sclass BackupThinner {
  settable File dir;
  
  settable TimeZone timeZone = defaultTimeZone();
  settable long currentTime = now();
  
  // all files in the backup directory
  settable L<File> allFiles;
  
  // all backup files
  // sorted by date (youngest first)
  L<File> files;
  
  // backup files that can be deleted
  gettable L<File> deletableFiles;
  
  Map<File, Double> ageMap;
  
  // age = age in days
  swappable bool shouldKeep(double age, double lastAge) {
    ret defaultAgeBasedBackupRetentionStrategy_shouldKeep(age, lastAge);
  }
  
  void scan {
    if (allFiles == null)
      allFiles = listFilesNotDirs(dir);
      
    deletableFiles = new L;
    files = new L;
    ageMap = new Map;
      
    for (File f : allFiles) {
      var bfa = new BackupFileAge().timeZone(timeZone).currentTime(currentTime);
      if (!bfa.fileName(f.getName()).parse()) continue;
      
      continue unless eq(bfa.originalName, "concepts.structure.gz");
      ageMap.put(f, bfa.ageInDays());
      files.add(f);
    }

    sortByMap_inPlace(files, ageMap);
    double lastAge = -1;
    for (File f : files) {
      double age = ageMap.get(f);
      if (!shouldKeep(age, lastAge))
        deletableFiles.add(f);
      else
        lastAge = age;
    }
  }    
}

Began life as a copy of #1011269