Files with no content should have no piece root.

WHen checking the hash of piece 0, we should skip past files
with a length of 0. This is important as if the file is smaller
than PieceLength in size, the 'PieceRoot' of the file is used
instead of the piece hashes, and so we need to ensure we pick
the pieces root from the file which has the actual content.

src/MonoTorrent.Client/MonoTorrent/PieceHashesV2.cs

@@ -74,7 +74,8 @@ public ReadOnlyPieceHash GetHash (int hashIndex)
                 throw new ArgumentOutOfRangeException (nameof (hashIndex), $"Value must be grater than or equal to '0' and less than '{Count}'");
 
             for (int i = 0; i < Files.Count; i++) {
-                if (hashIndex < Files[i].StartPieceIndex || hashIndex > Files[i].EndPieceIndex)
+                // Empty files have no root hash, as they have no data. Skip them.
+                if (hashIndex < Files[i].StartPieceIndex || hashIndex > Files[i].EndPieceIndex || Files[i].Length == 0)
                     continue;
 
                 // If the file has 2 or more pieces then we'll need to grab the appropriate sha from the layer

src/Tests/Tests.MonoTorrent.Client/MonoTorrent/PieceHashesTests.cs

@@ -27,11 +27,14 @@
 //
 
 
+using System;
 using System.Linq;
 
+using MonoTorrent.BEncoding;
+
 using NUnit.Framework;
 
-namespace MonoTorrent.Common
+namespace MonoTorrent
 {
     [TestFixture]
     public class PieceHashesTests
@@ -56,5 +59,20 @@ public void V2Only_NoHashes_Count ()
         {
             Assert.AreEqual (0, new PieceHashes (null, null).Count);
         }
+
+        [Test]
+        public void V2Only_WithEmptyFiles ()
+        {
+            var files = new[] {
+                new TorrentFile("a", length: 0, startIndex: 0, endIndex: 0, 0, TorrentFileAttributes.None, 0),
+                new TorrentFile("b", length: 1, startIndex: 0, endIndex: 0, 0, new MerkleRoot(Enumerable.Repeat<byte>(1, 32).ToArray ()), TorrentFileAttributes.None, 0),
+                new TorrentFile("c", length: 2, startIndex: 1, endIndex: 1, 1, new MerkleRoot(Enumerable.Repeat<byte>(2, 32).ToArray ()), TorrentFileAttributes.None, 0),
+            };
+
+            var pieceHashesV2 = new PieceHashesV2 (Constants.BlockSize * 4, files, new BEncodedDictionary ());
+            var hash = pieceHashesV2.GetHash (0);
+            Assert.IsTrue (files[1].PiecesRoot.AsMemory ().Span.SequenceEqual (hash.V2Hash.Span));
+            Assert.IsTrue (hash.V1Hash.IsEmpty);
+        }
     }
 }