added AssetInfo; this is the start of syncing files over netplay

OpenRa.DataStructures/AssetInfo.cs

@@ -0,0 +1,33 @@
+using System.IO;
+using System.Linq;
+using System.Security.Cryptography;
+
+namespace OpenRa
+{
+	class AssetInfo
+	{
+		public readonly string Filename;
+		public readonly string Hash;
+
+		public AssetInfo(string filename, string hash)
+		{
+			Filename = filename;
+			Hash = hash;
+		}
+
+		static string GetHash(string filename)
+		{
+			using (var csp = SHA1.Create())
+				return new string(csp.ComputeHash(File.ReadAllBytes(filename))
+					.SelectMany(a => a.ToString("x2")).ToArray());
+		}
+
+		public static AssetInfo FromLocalFile(string filename)
+		{
+			return new AssetInfo(filename, GetHash(filename));
+		}
+
+		/* todo: perf: cache this */
+		public bool IsPresent() { return File.Exists(Filename) && (Hash == GetHash(Filename)); }
+	}
+}