don't rebuild parser on every dice roll

dice/lib.py

@@ -35,6 +35,10 @@ class DiceRoller(object):
 
     output = ""
 
+    def __init__(self):
+        """Build the parsing tables."""
+        self.build()
+
     def roll_dice(self, keep, dice, size):
         """Takes the parsed dice string for a single roll (eg 3/4d20) and performs
         the actual roll. Returns a string representing the result.
@@ -253,7 +257,6 @@ class DiceRoller(object):
             dS: roll a S-sided die
             +M: add M to the result (-M for subtraction) (optional)
         """
-        self.build()
         yacc.parse(dicestr)
         return self.get_result()