Match C* sorting of TimeUUIDs when timestamps tie

Cassandra uses a signed byte array comparison to break
ties when two UUIDs have matching timestamp components.
The slice end templates we use to make the lowest or highest
possible UUID with the same timestamp needed to be changed
in order to match this.

pycassa/util.py

@@ -79,16 +79,26 @@ def convert_time_to_uuid(time_arg, lowest_val=True, randomize=False):
         clock_seq_hi_variant = (rand_bits & 0xff00L) / 0x100 # 8 bits, 8 offset
         node = (rand_bits & 0xffffffffffff0000L) / 0x10000L # 48 bits, 16 offset
     else:
+        # In the event of a timestamp tie, Cassandra compares the two
+        # byte arrays directly. This is a *signed* comparison of each byte
+        # in the two arrays.  So, we have to make each byte -128 or +127 for
+        # this to work correctly.
+        #
+        # For the clock_seq_hi_variant, we don't get to pick the two most
+        # significant bits (they're always 01), so we are dealing with a
+        # positive byte range for this particular byte.
         if lowest_val:
             # Make the lowest value UUID with the same timestamp
-            clock_seq_low = 0 & 0xffL
-            clock_seq_hi_variant = 0 & 0x3fL
-            node = 0 & 0xffffffffffffL # 48 bits
+            clock_seq_low = 0x80L
+            clock_seq_hi_variant = 0 & 0x3fL # The two most significant bits
+                                             # will be 0 and 1, no matter what
+            node = 0x808080808080L # 48 bits
         else:
             # Make the highest value UUID with the same timestamp
-            clock_seq_low = 0xffL
-            clock_seq_hi_variant = 0x3fL
-            node = 0xffffffffffffL # 48 bits
+            clock_seq_low = 0x7fL
+            clock_seq_hi_variant = 0x3fL # The two most significant bits will
+                                         # 0 and 1, no matter what
+            node = 0x7f7f7f7f7f7fL # 48 bits
     return uuid.UUID(fields=(time_low, time_mid, time_hi_version,
                         clock_seq_hi_variant, clock_seq_low, node), version=1)