sclass FileBasedStringMap extends AbstractMap<S> {

  File dir;

  bool compress; // use .gz

  *() {}

  *(File *dir) {}

  public S get(O a) {

    File f = fileForKey(a);

    ret dropFirstLine(compress ? loadGZTextFile(f) : loadTextFile(f));

  }

  public synchronized Set<Map.Entry<S,S>> entrySet() {

    Set<Map.Entry<S,S>> set = new HashSet;

    for (File f : listFiles(dir)) {

      S md5 = md5FromUniqueFile(f);

      if (md5 == null) continue;

      fS key = unquote(compress ? firstLineFromGZTextFile(f) : firstLineFromFile(f));

      set.add(new Map.Entry<S,S>() {

        public S getKey() { ret key; }

        public S getValue() { ret get(key); }

        public S setValue(S s) { S old = get(key); put(key, s); ret old; }

        public bool equals(O o) {

          if (!(o instanceof Map.Entry)) false;

          Map.Entry e = (Map.Entry) o;

          ret eq(getKey(), e.getKey()) && eq(getValue(), e.getValue());

        }

        public int hashCode() {

          S s = getValue();

          ret (getKey()==null ? 0 : getKey().hashCode())

            ^ (s == null ? 0 : s.hashCode());

        }

      });

    }

    ret set;

  }

  public S put(S a, S b) {

    S old = get(a);

    S text = b == null ? null : quote(a) + "\n" + b;

    if (compress)

      saveGZTextFile(fileForKey(a), text);

    else

      saveTextFile(fileForKey(a), text);

    ret old;

  }

  public S remove(O a) {

    if (!isString(a)) null;

    ret put((S) a, null);

  }

  File fileForKey(O key) {

    if (!isString(key)) null;

    ret new File(assertNotNull(dir), uniqueFileNameUsingMD5_80((S) key) + (compress ? ".gz" : ".txt"));

  }

}