Add helper method to convert to legacy mods enums

osu.Game/Scoring/Legacy/LegacyScoreEncoder.cs

@@ -0,0 +1,35 @@
+// Copyright (c) ppy Pty Ltd <contact@ppy.sh>. Licensed under the MIT Licence.
+// See the LICENCE file in the repository root for full licence text.
+
+using System;
+using System.IO;
+
+namespace osu.Game.Scoring.Legacy
+{
+    public class LegacyScoreEncoder
+    {
+        public const int LATEST_VERSION = 128;
+
+        private readonly Score score;
+
+        public LegacyScoreEncoder(Score score)
+        {
+            this.score = score;
+
+            if (score.ScoreInfo.Beatmap.RulesetID < 0 || score.ScoreInfo.Beatmap.RulesetID > 3)
+                throw new ArgumentException("Only scores in the osu, taiko, catch, or mania rulesets can be encoded to the legacy score format.", nameof(score));
+        }
+
+        public void Encode(TextWriter writer)
+        {
+            writer.WriteLine($"osu file format v{LATEST_VERSION}");
+
+            writer.WriteLine();
+            handleGeneral(writer);
+        }
+
+        private void handleGeneral(TextWriter writer)
+        {
+        }
+    }
+}