From 2b55f8a5dfc2050b9166182f54bec271af6a0b35 Mon Sep 17 00:00:00 2001
From: Junzi Sun <junzi.sun@gmail.com>
Date: Thu, 17 Sep 2015 20:24:32 +0200
Subject: [PATCH] work in progress

---
 decoder.py | 50 ++++++++++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 50 insertions(+)

diff --git a/decoder.py b/decoder.py
index 430a84f..af903c0 100644
--- a/decoder.py
+++ b/decoder.py
@@ -8,6 +8,24 @@ Date        : March 2015
 
 import math
 
+MODES_CHECKSUM_TABLE = [
+    0x3935ea, 0x1c9af5, 0xf1b77e, 0x78dbbf, 0xc397db, 0x9e31e9, 0xb0e2f0, 0x587178,
+    0x2c38bc, 0x161c5e, 0x0b0e2f, 0xfa7d13, 0x82c48d, 0xbe9842, 0x5f4c21, 0xd05c14,
+    0x682e0a, 0x341705, 0xe5f186, 0x72f8c3, 0xc68665, 0x9cb936, 0x4e5c9b, 0xd8d449,
+    0x939020, 0x49c810, 0x24e408, 0x127204, 0x093902, 0x049c81, 0xfdb444, 0x7eda22,
+    0x3f6d11, 0xe04c8c, 0x702646, 0x381323, 0xe3f395, 0x8e03ce, 0x4701e7, 0xdc7af7,
+    0x91c77f, 0xb719bb, 0xa476d9, 0xadc168, 0x56e0b4, 0x2b705a, 0x15b82d, 0xf52612,
+    0x7a9309, 0xc2b380, 0x6159c0, 0x30ace0, 0x185670, 0x0c2b38, 0x06159c, 0x030ace,
+    0x018567, 0xff38b7, 0x80665f, 0xbfc92b, 0xa01e91, 0xaff54c, 0x57faa6, 0x2bfd53,
+    0xea04ad, 0x8af852, 0x457c29, 0xdd4410, 0x6ea208, 0x375104, 0x1ba882, 0x0dd441,
+    0xf91024, 0x7c8812, 0x3e4409, 0xe0d800, 0x706c00, 0x383600, 0x1c1b00, 0x0e0d80,
+    0x0706c0, 0x038360, 0x01c1b0, 0x00e0d8, 0x00706c, 0x003836, 0x001c1b, 0xfff409,
+    0x000000, 0x000000, 0x000000, 0x000000, 0x000000, 0x000000, 0x000000, 0x000000,
+    0x000000, 0x000000, 0x000000, 0x000000, 0x000000, 0x000000, 0x000000, 0x000000,
+    0x000000, 0x000000, 0x000000, 0x000000, 0x000000, 0x000000, 0x000000, 0x000000
+]
+
+
 def hex2bin(hexstr):
     """Convert a hexdecimal string to binary string, with zero fillings. """
     length = len(hexstr) * 4
@@ -19,6 +37,38 @@ def hex2bin(hexstr):
 def bin2int(msgbin):
     return int(msgbin, 2)
 
+
+def checksum(msg):
+    nbits = len(msg)
+
+    if nbits == 28: 
+        offset = 0
+    elif nbits == 14:
+        offset = 112-56
+    else:
+        # raise exception
+        return False
+
+    print msg
+    msgbin = hex2bin(msg)
+    checksumhex = msg[22:28]
+    checksum = int(checksumhex, 16)
+
+    crc = 0
+    for i in xrange(nbits):
+        if int(msgbin[i]):
+            crc ^= MODES_CHECKSUM_TABLE[i+offset]
+
+    print bin(crc)
+    print bin(checksum)
+    print
+
+    if crc == checksum:
+        return True
+    else:
+        return False
+
+
 def get_df(msg):
     """Decode Downlink Format vaule, bits 1 to 5."""
     msgbin = hex2bin(msg)