private void InitializeFromOneFile()

in src/log4net/Appender/RollingFileAppender.cs [923:1001]
61 lines of code
15 McCabe index (conditional complexity)

		private void InitializeFromOneFile(string baseFile, string curFileName)
		{
			curFileName = curFileName.ToLower();
			baseFile = baseFile.ToLower();
            if (curFileName.StartsWith(Path.GetFileNameWithoutExtension(baseFile)) == false)
			{
				// This is not a log file, so ignore
				return;
			}
			if (curFileName.Equals(baseFile)) 
			{
				// Base log file is not an incremented logfile (.1 or .2, etc)
				return;
			}
	
			// Only look for files in the current roll point
			if (m_rollDate && !m_staticLogFileName)
			{
				var date = m_dateTime.Now.ToString(m_datePattern, DateTimeFormatInfo.InvariantInfo).ToLower();
				var prefix = (m_preserveLogFileNameExtension 
					? Path.GetFileNameWithoutExtension(baseFile) + date 
					: baseFile + date).ToLower();
				var suffix = m_preserveLogFileNameExtension 
					? Path.GetExtension(baseFile).ToLower()
					: "";
				if (!curFileName.StartsWith(prefix) || !curFileName.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
                var 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 > m_curSizeRollBackups)
                {
                    if (0 == m_maxSizeRollBackups)
                    {
                        // Stay at zero when zero backups are desired
                    }
                    else if (-1 == m_maxSizeRollBackups)
                    {
                        // Infinite backups, so go as high as the highest value
                        m_curSizeRollBackups = backup;
                    }
                    else
                    {
                        // Backups limited to a finite number
                        if (m_countDirection >= 0)
                        {
                            // Go with the highest file when counting up
                            m_curSizeRollBackups = backup;
                        }
                        else
                        {
                            // Clip to the limit when counting down
                            if (backup <= m_maxSizeRollBackups)
                            {
                                m_curSizeRollBackups = backup;
                            }
                        }
                    }
                    LogLog.Debug(declaringType, "File name [" + curFileName + "] moves current count to [" + m_curSizeRollBackups + "]");
                }
			} 
			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+"]");
			}
		}