improve mime type detection

src/Main/Base/Project/Src/Services/MimeTypeDetection.cs

@@ -3,6 +3,7 @@
 
 using System;
 using System.IO;
+using System.Linq;
 using System.Runtime.InteropServices;
 using System.Text;
 
@@ -12,6 +13,13 @@ public static class MimeTypeDetection
 	{
 		const int BUFFER_SIZE = 4 * 1024;
 
+		// Known BOMs
+		public static readonly byte[] UTF8 = new byte[] { 0xEF, 0xBB, 0xBF };
+		public static readonly byte[] UTF16BE = new byte[] { 0xFE, 0xFF };
+		public static readonly byte[] UTF16LE = new byte[] { 0xFF, 0xFE };
+		public static readonly byte[] UTF32BE = new byte[] { 0x00, 0x00, 0xFE, 0xFF };
+		public static readonly byte[] UTF32LE = new byte[] { 0xFF, 0xFE, 0x00, 0x00 };
+
 		[DllImport("urlmon.dll", CharSet = CharSet.Unicode, ExactSpelling = true, SetLastError = false)]
 		static extern unsafe int FindMimeFromData(
 			IntPtr pBC,
@@ -23,15 +31,45 @@ static extern unsafe int FindMimeFromData(
 			out IntPtr ppwzMimeOut,
 			int dwReserved);
 
+
+		static byte[] DetectAndRemoveBOM(byte[] buffer, out int len)
+		{
+			len = UTF8.Length;
+			if (buffer.StartsWith(UTF8))
+				return buffer.Skip(UTF8.Length).ToArray();
+			len = UTF32BE.Length;
+			if (buffer.StartsWith(UTF32BE))
+				return buffer.Skip(UTF32BE.Length).ToArray();
+			len = UTF32LE.Length;
+			if (buffer.StartsWith(UTF32LE))
+				return buffer.Skip(UTF32LE.Length).ToArray();
+			len = UTF16LE.Length;
+			if (buffer.StartsWith(UTF16LE))
+				return buffer.Skip(UTF16LE.Length).ToArray();
+			len = UTF16BE.Length;
+			if (buffer.StartsWith(UTF16BE))
+				return buffer.Skip(UTF16BE.Length).ToArray();
+			len = 0;
+			return buffer;
+		}
+
+		static bool StartsWith(this byte[] buffer, byte[] start)
+		{
+			if (buffer.Length < start.Length)
+				return false;
+			int i = 0;
+			while (i < start.Length && buffer[i] == start[i])
+				i++;
+			return i >= start.Length;
+		}
+
 		static unsafe string FindMimeType(byte[] buffer, int offset, int length)
 		{
-			if (length == 0 ||
-				// UTF-16 Big Endian
-				(buffer.Length >= 2 && buffer[0] == 0xFE && buffer[1] == 0xFF) ||
-				// UTF-16 Little Endian
-				(buffer.Length >= 2 && buffer[0] == 0xFF && buffer[1] == 0xFE) ||
-				// UTF-32 Big Endian
-				(buffer.Length >= 4 && buffer[0] == 0x00 && buffer[1] == 0x00 && buffer[2] == 0xFE && buffer[3] == 0xFF))
+			int len;
+			buffer = DetectAndRemoveBOM(buffer, out len);
+			length -= len;
+			offset = (offset < len) ? 0 : offset - len;
+			if (length == 0)
 				return "text/plain";
 
 			fixed (byte *b = &buffer[offset]) {

src/Main/Base/Test/MimeDetectionTests.cs

@@ -2,6 +2,8 @@
 // This code is distributed under the GNU LGPL (for details please see \doc\license.txt)
 
 using System;
+using System.Linq;
+using System.Text;
 using NUnit.Framework;
 
 namespace ICSharpCode.SharpDevelop.Tests
@@ -15,15 +17,27 @@ public void TextPlain()
 			// always open empty files with text editor
 			TestMime(new byte[] {}, "text/plain");
 			// UTF-8
-			TestMime(new byte[] { 0xEF, 0xBB, 0xBF }, "text/plain");
+			TestMime(MimeTypeDetection.UTF8, "text/plain");
 			// UTF-16 Big Endian
-			TestMime(new byte[] { 0xFE, 0xFF }, "text/plain");
+			TestMime(MimeTypeDetection.UTF16BE, "text/plain");
 			// UTF-16 Little Endian
-			TestMime(new byte[] { 0xFF, 0xFE }, "text/plain");
+			TestMime(MimeTypeDetection.UTF16LE, "text/plain");
 			// UTF-32 Big Endian
-			TestMime(new byte[] { 0x00, 0x00, 0xFE, 0xFF }, "text/plain");
+			TestMime(MimeTypeDetection.UTF32BE, "text/plain");
 			// UTF-32 Little Endian
-			TestMime(new byte[] { 0xFF, 0xFE, 0x00, 0x00 }, "text/plain");
+			TestMime(MimeTypeDetection.UTF32LE, "text/plain");
+		}
+
+		[Test]
+		public void TextXml()
+		{
+			string xml = "<?xml version=\"1.0\" ?>";
+			TestMime(Encoding.Default.GetBytes(xml), "text/xml");
+			TestMime(MimeTypeDetection.UTF8.Concat(Encoding.Default.GetBytes(xml)).ToArray(), "text/xml");
+			TestMime(MimeTypeDetection.UTF16BE.Concat(Encoding.Default.GetBytes(xml)).ToArray(), "text/xml");
+			TestMime(MimeTypeDetection.UTF16LE.Concat(Encoding.Default.GetBytes(xml)).ToArray(), "text/xml");
+			TestMime(MimeTypeDetection.UTF32BE.Concat(Encoding.Default.GetBytes(xml)).ToArray(), "text/xml");
+			TestMime(MimeTypeDetection.UTF32LE.Concat(Encoding.Default.GetBytes(xml)).ToArray(), "text/xml");
 		}
 
 		void TestMime(byte[] bytes, string expectedMime)