private void InitializeFromOneFile()

in src/log4net/Appender/RollingFileAppender.cs [790:870]
63 lines of code
15 McCabe index (conditional complexity)

  private void InitializeFromOneFile(string baseFile, string curFileName)
  {
    curFileName = curFileName.ToLowerInvariant();
    baseFile = baseFile.ToLowerInvariant();
    var baseFileWithoutExtension = Path.Combine(Path.GetDirectoryName(baseFile) ?? "", Path.GetFileNameWithoutExtension(baseFile));
    
    if (curFileName.StartsWith(baseFileWithoutExtension) == false)
    {
      return; // This is not a log file, so ignore
    }

    if (curFileName.Equals(baseFile, StringComparison.Ordinal))
    {
      return; // Base log file is not an incremented logfile (.1 or .2, etc.)
    }

    // Only look for files in the current roll point
    if (_rollDate && !StaticLogFileName)
    {
      string date = DateTimeStrategy.Now.ToString(DatePattern, DateTimeFormatInfo.InvariantInfo).ToLowerInvariant();
      string prefix = (PreserveLogFileNameExtension
        ? Path.GetFileNameWithoutExtension(baseFile) + date
        : baseFile + date).ToLowerInvariant();
      string suffix = PreserveLogFileNameExtension
        ? Path.GetExtension(baseFile).ToLowerInvariant()
        : "";
      var curFileNameWithoutDir = Path.GetFileName(curFileName);
      if (!curFileNameWithoutDir.StartsWith(prefix) || !curFileNameWithoutDir.EndsWith(suffix))
      {
        LogLog.Debug(_declaringType, $"Ignoring file [{curFileName}] because it is from a different date period");
        return;
      }
    }

    try
    {
      // Bump the counter up to the highest count seen so far
      int backup = GetBackupIndex(curFileName);

      // caution: we might get a false positive when certain
      // date patterns such as yyyyMMdd are used...those are
      // valid number but aren't the kind of back up index
      // we're looking for
      if (backup > CurrentSizeRollBackups)
      {
        if (0 == MaxSizeRollBackups)
        {
          // Stay at zero when zero backups are desired
        }
        else if (-1 == MaxSizeRollBackups)
        {
          // Infinite backups, so go as high as the highest value
          CurrentSizeRollBackups = backup;
        }
        else
        {
          // Backups limited to a finite number
          if (CountDirection >= 0)
          {
            // Go with the highest file when counting up
            CurrentSizeRollBackups = backup;
          }
          else
          {
            // Clip to the limit when counting down
            if (backup <= MaxSizeRollBackups)
            {
              CurrentSizeRollBackups = backup;
            }
          }
        }
        LogLog.Debug(_declaringType, $"File name [{curFileName}] moves current count to [{CurrentSizeRollBackups}]");
      }
    }
    catch (FormatException)
    {
      //this happens when file.log -> file.log.yyyy-MM-dd which is normal
      //when staticLogFileName == false
      LogLog.Debug(_declaringType, $"Encountered a backup file not ending in .x [{curFileName}]");
    }
  }