Initial support code for beatmap loading

osu.Desktop/Beatmaps/IO/LegacyFilesystemReader.cs

@@ -0,0 +1,51 @@
+using System;
+using System.IO;
+using System.Collections.Generic;
+using System.Linq;
+using osu.Game.Beatmaps.Formats;
+using osu.Game.Beatmaps.IO;
+using osu.Game.Beatmaps;
+
+namespace osu.Desktop.Beatmaps.IO
+{
+    /// <summary>
+    /// Reads an extracted legacy beatmap from disk.
+    /// </summary>
+    public class LegacyFilesystemReader : ArchiveReader
+    {
+        static LegacyFilesystemReader()
+        {
+            AddReader<LegacyFilesystemReader>((storage, path) => Directory.Exists(path));
+        }
+    
+        private string BasePath { get; set; }
+        private Beatmap FirstMap { get; set; }
+    
+        public LegacyFilesystemReader(string path)
+        {
+            BasePath = path;
+            var maps = ReadBeatmaps();
+            if (maps.Length == 0)
+                throw new FileNotFoundException("This directory contains no beatmaps");
+            using (var stream = new StreamReader(ReadFile(maps[0])))
+            {
+                var decoder = BeatmapDecoder.GetDecoder(stream);
+                FirstMap = decoder.Decode(stream);
+            }
+        }
+
+        public override string[] ReadBeatmaps()
+        {
+            return Directory.GetFiles(BasePath, "*.osu").Select(f => Path.GetFileName(f)).ToArray();
+        }
+
+        public override Stream ReadFile(string name)
+        {
+            return File.OpenRead(Path.Combine(BasePath, name));
+        }
+
+        public override Metadata ReadMetadata()
+        {
+            return FirstMap.Metadata;
+        }
+    }