3 files changed, 27 insertions, 16 deletions

diff --git a/glucometerutils/drivers/lifescan_common.py b/glucometerutils/drivers/lifescan_common.py
index 9a60ab6..f37903a 100644
--- a/glucometerutils/drivers/lifescan_common.py
+++ b/glucometerutils/drivers/lifescan_common.py
@@ -26,3 +26,27 @@ class InvalidSerialNumber(exceptions.Error):
   """The serial number is not as expected."""
   def __init__(self, serial_number):
     self.message = 'Serial number %s is invalid.' % serial_number
+
+
+def crc_ccitt(data):
+  """Calculate the CRC-16-CCITT with LifeScan's common seed.
+
+  Args:
+    data: (bytes) the data to calculate the checksum of
+
+  Returns:
+    (int) The 16-bit integer value of the CRC-CCITT calculated.
+
+  This function uses the non-default 0xFFFF seed as used by multiple
+  LifeScan meters.
+  """
+  crc = 0xffff
+
+  for byte in data:
+    crc = (crc >> 8) & 0xffff | (crc << 8) & 0xffff
+    crc ^= byte
+    crc ^= (crc & 0xff) >> 4
+    crc ^= (((crc << 8) & 0xffff) << 4) & 0xffff
+    crc ^= (crc & 0xff) << 5
+
+  return (crc & 0xffff)
diff --git a/glucometerutils/drivers/otultraeasy.py b/glucometerutils/drivers/otultraeasy.py
index 422a7c1..0028ff6 100644
--- a/glucometerutils/drivers/otultraeasy.py
+++ b/glucometerutils/drivers/otultraeasy.py
@@ -68,19 +68,6 @@ def _convert_timestamp(timestamp_bytes):
 class _Packet(object):
   _STRUCT = struct.Struct('<H')
 
-  @staticmethod
-  def _crc(cmd):
-    crc = 0xffff
-
-    for byte in cmd:
-      crc = (crc >> 8) & 0xffff | (crc << 8) & 0xffff
-      crc ^= byte
-      crc ^= (crc & 0xff) >> 4
-      crc ^= (((crc << 8) & 0xffff) << 4) & 0xffff
-      crc ^= (crc & 0xff) << 5
-
-    return (crc & 0xffff)
-
   def __init__(self):
     self.cmd = array.array('B')
 
@@ -148,7 +135,7 @@ class _Packet(object):
 
   @property
   def checksum(self):
-    return self._crc(self.cmd[:_IDX_CHECKSUM].tobytes())
+    return lifescan_common.crc_ccitt(self.cmd[:_IDX_CHECKSUM].tobytes())
 
   @property
   def acknowledge(self):
diff --git a/test/test_otultraeasy.py b/test/test_otultraeasy.py
index 326aeea..f263f12 100644
--- a/test/test_otultraeasy.py
+++ b/test/test_otultraeasy.py
@@ -31,12 +31,12 @@ class TestOTUltraMini(unittest.TestCase):
     def testCrc(self):
         self.assertEqual(
             0x41cd,
-            otultraeasy._Packet._crc(b'\x02\x06\x06\x03'))
+            lifescan_common.crc_ccitt(b'\x02\x06\x06\x03'))
 
         cmd_array = array.array('B', b'\x02\x06\x08\x03')
         self.assertEqual(
             0x62C2,
-            otultraeasy._Packet._crc(cmd_array))
+            lifescan_common.crc_ccitt(cmd_array))
 
     def testPacketUpdateChecksum(self):
         packet = otultraeasy._Packet()