Rewrote checksum calculation into class.

grmn/chksum.py

@@ -0,0 +1,34 @@
+# -*- coding: utf-8 -*-
+
+class ChkSum:
+
+    def __init__(self):
+        self.chksum = 0
+        self.chksum_without_last = 0
+        self.last_byte = 0xff
+
+    def add(self, data):
+        for c in data[:-1]:
+            self.chksum += c
+        self.chksum_without_last = self.chksum
+        self.chksum_without_last &= 0xff
+        self.last_byte = data[-1]
+        self.chksum += self.last_byte
+        self.chksum &= 0xff
+
+    def get(self):
+        remainder = ( 0x100 - self.chksum ) & 0xff
+        return remainder
+
+    def get_sum(self):
+        return self.chksum
+
+    def is_valid(self):
+        return (self.chksum == 0)
+
+    def get_expected(self):
+        expected = ( 0x100 - self.chksum_without_last ) & 0xff
+        return expected
+
+    def get_last_byte(self):
+        return self.last_byte