public int hilbertEncoded() {
    return startIndexForZoom(this.z) +
      Hilbert.hilbertXYToIndex(this.z, this.x, this.y);
  }
Return the equivalent tile index using Hilbert ordering on a single level instead of TMS.
planetiler-core/src/main/java/com/onthegomap/planetiler/geo/TileCoord.java