Integer key(String key) {
      Integer i = keys.get(key);
      if (i == null) {
        i = keys.size();
        keys.put(key, i);
      }
      return i;
    }
Returns the ID associated with {@code key} or adds a new one if not present.
planetiler-core/src/main/java/com/onthegomap/planetiler/VectorTile.java