update dice parser

src/dice/dice.py

@@ -5,18 +5,19 @@ from src.dice.parser import DieParser
 
 class DiceRoller:
     """
-        DiceRoller is a simple class that allows you to roll dices.
+    DiceRoller is a simple class that allows you to roll dices.
 
-        A die can be rolled using the following expression:
-        - 1d20 will roll a 20-faceted die and output the result a random number between 1 and 20.
-        - 1d100 will roll a 100 faceted die.
-        - 2d20 will roll a two d20 dies and multiply the result by two.
-        - 2d20+5 will roll a two d20 dies and multiply the result by two and ads 5.
+    A die can be rolled using the following expression:
+    - 1d20 will roll a 20-faceted die and output the result a random number between 1 and 20.
+    - 1d100 will roll a 100 faceted die.
+    - 2d20 will roll a two d20 dies and multiply the result by two.
+    - 2d20+5 will roll a two d20 dies and multiply the result by two and ads 5.
     """
+
     _parser = DieParser()
 
     @staticmethod
-    def roll(expression: str, advantage: typing.Optional[bool]) -> int:
+    def roll(expression: str, advantage: typing.Optional[bool] = None) -> int:
         """
         Roll die and return the result.
         :param expression: The die expression.

src/dice/parser.py

@@ -8,9 +8,9 @@ DIE_GRAMMAR = """
     @@grammar::Die
     @@whitespace :: None
 
-    start = die:die [modifier:die_modifier] $;
+    start = die:die $;
 
-    die = [number_of_dies:number] die_type:die_type die_number:number;
+    die = [number_of_dies:number] die_type:die_type die_number:number [modifier:die_modifier];
     die_modifier = op:operator modifier:number;
 
     operator = '+' | '-';

src/dice/semantics.py

@@ -8,9 +8,7 @@ class DieSemantics:
         return int(ast)
 
     def start(self, ast):
-        modifier = ast.get("modifier") or 0
-        die = ast.get("die")
-        return die + modifier
+        return ast.get("die")
 
     def die(self, ast):
         if not isinstance(ast, AST):
@@ -18,14 +16,22 @@ class DieSemantics:
         number_of_dies = ast.get("number_of_dies") or 1
         die_type = ast.get("die_type")
         die_number = ast.get("die_number") or 1
+        die_modifier = ast.get("modifier") or 0
         if die_number <= 0:
             return 0
         # normal die
         if die_type == "d":
-            return sum([random.randint(1, die_number) for _ in range(number_of_dies)])
+            die_sum = sum(
+                [random.randint(1, die_number) for _ in range(number_of_dies)]
+            )
+            # do not let die to underflow
+            return max(die_sum + die_modifier, 1)
         # zero-based die can output 0.
         if die_type == "zd":
-            return sum([random.randint(0, die_number) for _ in range(number_of_dies)])
+            die_sum = sum(
+                [random.randint(0, die_number) for _ in range(number_of_dies)]
+            )
+            return max(die_sum + die_modifier, 0)
         raise ValueError(f"Invalid die type: {die_type}")
 
     def die_modifier(self, ast):