From 91fb2d40f286c1f3b0af9146d66d424f4e8c5c6d Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Diego=20Elio=20Petten=C3=B2?= <flameeyes@flameeyes.eu>
Date: Sat, 3 Aug 2013 22:18:11 +0100
Subject: Implement the correct checksum algorithm.

Unlike what I though, OneTouch2 does not use CRC32 Sick algorithm, but
a very stupid full 16-bit modulo sum of the bytes in the reply.

This is very simple and does not require GPLv3 code at all.
---
 glucometerutils/drivers/lifescan_common.py | 38 +++++++-----------------------
 test/test_lifescan.py                      |  7 +++---
 test/test_otultra2.py                      |  2 +-
 3 files changed, 13 insertions(+), 34 deletions(-)

diff --git a/glucometerutils/drivers/lifescan_common.py b/glucometerutils/drivers/lifescan_common.py
index ac25d22..5b42c33 100644
--- a/glucometerutils/drivers/lifescan_common.py
+++ b/glucometerutils/drivers/lifescan_common.py
@@ -6,8 +6,6 @@ __email__ = 'flameeyes@flameeyes.eu'
 __copyright__ = 'Copyright © 2013, Diego Elio Pettenò'
 __license__ = 'GPL v3 or later'
 
-import ctypes
-
 from glucometerutils import exceptions
 
 
@@ -31,40 +29,20 @@ class InvalidSerialNumber(exceptions.Error):
 
 
 def calculate_checksum(bytestring):
-  """Calculate the "CRC16 Sick" style checksum for LifeScan protocols.
+  """Calculate the checksum used by OneTouch Ultra and Ultra2 devices
 
   Args:
     bytestring: the string of which the checksum has to be calculated.
 
   Returns:
-    A 16-bit integer that is the checksum for the input.
+    A string with the hexdecimal representation of the checksum for the input.
 
-  Credits for this code go to Christian Navalici, who implemented it in his
-  library at https://github.com/cristianav/PyCRC/ .
+  The checksum is a very stupid one: it just sums all the bytes,
+  modulo 16-bit, without any parity.
   """
-  crcValue = 0x0000
-  prev_c = 0x0000
-
-  for idx, c in enumerate(bytestring):
-    short_c  =  0x00ff & c
-
-    idx_previous = idx - 1
-    short_p  = ( 0x00ff & prev_c) << 8;
-
-    if ( crcValue & 0x8000 ):
-      crcValue = ctypes.c_ushort(crcValue << 1).value ^ 0x8005
-    else:
-      crcValue = ctypes.c_ushort(crcValue << 1).value
-
-    crcValue &= 0xffff
-    crcValue ^= ( short_c | short_p )
-
-    prev_c = short_c
+  checksum = 0
 
-  # After processing, the one's complement of the CRC is calcluated and the
-  # two bytes of the CRC are swapped.
-  low_byte   = (crcValue & 0xff00) >> 8
-  high_byte  = (crcValue & 0x00ff) << 8
-  crcValue   = low_byte | high_byte;
+  for byte in bytestring:
+    checksum = (checksum + byte) & 0xffff
 
-  return crcValue
+  return checksum
diff --git a/test/test_lifescan.py b/test/test_lifescan.py
index 774567d..a7446e0 100644
--- a/test/test_lifescan.py
+++ b/test/test_lifescan.py
@@ -20,10 +20,11 @@ from glucometerutils import exceptions
 class TestOTUltra2(unittest.TestCase):
   def testChecksum(self):
     checksum = lifescan_common.calculate_checksum(bytes('T', 'ascii'))
-    self.assertEqual(0x5400, checksum)
+    self.assertEqual(0x0054, checksum)
 
-    checksum = lifescan_common.calculate_checksum(bytes('TestString', 'ascii'))
-    self.assertEqual(0x0643, checksum)
+    checksum = lifescan_common.calculate_checksum(
+      bytes('T "SAT","08/03/13","22:12:00   "', 'ascii'))
+    self.assertEqual(0x0608, checksum)
 
 if __name__ == '__main__':
     unittest.main()
diff --git a/test/test_otultra2.py b/test/test_otultra2.py
index 6ffea83..98fffc8 100644
--- a/test/test_otultra2.py
+++ b/test/test_otultra2.py
@@ -48,7 +48,7 @@ class TestOTUltra2(unittest.TestCase):
 
   def testInvalidSerialNumber(self):
     self.mock_readline.return_value = bytes(
-      '@ "12345678O" E105\r', 'ascii')
+      '@ "12345678O" 0297\r', 'ascii')
 
     self.assertRaises(lifescan_common.InvalidSerialNumber,
                       self.device.get_serial_number)
-- 
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