From 0c9528fd4f66a175c737491959974a715abce6f0 Mon Sep 17 00:00:00 2001
From: Markus Birth <mbirth@gmail.com>
Date: Tue, 16 Oct 2018 23:44:30 +0200
Subject: [PATCH] Further optimisations in ChkSum.

---
 grmn/chksum.py | 9 +++------
 1 file changed, 3 insertions(+), 6 deletions(-)

diff --git a/grmn/chksum.py b/grmn/chksum.py
index 7a01c19..1892705 100644
--- a/grmn/chksum.py
+++ b/grmn/chksum.py
@@ -4,16 +4,12 @@ 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]:
+        for c in data:
             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 add_from_file(self, filename: str, print_progress: bool = False, blocksize: int=16384):
@@ -38,7 +34,8 @@ class ChkSum:
         return (self.chksum == 0)
 
     def get_expected(self):
-        expected = ( 0x100 - self.chksum_without_last ) & 0xff
+        chksum_without_last = ( 0x100 + self.chksum - self.last_byte ) & 0xff
+        expected = ( 0x100 - chksum_without_last ) & 0xff
         return expected
 
     def get_last_byte(self):