Move MapDifficultyRange into BaseDifficulty.

osu.Game/Database/BaseDifficulty.cs

@@ -15,6 +15,23 @@ namespace osu.Game.Database
         public float ApproachRate { get; set; }
         public float SliderMultiplier { get; set; }
         public float SliderTickRate { get; set; }
+
+        /// <summary>
+        /// Maps a difficulty value [0, 10] to a two-piece linear range of values.
+        /// </summary>
+        /// <param name="difficulty">The difficulty value to be mapped.</param>
+        /// <param name="min">Minimum of the resulting range which will be achieved by a difficulty value of 0.</param>
+        /// <param name="mid">Midpoint of the resulting range which will be achieved by a difficulty value of 5.</param>
+        /// <param name="max">Maximum of the resulting range which will be achieved by a difficulty value of 10.</param>
+        /// <returns>Value to which the difficulty value maps in the specified range.</returns>
+        public static double DifficultyRange(double difficulty, double min, double mid, double max)
+        {
+            if (difficulty > 5)
+                return mid + (max - mid) * (difficulty - 5) / 5;
+            if (difficulty < 5)
+                return mid - (mid - min) * (5 - difficulty) / 5;
+            return mid;
+        }
     }
 }