Core: Fix the distribution of Options.Range.triangular() (#4283)

Options.py

@@ -689,9 +689,9 @@ class Range(NumericOption):
     @classmethod
     def weighted_range(cls, text) -> Range:
         if text == "random-low":
-            return cls(cls.triangular(cls.range_start, cls.range_end, cls.range_start))
+            return cls(cls.triangular(cls.range_start, cls.range_end, 0.0))
         elif text == "random-high":
-            return cls(cls.triangular(cls.range_start, cls.range_end, cls.range_end))
+            return cls(cls.triangular(cls.range_start, cls.range_end, 1.0))
         elif text == "random-middle":
             return cls(cls.triangular(cls.range_start, cls.range_end))
         elif text.startswith("random-range-"):
@@ -717,11 +717,11 @@ class Range(NumericOption):
                 f"{random_range[0]}-{random_range[1]} is outside allowed range "
                 f"{cls.range_start}-{cls.range_end} for option {cls.__name__}")
         if text.startswith("random-range-low"):
-            return cls(cls.triangular(random_range[0], random_range[1], random_range[0]))
+            return cls(cls.triangular(random_range[0], random_range[1], 0.0))
         elif text.startswith("random-range-middle"):
             return cls(cls.triangular(random_range[0], random_range[1]))
         elif text.startswith("random-range-high"):
-            return cls(cls.triangular(random_range[0], random_range[1], random_range[1]))
+            return cls(cls.triangular(random_range[0], random_range[1], 1.0))
         else:
             return cls(random.randint(random_range[0], random_range[1]))
 
@@ -739,8 +739,16 @@ class Range(NumericOption):
         return str(self.value)
 
     @staticmethod
-    def triangular(lower: int, end: int, tri: typing.Optional[int] = None) -> int:
-        return int(round(random.triangular(lower, end, tri), 0))
+    def triangular(lower: int, end: int, tri: float = 0.5) -> int:
+        """
+        Integer triangular distribution for `lower` inclusive to `end` inclusive.
+
+        Expects `lower <= end` and `0.0 <= tri <= 1.0`. The result of other inputs is undefined.
+        """
+        # Use the continuous range [lower, end + 1) to produce an integer result in [lower, end].
+        # random.triangular is actually [a, b] and not [a, b), so there is a very small chance of getting exactly b even
+        # when a != b, so ensure the result is never more than `end`.
+        return min(end, math.floor(random.triangular(0.0, 1.0, tri) * (end - lower + 1) + lower))
 
 
 class NamedRange(Range):