Refactor _safe_eval to extract AST evaluation logic

- Extract _eval_ast_node as a module-level function for better testability
- Simplify _safe_eval by removing nested function definition
- Improve code readability and maintainability
- Preserve all security benefits of AST-based evaluation

🤖 Generated with [Claude Code](https://claude.ai/code)

Co-Authored-By: Claude <noreply@anthropic.com>

Author: alexanderthclark

freeride/formula.py

@@ -8,6 +8,40 @@
 from freeride.exceptions import FormulaParseError
 
 
+def _eval_ast_node(node, allowed_names, values):
+    """Recursively evaluate an AST node with restricted operations."""
+    if isinstance(node, ast.Expression):
+        return _eval_ast_node(node.body, allowed_names, values)
+    
+    if isinstance(node, ast.BinOp):
+        if isinstance(node.op, (ast.Add, ast.Sub, ast.Mult)):
+            left = _eval_ast_node(node.left, allowed_names, values)
+            right = _eval_ast_node(node.right, allowed_names, values)
+            if isinstance(node.op, ast.Add):
+                return left + right
+            if isinstance(node.op, ast.Sub):
+                return left - right
+            if isinstance(node.op, ast.Mult):
+                return left * right
+    
+    if isinstance(node, ast.UnaryOp) and isinstance(node.op, (ast.UAdd, ast.USub)):
+        operand = _eval_ast_node(node.operand, allowed_names, values)
+        return operand if isinstance(node.op, ast.UAdd) else -operand
+    
+    if isinstance(node, ast.Num):  # Python <3.8 compatibility
+        return node.n
+    
+    if isinstance(node, ast.Constant) and isinstance(node.value, (int, float)):
+        return node.value
+    
+    if isinstance(node, ast.Name):
+        if node.id not in allowed_names:
+            raise FormulaParseError(f"Invalid variable '{node.id}' in equation")
+        return values.get(node.id, 0.0)
+    
+    raise FormulaParseError("Invalid syntax in equation")
+
+
 def _formula(equation: str):
     """
     Parse a linear equation string and return an Affine object.
@@ -68,49 +102,18 @@ def _formula(equation: str):
     lhs, rhs = equation.split("=")
 
     def _safe_eval(expr: str, **values: float) -> float:
-        """Safely evaluate arithmetic expressions for ``x`` and ``y``."""
-
+        """Safely evaluate arithmetic expressions for x and y."""
         if not re.fullmatch(r"[0-9xy+\-*.()]+", expr):
             raise FormulaParseError("Invalid characters in equation")
 
         try:
-            node = ast.parse(expr, mode="eval")
+            ast_tree = ast.parse(expr, mode="eval")
         except SyntaxError as exc:
             raise FormulaParseError("Invalid syntax in equation") from exc
 
         allowed_names = {"x", "y"}
-
-        def _eval(n):
-            if isinstance(n, ast.Expression):
-                return _eval(n.body)
-            if isinstance(n, ast.BinOp) and isinstance(
-                n.op, (ast.Add, ast.Sub, ast.Mult)
-            ):
-                left = _eval(n.left)
-                right = _eval(n.right)
-                if isinstance(n.op, ast.Add):
-                    return left + right
-                if isinstance(n.op, ast.Sub):
-                    return left - right
-                if isinstance(n.op, ast.Mult):
-                    return left * right
-            if isinstance(n, ast.UnaryOp) and isinstance(n.op, (ast.UAdd, ast.USub)):
-                val = _eval(n.operand)
-                return val if isinstance(n.op, ast.UAdd) else -val
-            if isinstance(n, ast.Num):  # pragma: no cover - for Python <3.8
-                return n.n
-            if isinstance(n, ast.Constant):  # for Python >=3.8
-                if isinstance(n.value, (int, float)):
-                    return n.value
-            if isinstance(n, ast.Name):
-                if n.id not in allowed_names:
-                    raise FormulaParseError(
-                        f"Invalid variable '{n.id}' in equation"
-                    )
-                return values.get(n.id, 0.0)
-            raise FormulaParseError("Invalid syntax in equation")
-
-        return float(_eval(node))
+        result = _eval_ast_node(ast_tree, allowed_names, values)
+        return float(result)
 
     # If equation is given explicitly as y = f(x)
     if lhs == "y" or rhs == "y":