‡     |
-| Abbott       | FreeStyle Optium Neo    | `fsprecisionneo`   | [hidapi]‡     |
-| Abbott       | FreeStyle Optium Neo H  | `fsprecisionneo`   | [hidapi]‡     |
-| Roche        | Accu-Chek Mobile        | `accuchek_reports` |               |
-| SD Biosensor | SD CodeFree             | `sdcodefree`       | [pyserial]    |
+| Manufacturer | Model Name              | Driver             | Dependencies          |
+| ---          | ---                     | ---                | ---                   |
+| LifeScan     | OneTouch Ultra 2        | `otultra2`         | [pyserial]            |
+| LifeScan     | OneTouch Ultra Easy     | `otultraeasy`      | [pyserial]            |
+| LifeScan     | OneTouch Ultra Mini     | `otultraeasy`      | [pyserial]            |
+| LifeScan     | OneTouch Verio (USB)    | `otverio2015`      | [python-scsi]         |
+| LifeScan     | OneTouch Select Plus    | `otverio2015`      | [python-scsi]         |
+| Abbott       | FreeStyle InsuLinx†     | `fsinsulinx`       | [construct] [hidapi]‡ |
+| Abbott       | FreeStyle Libre         | `fslibre`          | [construct] [hidapi]‡ |
+| Abbott       | FreeStyle Optium        | `fsoptium`         | [pyserial]            |
+| Abbott       | FreeStyle Precision Neo | `fsprecisionneo`   | [construct] [hidapi]‡ |
+| Abbott       | FreeStyle Optium Neo    | `fsprecisionneo`   | [construct] [hidapi]‡ |
+| Abbott       | FreeStyle Optium Neo H  | `fsprecisionneo`   | [construct] [hidapi]‡ |
+| Roche        | Accu-Chek Mobile        | `accuchek_reports` |                       |
+| SD Biosensor | SD CodeFree             | `sdcodefree`       | [pyserial]            |
 
 † Untested.
 ‡ Optional dependency on Linux; required on other operating systems.
@@ -60,6 +60,7 @@ If you have knowledge of a protocol of a glucometer you would have supported,
 please provide a reference, possibly by writing a specification and contribute
 it to https://github.com/Flameeyes/glucometer-protocols/.
 
+[construct]: https://construct.readthedocs.io/en/latest/
 [pyserial]: https://pythonhosted.org/pyserial/
 [python-scsi]: https://github.com/rosjat/python-scsi
 [hidapi]: https://pypi.python.org/pypi/hidapi
diff --git a/glucometerutils/support/freestyle.py b/glucometerutils/support/freestyle.py
index d722c35..7610729 100644
--- a/glucometerutils/support/freestyle.py
+++ b/glucometerutils/support/freestyle.py
@@ -15,7 +15,8 @@ import csv
 import datetime
 import logging
 import re
-import struct
+
+import construct
 
 from glucometerutils import exceptions
 from glucometerutils.support import hiddevice
@@ -24,7 +25,13 @@ from glucometerutils.support import hiddevice
 # protocol.
 _INIT_SEQUENCE = (0x04, 0x05, 0x15, 0x01)
 
-_STRUCT_PREAMBLE = struct.Struct('<BB')
+_LIFESCAN_MESSAGE = construct.Struct(
+    'hid_report' / construct.Const(construct.Byte, 0),
+    'message_type' / construct.Byte,
+    'command' / construct.Padded(
+        63,  # command can only be up to 62 bytes, but one is used for length.
+        construct.Prefixed(construct.Byte, construct.GreedyBytes)),
+)
 
 _TEXT_COMPLETION_RE = re.compile('CMD (?:OK|Fail!)')
 _TEXT_REPLY_FORMAT = re.compile(
@@ -93,12 +100,8 @@ class FreeStyleHidDevice(hiddevice.HidDevice):
           message_type: (int) The first byte sent with the report to the device.
           command: (bytes) The command to send out the device.
         """
-        cmdlen = len(command)
-        assert cmdlen <= 62
-
-        # First byte in the written buffer is the report number.
-        usb_packet = b'\x00' + _STRUCT_PREAMBLE.pack(
-            message_type, cmdlen) + command + bytes(62 - cmdlen)
+        usb_packet = _LIFESCAN_MESSAGE.build(
+            {'message_type': message_type, 'command': command})
 
         self._write(usb_packet)
 
diff --git a/setup.py b/setup.py
index 5da8086..5e65e1e 100644
--- a/setup.py
+++ b/setup.py
@@ -34,10 +34,10 @@ setup(
         'otultra2': ['pyserial'],
         'otultraeasy': ['pyserial'],
         'otverio2015': ['python-scsi'],
-        'fsinsulinx': ['hidapi'],
-        'fslibre': ['hidapi'],
+        'fsinsulinx': ['construct', 'hidapi'],
+        'fslibre': ['construct', 'hidapi'],
         'fsoptium': ['pyserial'],
-        'fsprecisionneo': ['hidapi'],
+        'fsprecisionneo': ['construct', 'hidapi'],
         'accucheck_reports': [],
         'sdcodefree': ['pyserial'],
     },