From 1c060d27fa14428f802ac58d68d02ea0b7e342cf Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Diego=20Elio=20Petten=C3=B2?= <flameeyes@flameeyes.eu>
Date: Thu, 28 Dec 2017 20:16:22 +0000
Subject: freestyle: replace the custom struct and bytemangling with construct.

This simplifies the code and enforces the validation within the format
documentation too.

construct can handle the full package verification, including dealing with
padding.
---
 glucometerutils/support/freestyle.py | 19 +++++++++++--------
 1 file changed, 11 insertions(+), 8 deletions(-)

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)
 
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