Implement loader for MegV3 file format

OpenRA.Mods.Cnc/FileSystem/MegFile.cs

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+#region Copyright & License Information
+/*
+ * Copyright 2007-2020 The OpenRA Developers (see AUTHORS)
+ * This file is part of OpenRA, which is free software. It is made
+ * available to you under the terms of the GNU General Public License
+ * as published by the Free Software Foundation, either version 3 of
+ * the License, or (at your option) any later version. For more
+ * information, see COPYING.
+ */
+#endregion
+
+using System;
+using System.Collections.Generic;
+using System.IO;
+using OpenRA.FileSystem;
+using OpenRA.Primitives;
+
+namespace OpenRA.Mods.Cnc.FileSystem
+{
+	/// <summary>
+	/// This class supports loading unencrypted V3 .meg files using
+	/// reference documentation from here https://modtools.petrolution.net/docs/MegFileFormat
+	/// </summary>
+	public class MegV3Loader : IPackageLoader
+	{
+		const uint UnencryptedMegID = 0xFFFFFFFF;
+
+		// Float value 0.99, but it is simpler to read and compare as an integer
+		const uint MegVersion = 0x3F7D70A4;
+
+		public bool TryParsePackage(Stream s, string filename, OpenRA.FileSystem.FileSystem context, out IReadOnlyPackage package)
+		{
+			var position = s.Position;
+
+			var id = s.ReadUInt32();
+			var version = s.ReadUInt32();
+
+			s.Position = position;
+
+			if (id != UnencryptedMegID || version != MegVersion)
+			{
+				package = null;
+				return false;
+			}
+
+			package = new MegFile(s, filename);
+			return true;
+		}
+
+		public sealed class MegFile : IReadOnlyPackage
+		{
+			readonly Stream s;
+
+			readonly Dictionary<string, (uint Offset, int Length)> contents = new Dictionary<string, (uint Offset, int Length)>();
+
+			public MegFile(Stream s, string filename)
+			{
+				Name = filename;
+				this.s = s;
+
+				var id = s.ReadUInt32();
+				var version = s.ReadUInt32();
+
+				if (id != UnencryptedMegID || version != MegVersion)
+					throw new Exception("Invalid file signature for meg file");
+
+				var headerSize = s.ReadUInt32();
+				var numStrings = s.ReadUInt32();
+				var numFiles = s.ReadUInt32();
+				var stringsSize = s.ReadUInt32();
+				var stringsStart = s.Position;
+
+				var filenames = new List<string>();
+
+				// The file names are an indexed array of strings
+				for (var i = 0; i < numStrings; i++)
+				{
+					var length = s.ReadUInt16();
+					filenames.Add(s.ReadASCII(length));
+				}
+
+				// The header indicates where we should be, so verify it
+				if (s.Position != stringsSize + stringsStart)
+					throw new Exception("File name table in .meg file inconsistent");
+
+				// Now we load each file entry and associated info
+				for (var i = 0; i < numFiles; i++)
+				{
+					// Ignore flags, crc, index
+					s.Position += 10;
+					var size = s.ReadUInt32();
+					var offset = s.ReadUInt32();
+					var nameIndex = s.ReadUInt16();
+					contents[filenames[nameIndex]] = (offset, (int)size);
+				}
+
+				if (s.Position != headerSize)
+					throw new Exception("Expected to be at data start offset");
+			}
+
+			public string Name { get; }
+
+			public IEnumerable<string> Contents => contents.Keys;
+
+			public bool Contains(string filename)
+			{
+				return contents.ContainsKey(filename);
+			}
+
+			public void Dispose()
+			{
+				s.Dispose();
+			}
+
+			public Stream GetStream(string filename)
+			{
+				// Look up the index of the filename
+				if (!contents.TryGetValue(filename, out var index))
+					return null;
+
+				return SegmentStream.CreateWithoutOwningStream(s, index.Offset, index.Length);
+			}
+
+			public IReadOnlyPackage OpenPackage(string filename, OpenRA.FileSystem.FileSystem context)
+			{
+				var childStream = GetStream(filename);
+				if (childStream == null)
+					return null;
+
+				if (context.TryParsePackage(childStream, filename, out var package))
+					return package;
+
+				childStream.Dispose();
+				return null;
+			}
+		}
+	}
+}