Add legacy beatmap flag.

osu.Game/Beatmaps/Beatmap.cs

@@ -70,5 +70,14 @@ namespace osu.Game.Beatmaps
         /// </summary>
         /// <returns>The star difficulty.</returns>
         public double CalculateStarDifficulty() => Ruleset.GetRuleset(BeatmapInfo.Mode).CreateDifficultyCalculator(this).Calculate();
+
+        /// <summary>
+        /// Constructs a new beatmap.
+        /// </summary>
+        /// <param name="original">The original beatmap to use the parameters of.</param>
+        public Beatmap(Beatmap original = null)
+            : base(original)
+        {
+        }
     }
 }

osu.Game/Beatmaps/Formats/OsuLegacyDecoder.cs

@@ -10,6 +10,7 @@ using osu.Game.Beatmaps.Samples;
 using osu.Game.Beatmaps.Timing;
 using osu.Game.Modes;
 using osu.Game.Modes.Objects;
+using osu.Game.Beatmaps.Legacy;
 
 namespace osu.Game.Beatmaps.Formats
 {
@@ -242,6 +243,16 @@ namespace osu.Game.Beatmaps.Formats
             }
         }
 
+        protected override Beatmap ParseFile(TextReader stream)
+        {
+            return new LegacyBeatmap(base.ParseFile(stream));
+        }
+
+        public override Beatmap Decode(TextReader stream)
+        {
+            return new LegacyBeatmap(base.Decode(stream));
+        }
+
         protected override void ParseFile(TextReader stream, Beatmap beatmap)
         {
             HitObjectParser parser = null;

osu.Game/Beatmaps/Legacy/IIsLegacy.cs

@@ -0,0 +1,9 @@
+namespace osu.Game.Beatmaps.Legacy
+{
+    /// <summary>
+    /// A Beatmap that was loaded from a legacy .osu beatmap file (version &lt;=15).
+    /// </summary>
+    public interface IIsLegacy
+    {
+    }
+}

osu.Game/Beatmaps/Legacy/LegacyBeatmap.cs

@@ -0,0 +1,17 @@
+namespace osu.Game.Beatmaps.Legacy
+{
+    /// <summary>
+    /// A type of Beatmap loaded from a legacy .osu beatmap file (version &lt;=15).
+    /// </summary>
+    internal class LegacyBeatmap : Beatmap, IIsLegacy
+    {
+        /// <summary>
+        /// Constructs a new beatmap.
+        /// </summary>
+        /// <param name="original">The original beatmap to use the parameters of.</param>
+        public LegacyBeatmap(Beatmap original = null)
+            : base(original)
+        {
+        }
+    }
+}

osu.Game/osu.Game.csproj

@@ -75,7 +75,9 @@
     <Compile Include="Beatmaps\DifficultyCalculator.cs" />
     <Compile Include="Beatmaps\IBeatmapCoverter.cs" />
     <Compile Include="Beatmaps\IBeatmapProcessor.cs" />
+    <Compile Include="Beatmaps\Legacy\LegacyBeatmap.cs" />
     <Compile Include="Beatmaps\Timing\TimingInfo.cs" />
+    <Compile Include="Beatmaps\Legacy\IIsLegacy.cs" />
     <Compile Include="Database\ScoreDatabase.cs" />
     <Compile Include="Graphics\Backgrounds\Triangles.cs" />
     <Compile Include="Graphics\Cursor\CursorTrail.cs" />