added countdown score answer impl

reasoning_gym/games/countdown.py

@@ -1,9 +1,10 @@
 from dataclasses import dataclass
 from random import Random
-from typing import List, Optional, Tuple
+from typing import List, Optional, Tuple, Dict, Any
 
 import sympy
 from sympy import Symbol, symbols
+from sympy.parsing.sympy_parser import parse_expr
 
 from ..factory import ProceduralDataset, register_dataset
 
@@ -157,6 +158,23 @@ class CountdownDataset(ProceduralDataset):
                 continue
 
         raise ValueError(f"Failed to generate valid expression after {max_attempts} attempts")
+    
+    def score_answer(self, answer: Optional[str], metadata: Dict[str, Any]) -> float:
+        """Determine if the solution provided solves the problem"""
+        reward = 0.0
+        if answer is not None:
+            try:
+                user_answer = int(parse_expr(answer))
+                solved = user_answer == metadata["target"]
+                if solved:
+                    reward = 1.0
+                elif (len(answer.strip()) > 0): # encourage partial solutions
+                    reward = 0.05
+                else:
+                    reward = 0.01
+            except:
+                reward = 0.01
+        return reward
 
 
 # Register the dataset