rewrite CRC function

pyModeS/decoder/common.py

@@ -1,8 +1,6 @@
 from __future__ import absolute_import, print_function, division
 import numpy as np
-
-from pyModeS.decoder import fastcrc
-
+from textwrap import wrap
 
 def hex2bin(hexstr):
     """Convert a hexdecimal string to binary string, with zero fillings."""
@@ -60,17 +58,37 @@ def crc(msg, encode=False):
         int: message checksum, or partity bits (encoder)
 
     """
+    # the CRC generator
+    G = [
+        int("11111111", 2), int("11111010", 2),
+        int("00000100", 2), int("10000000", 2)
+    ]
+
     if encode:
         msg = msg[:-6] + "000000"
 
-    reminder_int = fastcrc.crc(msg)
+    msgbin = hex2bin(msg)
+    msgbin_split = wrap(msgbin, 8)
+    mbytes = list(map(bin2int, msgbin_split))
+
+    for ibyte in range(len(mbytes)-3):
+        for ibit in range(8):
+            mask = 0x80 >> ibit
+            bits = mbytes[ibyte] & mask
 
-    return reminder_int
+            if bits > 0:
+                mbytes[ibyte] = mbytes[ibyte] ^ (G[0] >> ibit)
+                mbytes[ibyte+1] = mbytes[ibyte+1] ^ (0xFF & ((G[0] << 8-ibit) | (G[1] >> ibit)))
+                mbytes[ibyte+2] = mbytes[ibyte+2] ^ (0xFF & ((G[1] << 8-ibit) | (G[2] >> ibit)))
+                mbytes[ibyte+3] = mbytes[ibyte+3] ^ (0xFF & ((G[2] << 8-ibit) | (G[3] >> ibit)))
 
+    result = (mbytes[-3] << 16) | (mbytes[-2] << 8) | mbytes[-1]
 
-def crc_legacy(msg, encode=False):
-    """Mode-S Cyclic Redundancy Check. (Legacy code, slow)."""
+    return result
 
+
+def crc_legacy(msg, encode=False):
+    """Mode-S Cyclic Redundancy Check. (Legacy code, 2x slow)."""
     # the polynominal generattor code for CRC [1111111111111010000001001]
     generator = np.array([1,1,1,1,1,1,1,1,1,1,1,1,1,0,1,0,0,0,0,0,0,1,0,0,1])
     ng = len(generator)

pyModeS/decoder/fastcrc.py

@@ -1,47 +0,0 @@
-"""Compute CRC checksum of a hexadecimal string."""
-
-GENERATOR = [
-    int("11111111", 2), int("11111010", 2),
-    int("00000100", 2), int("10000000", 2)
-]
-
-
-class BytesWrapper:
-
-    def __init__(self, hexstr):
-        self._bytes = [int(hexstr[i:i+2], 16) for i in range(0, len(hexstr), 2)]
-
-    def byte_count(self):
-        return len(self._bytes) - 3
-
-    def get_bit(self, byte_index, bit_index):
-        mask = 0x80 >> bit_index
-        bits = self._bytes[byte_index] & mask
-        return 0 if bits == 0 else 1
-
-    def apply_matrix(self, byte_index, bit_index):
-        self._bytes[byte_index] = self._bytes[byte_index] ^ (GENERATOR[0] >> bit_index)
-        self._bytes[byte_index + 1] = self._bytes[byte_index + 1] ^ \
-            (0xFF & ((GENERATOR[0] << 8 - bit_index) | (GENERATOR[1] >> bit_index)))
-        self._bytes[byte_index + 2] = self._bytes[byte_index + 2] ^ \
-            (0xFF & ((GENERATOR[1] << 8 - bit_index) | (GENERATOR[2] >> bit_index)))
-        self._bytes[byte_index + 3] = self._bytes[byte_index + 3] ^ \
-            (0xFF & ((GENERATOR[2] << 8 - bit_index) | (GENERATOR[3] >> bit_index)))
-
-    def get_suffix(self):
-        return (self._bytes[-3] << 16) | (self._bytes[-2] << 8) | self._bytes[-1]
-
-
-def crc(msg):
-    msgbin = BytesWrapper(msg)
-
-    for byte_index in range(msgbin.byte_count()):
-        for bit_index in range(8):
-            b = msgbin.get_bit(byte_index, bit_index)
-
-            if b == 1:
-                msgbin.apply_matrix(byte_index, bit_index)
-
-    result = msgbin.get_suffix()
-
-    return result