update CRC code

pyModeS/decoder/common.py

@@ -5,48 +5,61 @@ from pyModeS.decoder import fastcrc
 
 
 def hex2bin(hexstr):
-    """Convert a hexdecimal string to binary string, with zero fillings. """
+    """Convert a hexdecimal string to binary string, with zero fillings."""
     num_of_bits = len(hexstr) * 4
     binstr = bin(int(hexstr, 16))[2:].zfill(int(num_of_bits))
     return binstr
 
 
-def bin2int(binstr):
-    """Convert a binary string to integer. """
-    return int(binstr, 2)
-
-
 def hex2int(hexstr):
-    """Convert a hexdecimal string to integer. """
+    """Convert a hexdecimal string to integer."""
     return int(hexstr, 16)
 
 
+def int2hex(n):
+    """Convert a integer to hexadecimal string."""
+    return hex(n)[2:].rjust(6, '0').upper()
+
+
+def bin2int(binstr):
+    """Convert a binary string to integer."""
+    return int(binstr, 2)
+
+
+def bin2hex(hexstr):
+    """Convert a hexdecimal string to integer."""
+    return int2hex(bin2int(hexstr))
+
+
 def bin2np(binstr):
-    """Convert a binary string to numpy array. """
+    """Convert a binary string to numpy array."""
     return np.array([int(i) for i in binstr])
 
 
 def np2bin(npbin):
-    """Convert a binary numpy array to string. """
+    """Convert a binary numpy array to string."""
     return np.array2string(npbin, separator='')[1:-1]
 
 
 def df(msg):
     """Decode Downlink Format vaule, bits 1 to 5."""
     msgbin = hex2bin(msg)
-    return min( bin2int(msgbin[0:5]) , 24 )
+    return min(bin2int(msgbin[0:5]), 24)
 
 
 def crc(msg, encode=False):
-    """Mode-S Cyclic Redundancy Check
+    """Mode-S Cyclic Redundancy Check.
-    Detect if bit error occurs in the Mode-S message
+
+    Detect if bit error occurs in the Mode-S message. When encode option is on,
+    the checksum is generated.
+
     Args:
         msg (string): 28 bytes hexadecimal message string
         encode (bool): True to encode the date only and return the checksum
     Returns:
         int: message checksum, or partity bits (encoder)
-    """
 
+    """
     if encode:
         msg = msg[:-6] + "000000"
 
@@ -55,27 +68,53 @@ def crc(msg, encode=False):
     return reminder_int
 
 
+def crc_legacy(msg, encode=False):
+    """Mode-S Cyclic Redundancy Check. (Legacy code, 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)
+
+    msgnpbin = bin2np(hex2bin(msg))
+
+    if encode:
+        msgnpbin[-24:] = [0] * 24
+
+    # loop all bits, except last 24 piraty bits
+    for i in range(len(msgnpbin)-24):
+        if msgnpbin[i] == 0:
+            continue
+
+        # perform XOR, when 1
+        msgnpbin[i:i+ng] = np.bitwise_xor(msgnpbin[i:i+ng], generator)
+
+    # last 24 bits
+    reminder = bin2int(np2bin(msgnpbin[-24:]))
+    return reminder
+
+
 def floor(x):
-    """ Mode-S floor function
+    """Mode-S floor function.
 
     Defined as the greatest integer value k, such that k <= x
+    For example: floor(3.6) = 3 and floor(-3.6) = -4
 
-        eg.: floor(3.6) = 3, while floor(-3.6) = -4
     """
     return int(np.floor(x))
 
 
 def icao(msg):
-    """Calculate the ICAO address from an Mode-S message
+    """Calculate the ICAO address from an Mode-S message.
-    with DF4, DF5, DF20, DF21
+
+    Applicable only with DF4, DF5, DF20, DF21 messages.
 
     Args:
         msg (String): 28 bytes hexadecimal message string
 
     Returns:
         String: ICAO address in 6 bytes hexadecimal string
-    """
 
+    """
     DF = df(msg)
 
     if DF in (11, 17, 18):
@@ -91,8 +130,7 @@ def icao(msg):
 
 
 def is_icao_assigned(icao):
-    """ Check whether the ICAO address is assigned (Annex 10, Vol 3)"""
+    """Check whether the ICAO address is assigned (Annex 10, Vol 3)."""
-
     if (icao is None) or (not isinstance(icao, str)) or (len(icao)!=6):
         return False
 
@@ -127,7 +165,7 @@ def typecode(msg):
 
 
 def cprNL(lat):
-    """NL() function in CPR decoding"""
+    """NL() function in CPR decoding."""
 
     if lat == 0:
         return 59
@@ -145,8 +183,11 @@ def cprNL(lat):
     NL = floor(nl)
     return NL
 
+
 def idcode(msg):
-    """Computes identity (squawk code) from DF5 or DF21 message, bit 20-32.
+    """Compute identity (squawk code).
+
+    Applicable only for DF5 or DF21 messages, bit 20-32.
     credit: @fbyrkjeland
 
     Args:
@@ -154,8 +195,8 @@ def idcode(msg):
 
     Returns:
         string: squawk code
-    """
 
+    """
     if df(msg) not in [5, 21]:
         raise RuntimeError("Message must be Downlink Format 5 or 21.")
 
@@ -184,7 +225,9 @@ def idcode(msg):
 
 
 def altcode(msg):
-    """Computes the altitude from DF4 or DF20 message, bit 20-32.
+    """Compute the altitude.
+
+    Applicable only for DF4 or DF20 message, bit 20-32.
     credit: @fbyrkjeland
 
     Args:
@@ -192,8 +235,8 @@ def altcode(msg):
 
     Returns:
         int: altitude in ft
-    """
 
+    """
     if df(msg) not in [0, 4, 16, 20]:
         raise RuntimeError("Message must be Downlink Format 0, 4, 16, or 20.")
 
@@ -255,7 +298,7 @@ def gray2alt(codestr):
 
 
 def gray2int(graystr):
-    """Convert greycode to binary"""
+    """Convert greycode to binary."""
     num = bin2int(graystr)
     num ^= (num >> 8)
     num ^= (num >> 4)
@@ -265,18 +308,19 @@ def gray2int(graystr):
 
 
 def data(msg):
-    """Return the data frame in the message, bytes 9 to 22"""
+    """Return the data frame in the message, bytes 9 to 22."""
     return msg[8:-6]
 
 
 def allzeros(msg):
-    """check if the data bits are all zeros
+    """Check if the data bits are all zeros.
 
     Args:
         msg (String): 28 bytes hexadecimal message string
 
     Returns:
         bool: True or False
+
     """
     d = hex2bin(data(msg))
 
@@ -287,10 +331,11 @@ def allzeros(msg):
 
 
 def wrongstatus(data, sb, msb, lsb):
-    """Check if the status bit and field bits are consistency. This Function
+    """Check if the status bit and field bits are consistency.
-    is used for checking BDS code versions.
-    """
 
+    This Function is used for checking BDS code versions.
+
+    """
     # status bit, most significant bit, least significant bit
     status = int(data[sb-1])
     value = bin2int(data[msb-1:lsb])

pyModeS/decoder/fastcrc.py

@@ -1,13 +1,15 @@
-GENERATOR = [int("11111111", 2), int("11111010", 2), int("00000100", 2), int("10000000", 2)]
+"""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, hex: str):
+    def __init__(self, hexstr):
-        if len(hex) % 2 == 1:
+        self._bytes = [int(hexstr[i:i+2], 16) for i in range(0, len(hexstr), 2)]
-            hex += '0'
-
-        self._bytes = [b for b in bytes.fromhex(hex)]
 
     def byte_count(self):
         return len(self._bytes) - 3
@@ -26,7 +28,7 @@ class BytesWrapper:
         self._bytes[byte_index + 3] = self._bytes[byte_index + 3] ^ \
             (0xFF & ((GENERATOR[2] << 8 - bit_index) | (GENERATOR[3] >> bit_index)))
 
-    def get_suffix(self) -> int:
+    def get_suffix(self):
         return (self._bytes[-3] << 16) | (self._bytes[-2] << 8) | self._bytes[-1]
 
 

tests/test_common.py

@@ -1,10 +1,14 @@
 from pyModeS import common
 
 
-def test_hex2bin():
+def test_conversions():
     assert common.hex2bin('6E406B') == "011011100100000001101011"
+    assert common.bin2hex('011011100100000001101011') == "6E406B"
+    assert common.int2hex(11160538) == "AA4BDA"
 
 def test_crc_decode():
+    assert common.crc_legacy("8D406B902015A678D4D220AA4BDA") == 0
+
     assert common.crc("8D406B902015A678D4D220AA4BDA") == 0
     assert common.crc('8d8960ed58bf053cf11bc5932b7d') == 0
     assert common.crc('8d45cab390c39509496ca9a32912') == 0
@@ -22,7 +26,7 @@ def test_crc_decode():
 
 def test_crc_encode():
     parity = common.crc("8D406B902015A678D4D220AA4BDA", encode=True)
-    assert int("AA4BDA", 16) == parity
+    assert common.int2hex(parity) == "AA4BDA"
 
 def test_icao():
     assert common.icao("8D406B902015A678D4D220AA4BDA") == "406B90"