Fixes #10 - add padding if new seekPosition bytes are lower than init…

…ially

MatroskaLib/MatroskaLib.Test/MatroskaLib.Test.csproj

@@ -49,6 +49,10 @@
       <Link>mkv files\TestFile5_MkvProEdit.mkv</Link>
       <CopyToOutputDirectory>PreserveNewest</CopyToOutputDirectory>
     </Content>
+    <Content Include="..\..\TestMkvFiles\mkv files\TestFile6_SmallSeekHead.mkv">
+      <Link>mkv files\TestFile6_SmallSeekHead.mkv</Link>
+      <CopyToOutputDirectory>PreserveNewest</CopyToOutputDirectory>
+    </Content>
     <Content Include="mkvalidator.exe">
       <CopyToOutputDirectory>PreserveNewest</CopyToOutputDirectory>
     </Content>

MatroskaLib/MatroskaLib.Test/MatroskaLibTest.cs

@@ -13,6 +13,9 @@ namespace MatroskaLib.Test;
  *   Only first void with checksum elements
  * MkvProEdit
  *   Only second void and may need to change length of that void
+ * TestFile6_SmallSeekHead.mkv
+ *  SeekHead size of 2, which caused an exception (see github issue #10).
+ *  Do that this is not a valid mkv file accordant to MkValidator.
  */
 public class MatroskaLibTest
 {
@@ -172,4 +175,14 @@ public void WriteTestFile4(string file)
         lsTracks[2].Should().BeEquivalentTo(new { flagDefault = false, flagForced = false, language = "jpn", type = TrackTypeEnum.audio });
         MkvValidator.Validate(TestFilePath);
     }
+
+    [Fact]
+    public void FileWithSeekHeadSizeOf2ShouldNotThrow()
+    {
+        File.Copy("mkv files/TestFile6_SmallSeekHead.mkv", TestFilePath, true);
+        List<MkvFile> lsMkvFiles = MatroskaReader.ReadMkvFiles(new[] { TestFilePath });
+        lsMkvFiles[0].tracks[0].flagDefault = false;
+
+        MatroskaWriter.WriteMkvFile(lsMkvFiles[0]);
+    }
 }

MatroskaLib/MatroskaLib/Helpers/ByteHelper.cs

@@ -57,7 +57,8 @@ public static void ChangeLength(List<byte> lsBytes, int position, List<byte> lsL
 
             // Convert new length to bytes and strip bytes
             List<byte> lsNewBytes = ToBytes(ret);
-            if (lsNewBytes.Count != lsLengthBytes.Count) throw new Exception("New length doesn't fit into existing length element");
+            if (lsNewBytes.Count != lsLengthBytes.Count) 
+                throw new InvalidOperationException($"New length bytes are not the same length as the old ones. Old length: {lsLengthBytes.Count}, new length: {lsNewBytes.Count}");
 
             // Replace old length with new length bytes
             lsBytes.RemoveRange(position, lsNewBytes.Count);

MatroskaLib/MatroskaLib/Helpers/CustomExtensions.cs

@@ -16,7 +16,7 @@ public static void LocateElement(this EbmlReader reader, ulong descriptor)
                 }
             }
 
-            throw new Exception($"Cannot find descriptor 0x{descriptor:X}");
+            throw new InvalidOperationException($"Cannot find descriptor 0x{descriptor:X}");
         }
     }
 }

MatroskaLib/MatroskaLib/MatroskaWriter.cs

@@ -72,10 +72,14 @@ private static void _ChangeVoidLengthAndHeaders(List<Seek> seekList, int? seekHe
             foreach (var s in seekList.Where(s =>
                          s.seekId is MatroskaElements.Tracks or MatroskaElements.SegmentInfo))
             {
-                int desiredLength = Convert.ToInt32(lsBytes[s.seekPositionByteNumber - 1] - 0x80);
                 List<byte> lsNewBytes = ByteHelper.ToBytes(s.seekPosition - (ulong)offset);
-                if (desiredLength != lsNewBytes.Count)
-                    throw new Exception("New seekposition doesn't fit into existing element");
+                if (lsNewBytes.Count > s.elementLength)
+                    throw new InvalidOperationException($"New seekPosition bytes are bigger than the old one. Trying to fit {lsNewBytes.Count} bytes into {s.elementLength} bytes");
+                if (lsNewBytes.Count < s.elementLength)
+                {
+                    // The new seekPosition is smaller than the old one, add padding
+                    lsNewBytes.AddRange(new byte[s.elementLength - lsNewBytes.Count]);
+                }
 
                 lsBytes.RemoveRange(s.seekPositionByteNumber, lsNewBytes.Count);
                 lsBytes.InsertRange(s.seekPositionByteNumber, lsNewBytes);

MatroskaLib/MatroskaLib/Types/Seek.cs

@@ -9,6 +9,7 @@ public class Seek
     public ulong seekId { get; private set; }
     public ulong seekPosition { get; private set; }
     public int seekPositionByteNumber { get; private set; }
+    public int elementLength { get; private set; }
 
     public Seek(EbmlReader reader) =>
         _reader = reader;
@@ -23,6 +24,7 @@ public void ApplyElement(FileStream fileStream)
         {
             seekPositionByteNumber = (int)fileStream.Position;
             seekPosition = _reader.ReadUInt();
+            elementLength = (int)_reader.ElementSize;
         }
     }
 }