lib/packet/packet_utils: Add a function to calculate fletcher checksum

I'm resending the patches which appease pep8

fletcher checksum function for OSPF LSA checksum.
refer to RFC1008.

Signed-off-by: Wataru ISHIDA <ishida.wataru@lab.ntt.co.jp>
Signed-off-by: FUJITA Tomonori <fujita.tomonori@lab.ntt.co.jp>

ryu/lib/packet/packet_utils.py

@@ -97,3 +97,40 @@ def checksum_ip(ipvx, length, payload):
 
     buf = header + payload
     return checksum(buf)
+
+_MODX = 4102
+
+
+def fletcher_checksum(data, offset):
+    """
+    Fletcher Checksum -- Refer to RFC1008
+
+    calling with offset == _FLETCHER_CHECKSUM_VALIDATE will validate the
+    checksum without modifying the buffer; a valid checksum returns 0.
+    """
+    c0 = 0
+    c1 = 0
+    pos = 0
+    length = len(data)
+    data = bytearray(data)
+    data[offset:offset+2] = [0]*2
+
+    while pos < length:
+        tlen = min(length - pos, _MODX)
+        for d in data[pos:pos+tlen]:
+            c0 += d
+            c1 += c0
+        c0 %= 255
+        c1 %= 255
+        pos += tlen
+
+    x = ((length - offset - 1) * c0 - c1) % 255
+    if x <= 0:
+        x += 255
+    y = 510 - c0 - x
+    if y > 255:
+        y -= 255
+
+    data[offset] = x
+    data[offset+1] = y
+    return (x << 8) | (y & 0xff)