Add helper method to convert to legacy mods enums

osu.Game.Rulesets.Catch/CatchRuleset.cs

@@ -48,7 +48,7 @@ namespace osu.Game.Rulesets.Catch
             new KeyBinding(InputKey.Shift, CatchAction.Dash),
         };
 
-        public override IEnumerable<Mod> ConvertLegacyMods(LegacyMods mods)
+        public override IEnumerable<Mod> ConvertFromLegacyMods(LegacyMods mods)
         {
             if (mods.HasFlag(LegacyMods.Nightcore))
                 yield return new CatchModNightcore();

osu.Game.Rulesets.Mania/ManiaRuleset.cs

@@ -49,7 +49,7 @@ namespace osu.Game.Rulesets.Mania
 
         public override ISkin CreateLegacySkinProvider(ISkinSource source) => new ManiaLegacySkinTransformer(source);
 
-        public override IEnumerable<Mod> ConvertLegacyMods(LegacyMods mods)
+        public override IEnumerable<Mod> ConvertFromLegacyMods(LegacyMods mods)
         {
             if (mods.HasFlag(LegacyMods.Nightcore))
                 yield return new ManiaModNightcore();
@@ -118,6 +118,59 @@ namespace osu.Game.Rulesets.Mania
                 yield return new ManiaModRandom();
         }
 
+        public override LegacyMods ConvertToLegacyMods(Mod[] mods)
+        {
+            var value = base.ConvertToLegacyMods(mods);
+
+            foreach (var mod in mods)
+            {
+                switch (mod)
+                {
+                    case ManiaModKey1 _:
+                        value |= LegacyMods.Key1;
+                        break;
+
+                    case ManiaModKey2 _:
+                        value |= LegacyMods.Key2;
+                        break;
+
+                    case ManiaModKey3 _:
+                        value |= LegacyMods.Key3;
+                        break;
+
+                    case ManiaModKey4 _:
+                        value |= LegacyMods.Key4;
+                        break;
+
+                    case ManiaModKey5 _:
+                        value |= LegacyMods.Key5;
+                        break;
+
+                    case ManiaModKey6 _:
+                        value |= LegacyMods.Key6;
+                        break;
+
+                    case ManiaModKey7 _:
+                        value |= LegacyMods.Key7;
+                        break;
+
+                    case ManiaModKey8 _:
+                        value |= LegacyMods.Key8;
+                        break;
+
+                    case ManiaModKey9 _:
+                        value |= LegacyMods.Key9;
+                        break;
+
+                    case ManiaModFadeIn _:
+                        value |= LegacyMods.FadeIn;
+                        break;
+                }
+            }
+
+            return value;
+        }
+
         public override IEnumerable<Mod> GetModsFor(ModType type)
         {
             switch (type)

osu.Game.Rulesets.Osu/OsuRuleset.cs

@@ -52,7 +52,7 @@ namespace osu.Game.Rulesets.Osu
             new KeyBinding(InputKey.MouseRight, OsuAction.RightButton),
         };
 
-        public override IEnumerable<Mod> ConvertLegacyMods(LegacyMods mods)
+        public override IEnumerable<Mod> ConvertFromLegacyMods(LegacyMods mods)
         {
             if (mods.HasFlag(LegacyMods.Nightcore))
                 yield return new OsuModNightcore();

osu.Game.Rulesets.Taiko/TaikoRuleset.cs

@@ -50,7 +50,7 @@ namespace osu.Game.Rulesets.Taiko
             new KeyBinding(InputKey.K, TaikoAction.RightRim),
         };
 
-        public override IEnumerable<Mod> ConvertLegacyMods(LegacyMods mods)
+        public override IEnumerable<Mod> ConvertFromLegacyMods(LegacyMods mods)
         {
             if (mods.HasFlag(LegacyMods.Nightcore))
                 yield return new TaikoModNightcore();

osu.Game/Rulesets/Ruleset.cs

@@ -42,9 +42,63 @@ namespace osu.Game.Rulesets
         /// <summary>
         /// Converts mods from legacy enum values. Do not override if you're not a legacy ruleset.
         /// </summary>
-        /// <param name="mods">The legacy enum which will be converted</param>
-        /// <returns>An enumerable of constructed <see cref="Mod"/>s</returns>
-        public virtual IEnumerable<Mod> ConvertLegacyMods(LegacyMods mods) => Array.Empty<Mod>();
+        /// <param name="mods">The legacy enum which will be converted.</param>
+        /// <returns>An enumerable of constructed <see cref="Mod"/>s.</returns>
+        public virtual IEnumerable<Mod> ConvertFromLegacyMods(LegacyMods mods) => Array.Empty<Mod>();
+
+        /// <summary>
+        /// Converts mods to legacy enum values. Do not override if you're not a legacy ruleset.
+        /// </summary>
+        /// <param name="mods">The mods which will be converted.</param>
+        /// <returns>A single bitwise enumerable value representing (to the best of our ability) the mods.</returns>
+        public virtual LegacyMods ConvertToLegacyMods(Mod[] mods)
+        {
+            var value = LegacyMods.None;
+
+            foreach (var mod in mods)
+            {
+                switch (mod)
+                {
+                    case ModNoFail _:
+                        value |= LegacyMods.NoFail;
+                        break;
+
+                    case ModEasy _:
+                        value |= LegacyMods.Easy;
+                        break;
+
+                    case ModHidden _:
+                        value |= LegacyMods.Hidden;
+                        break;
+
+                    case ModHardRock _:
+                        value |= LegacyMods.HardRock;
+                        break;
+
+                    case ModSuddenDeath _:
+                        value |= LegacyMods.SuddenDeath;
+                        break;
+
+                    case ModDoubleTime _:
+                        value |= LegacyMods.DoubleTime;
+                        break;
+
+                    case ModRelax _:
+                        value |= LegacyMods.Relax;
+                        break;
+
+                    case ModHalfTime _:
+                        value |= LegacyMods.HalfTime;
+                        break;
+
+                    case ModFlashlight _:
+                        value |= LegacyMods.Flashlight;
+                        break;
+                }
+            }
+
+            return value;
+        }
 
         public ModAutoplay GetAutoplayMod() => GetAllMods().OfType<ModAutoplay>().First();
 

osu.Game/Scoring/Legacy/LegacyScoreDecoder.cs

@@ -66,7 +66,7 @@ namespace osu.Game.Scoring.Legacy
                 /* score.Perfect = */
                 sr.ReadBoolean();
 
-                scoreInfo.Mods = currentRuleset.ConvertLegacyMods((LegacyMods)sr.ReadInt32()).ToArray();
+                scoreInfo.Mods = currentRuleset.ConvertFromLegacyMods((LegacyMods)sr.ReadInt32()).ToArray();
 
                 /* score.HpGraphString = */
                 sr.ReadString();

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)
+        {
+        }
+    }
+}

osu.Game/Tests/Beatmaps/LegacyModConversionTest.cs

@@ -23,7 +23,7 @@ namespace osu.Game.Tests.Beatmaps
         protected void Test(LegacyMods legacyMods, Type[] expectedMods)
         {
             var ruleset = CreateRuleset();
-            var mods = ruleset.ConvertLegacyMods(legacyMods).ToList();
+            var mods = ruleset.ConvertFromLegacyMods(legacyMods).ToList();
             Assert.AreEqual(expectedMods.Length, mods.Count);
 
             foreach (var modType in expectedMods)