private void prepare() {
      if (!prepared) {
        synchronized (this) {
          if (!prepared) {
            doPrepare();
            prepared = true;
          }
        }
      }
    }
A map from {@code long} to {@code long} stored as a list of keys and values that uses binary search to find the
values for a key. Inserts do not need to be ordered, the first read will sort the array.
planetiler-core/src/main/java/com/onthegomap/planetiler/collection/LongLongMultimap.java