public PreviousBookmarkStartedOnLastFile()
                    {
                        var bookmarkProvider = Substitute.For <IBookmarkProvider>();

                        bookmarkProvider
                        .When(x => x.UpdateBookmark(Arg.Any <FileSetPosition>()))
                        .Do(x => _sut = x.ArgAt <FileSetPosition>(0));
                        bookmarkProvider.GetCurrentBookmarkPosition().Returns(new FileSetPosition(0, @"c:\a\file001.json"));

                        IFileSystemAdapter fsAdapter = CreateFileSystemAdapter();

                        var provider = new FileBufferDataProviderThatAllowscurrentBookmarkToBeSet(
                            BaseBufferFileName,
                            fsAdapter,
                            bookmarkProvider,
                            Utf8Encoder,
                            BatchLimit,
                            EventSizeLimit,
                            null);

                        //force the current Bookmark to be current file:
                        provider.DefineCurrentBookmark(123, @"c:\a\file003.json");

                        //excercise the SUT
                        provider.MoveBookmarkForward();
                    }
                public RetentionLimitLessThenNumberOfBufferFilesAndBookMarkOnSecondFile()
                {
                    var bookmarkProvider = Substitute.For <IBookmarkProvider>();

                    bookmarkProvider
                    .When(x => x.UpdateBookmark(Arg.Any <FileSetPosition>()))
                    .Do(x => _sut = x.ArgAt <FileSetPosition>(0));

                    IFileSystemAdapter fsAdapter = CreateFileSystemAdapter();

                    var provider = new FileBufferDataProviderThatAllowscurrentBookmarkToBeSet(
                        BaseBufferFileName,
                        fsAdapter,
                        bookmarkProvider,
                        Utf8Encoder,
                        BatchLimit,
                        EventSizeLimit,
                        Limit);

                    //force the current Bookmark to be second file:
                    provider.DefineCurrentBookmark(123, @"c:\a\file002.json");

                    provider.MoveBookmarkForward();
                }