Implement best-name-finding helper method

osu.Game/Utils/NamingUtils.cs

@@ -2,14 +2,60 @@
 // See the LICENCE file in the repository root for full licence text.
 
 using System.Collections.Generic;
+using System.Text.RegularExpressions;
 
 namespace osu.Game.Utils
 {
     public static class NamingUtils
     {
+        /// <summary>
+        /// Given a set of <paramref name="existingNames"/> and a target <paramref name="desiredName"/>,
+        /// finds a "best" name closest to <paramref name="desiredName"/> that is not in <paramref name="existingNames"/>.
+        /// </summary>
+        /// <remarks>
+        /// <para>
+        /// This helper is most useful in scenarios when creating new objects in a set
+        /// (such as adding new difficulties to a beatmap set, or creating a clone of an existing object that needs a unique name).
+        /// If <paramref name="desiredName"/> is already present in <paramref name="existingNames"/>,
+        /// this method will append the lowest possible number in brackets that doesn't conflict with <paramref name="existingNames"/>
+        /// to <paramref name="desiredName"/> and return that.
+        /// See <c>osu.Game.Tests.Utils.NamingUtilsTest</c> for concrete examples of behaviour.
+        /// </para>
+        /// <para>
+        /// <paramref name="desiredName"/> and <paramref name="existingNames"/> are compared in a case-insensitive manner,
+        /// so this method is safe to use for naming files in a platform-invariant manner.
+        /// </para>
+        /// </remarks>
         public static string GetNextBestName(IEnumerable<string> existingNames, string desiredName)
         {
-            return null;
+            string pattern = $@"^(?i){Regex.Escape(desiredName)}(?-i)( \((?<copyNumber>[1-9][0-9]*)\))?$";
+            var regex = new Regex(pattern, RegexOptions.Compiled);
+            var takenNumbers = new HashSet<int>();
+
+            foreach (string name in existingNames)
+            {
+                var match = regex.Match(name);
+                if (!match.Success)
+                    continue;
+
+                string copyNumberString = match.Groups[@"copyNumber"].Value;
+
+                if (string.IsNullOrEmpty(copyNumberString))
+                {
+                    takenNumbers.Add(0);
+                    continue;
+                }
+
+                takenNumbers.Add(int.Parse(copyNumberString));
+            }
+
+            int bestNumber = 0;
+            while (takenNumbers.Contains(bestNumber))
+                bestNumber += 1;
+
+            return bestNumber == 0
+                ? desiredName
+                : $"{desiredName} ({bestNumber})";
         }
     }
 }