// possible problem: case of keys (Windows only)
static class DiskTextMap {
  File dir;
  
  *(S dirName) {
    dir = getProgramFile(dirName);
    dir.mkdirs();
  }
  
  void clear() {
    deleteAllFilesWithSuffix(dir, ".text");
  }
  
  S get(S key) {
    ret loadTextFile(getFile(key));
  }
  
  File getFile(S key) {
    ret new File(dir, urlencode(key) + ".text");
  }
  
  void remove(S key) {
    getFile(key).delete();
  }
  
  void put(S key, S value) {
    saveTextFile(getFile(key), value);
  }
  
  int size() {
    ret l(listFilesWithSuffix(dir, ".text"));
  }
  
  Set<S> keySet() {
    new TreeSet<S> set;
    for (File f : listFilesWithSuffix(dir, ".text"))
      set.add(dropSuffix(".text", f.getName()));
    ret set;
  }
}