Add basic mania skin parsing

osu.Game/Beatmaps/Formats/LegacyDecoder.cs

@@ -41,6 +41,7 @@ protected override void ParseStreamInto(LineBufferedReader stream, T output)
                         section = Section.None;
                     }
 
+                    OnBeginNewSection(section);
                     continue;
                 }
 
@@ -57,6 +58,14 @@ protected override void ParseStreamInto(LineBufferedReader stream, T output)
 
         protected virtual bool ShouldSkipLine(string line) => string.IsNullOrWhiteSpace(line) || line.AsSpan().TrimStart().StartsWith("//".AsSpan(), StringComparison.Ordinal);
 
+        /// <summary>
+        /// Invoked when a new <see cref="Section"/> has been entered.
+        /// </summary>
+        /// <param name="section">The entered <see cref="Section"/>.</param>
+        protected virtual void OnBeginNewSection(Section section)
+        {
+        }
+
         protected virtual void ParseLine(T output, Section section, string line)
         {
             line = StripComments(line);
@@ -139,7 +148,8 @@ protected enum Section
             Colours,
             HitObjects,
             Variables,
-            Fonts
+            Fonts,
+            Mania
         }
 
         internal class LegacyDifficultyControlPoint : DifficultyControlPoint

osu.Game/Skinning/LegacyManiaSkinConfiguration.cs

@@ -0,0 +1,30 @@
+// 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;
+
+namespace osu.Game.Skinning
+{
+    public class LegacyManiaSkinConfiguration
+    {
+        public readonly int Keys;
+
+        public readonly float[] ColumnLineWidth;
+        public readonly float[] ColumnSpacing;
+        public readonly float[] ColumnWidth;
+
+        public float HitPosition = 124.8f; // (480 - 402) * 1.6f
+
+        public LegacyManiaSkinConfiguration(int keys)
+        {
+            Keys = keys;
+
+            ColumnLineWidth = new float[keys + 1];
+            ColumnSpacing = new float[keys - 1];
+            ColumnWidth = new float[keys];
+
+            ColumnLineWidth.AsSpan().Fill(2);
+            ColumnWidth.AsSpan().Fill(48);
+        }
+    }
+}

osu.Game/Skinning/LegacyManiaSkinDecoder.cs

@@ -0,0 +1,106 @@
+// 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.Collections.Generic;
+using System.Diagnostics;
+using System.Globalization;
+using osu.Game.Beatmaps.Formats;
+
+namespace osu.Game.Skinning
+{
+    public class LegacyManiaSkinDecoder : LegacyDecoder<List<LegacyManiaSkinConfiguration>>
+    {
+        private const float size_scale_factor = 1.6f;
+
+        public LegacyManiaSkinDecoder()
+            : base(1)
+        {
+        }
+
+        private readonly List<string> pendingLines = new List<string>();
+        private LegacyManiaSkinConfiguration currentConfig;
+
+        protected override void OnBeginNewSection(Section section)
+        {
+            base.OnBeginNewSection(section);
+
+            // If a new section is reached with pending lines remaining, they can all be discarded as there isn't a valid configuration to parse them into.
+            pendingLines.Clear();
+            currentConfig = null;
+        }
+
+        protected override void ParseLine(List<LegacyManiaSkinConfiguration> output, Section section, string line)
+        {
+            line = StripComments(line);
+
+            switch (section)
+            {
+                case Section.Mania:
+                    var pair = SplitKeyVal(line);
+
+                    switch (pair.Key)
+                    {
+                        case "Keys":
+                            currentConfig = new LegacyManiaSkinConfiguration(int.Parse(pair.Value, CultureInfo.InvariantCulture));
+                            output.Add(currentConfig);
+
+                            // All existing lines can be flushed now that we have a valid configuration.
+                            flushPendingLines();
+                            break;
+
+                        default:
+                            pendingLines.Add(line);
+
+                            // Hold all lines until a "Keys" item is found.
+                            if (currentConfig != null)
+                                flushPendingLines();
+                            break;
+                    }
+
+                    break;
+            }
+        }
+
+        private void flushPendingLines()
+        {
+            Debug.Assert(currentConfig != null);
+
+            foreach (var line in pendingLines)
+            {
+                var pair = SplitKeyVal(line);
+
+                switch (pair.Key)
+                {
+                    case "ColumnLineWidth":
+                        parseArrayValue(pair.Value, currentConfig.ColumnLineWidth);
+                        break;
+
+                    case "ColumnSpacing":
+                        parseArrayValue(pair.Value, currentConfig.ColumnSpacing);
+                        break;
+
+                    case "ColumnWidth":
+                        parseArrayValue(pair.Value, currentConfig.ColumnWidth);
+                        break;
+
+                    case "HitPosition":
+                        currentConfig.HitPosition = (480 - float.Parse(pair.Value, CultureInfo.InvariantCulture)) * size_scale_factor;
+                        break;
+                }
+            }
+        }
+
+        private void parseArrayValue(string value, float[] output)
+        {
+            string[] values = value.Split(',');
+
+            for (int i = 0; i < values.Length; i++)
+            {
+                if (i >= output.Length)
+                    break;
+
+                output[i] = float.Parse(values[i], CultureInfo.InvariantCulture) * size_scale_factor;
+            }
+        }
+    }
+}