adds commnets and some tests

crc.py

@@ -6,6 +6,14 @@ def __init__(self):
         self.table = {c: self._compute_entry(c) for c in range(256)}
 
     def _compute_entry(self, c):
+        ''' _compute_entry : Int -> Int
+        Builds an entry in the CRC table from an Int (0 <= c <= 255) using
+        the CRC algorithm described here: 
+        http://stigge.org/martin/pub/SAR-PR-2006-05.pdf
+        >>> assert crc._compute_entry(0) == 0
+        >>> assert crc._compute_entry(113) == 654459306
+        >>> assert crc._compute_entry(248) == 3009837614
+        '''
         for _ in range(8):
             if c & 1:
                 c = CRC_POLY ^ c >> 1
@@ -16,10 +24,16 @@ def _compute_entry(self, c):
 
     # create : bytes -> bytes
     def create(self, buf):
-        ''' Where buf is the contents of the chunk type and the chunk data
+        ''' create : Bytes -> Bytes
+        Where buf is the contents of the chunk type and the chunk data 
+        as a string of bytes.
         '''
         crc = INITIAL_CRC
         for i, byt in enumerate(buf):
             crc = self.table[(crc ^ byt) & 0xFF] ^ (crc >> 8)
 
         return (crc ^ INITIAL_CRC).to_bytes(4, 'big')
+
+if __name__ == '__main__':
+    import doctest
+    doctest.testmod(extraglobs={'crc': CRC()})