@Override
  public long estimateMemoryUsageBytes() {
    return values.estimateMemoryUsageBytes() + estimateSize(offsets) + estimateSize(offsetStartPad);
  }
A longlong map that only stores values and uses the key as an index into the array, with some tweaks to avoid storing
many sequential 0's.
planetiler-core/src/main/java/com/onthegomap/planetiler/collection/SparseArrayLongLongMap.java