feat: Add comprehensive unit tests for parse_string_to_complex() method

reasoning_gym/algebra/complex_arithmetic.py

@@ -96,37 +96,57 @@ class ComplexArithmeticDataset(ProceduralDataset):
 
     @staticmethod
     def parse_string_to_complex(answer: str) -> complex:
+        if answer is None:
+            return None
+
         try:
             # Normalize the answer string by removing spaces and converting to lowercase
             answer = answer.replace(" ", "").lower()
+
             # Convert mathematical notation 'i' to Python's 'j' for complex numbers
             answer = answer.replace("i", "j")
 
             # Handle real numbers (no imaginary part)
             if "j" not in answer:
-                student_result = complex(float(answer))
-            else:
-                # Handle cases like "j" or "2j" (implicit coefficient)
-                if answer[0] == "j":
-                    # Convert "j" to "1j", "2j" remains unchanged
-                    answer = "1" + answer
-                # Handle cases like "3j" where there's no explicit + or - before j
-                elif answer[-1] == "j" and not any(c in answer[:-1] for c in "+-"):
-                    # Convert "3j" to "3+1j"
-                    answer = answer.replace("j", "+1j")
+                return complex(float(answer))
 
-                # Ensure the string has an imaginary part, even if zero
-                if "j" not in answer:
-                    answer += "+0j"
+            # Handle pure imaginary numbers
+            if answer == "j":
+                return complex(0, 1)
+            if answer == "-j":
+                return complex(0, -1)
 
-                # Parse the normalized string into a complex number
-                student_result = complex(answer)
+            # Handle cases like "7j" or "-7j" (no real part)
+            if (
+                answer.endswith("j")
+                and not any(c in answer[:-1] for c in "+-")
+                or (answer.startswith("-") and not any(c in answer[1:-1] for c in "+-"))
+            ):
+                # Extract coefficient
+                coef = answer[:-1]
+                if coef == "":
+                    coef = "1"
+                elif coef == "-":
+                    coef = "-1"
+                return complex(0, float(coef))
 
-        except ValueError:
+            # Handle complex numbers with both parts
+            # Make sure there's a + or - before j if it's not at the beginning
+            if "j" in answer and not answer.endswith("j"):
+                return None  # Invalid format like "3j+2"
+
+            # Handle cases like "3+j" (implicit 1)
+            if "+j" in answer:
+                answer = answer.replace("+j", "+1j")
+            if "-j" in answer:
+                answer = answer.replace("-j", "-1j")
+
+            # Parse the normalized string into a complex number
+            return complex(answer)
+
+        except (ValueError, TypeError):
             return None
 
-        return student_result
-
     def score_answer(self, answer: Optional[str], entry: dict) -> float:
         """Score the answer using exponential distance-based scoring."""
         if answer is None: