sclass BackupThinner {

  settable File dir;

  settable TimeZone timeZone = defaultTimeZone();

  settable long currentTime = now();

  settable L<File> allFiles;

  gettable L<File> deletableFiles;

  L<File> files;

  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;

    }

  }    

}