Add a "SoftClamp" function which tolerates min > max, use for DrawTrainEngine.

src/core/math_func.hpp

@@ -144,6 +144,29 @@ static inline T Clamp(const T a, const T min, const T max)
 	return a;
 }
 
+/**
+ * Clamp a value between an interval.
+ *
+ * This function returns a value which is between the given interval of
+ * min and max. If the given value is in this interval the value itself
+ * is returned otherwise the border of the interval is returned, according
+ * which side of the interval was 'left'.
+ *
+ * @note If the min value is greater than the return value is the average of the min and max.
+ * @param a The value to clamp/truncate.
+ * @param min The minimum of the interval.
+ * @param max the maximum of the interval.
+ * @returns A value between min and max which is closest to a.
+ */
+template <typename T>
+static inline T SoftClamp(const T a, const T min, const T max)
+{
+	if (min > max) return (min + max) / 2;
+	if (a <= min) return min;
+	if (a >= max) return max;
+	return a;
+}
+
 /**
  * Clamp an integer between an interval.
  *