@Override
    public void put(long key, long val) {
      if (val <= 0) {
        throw new IllegalArgumentException("Invalid value: " + val + " must be >0");
      }
      if (prepared) {
        throw new IllegalArgumentException("Cannot insert after preparing");
      }
      keys.add(key);
      values.add(val);
    }
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