function linearRingss(flatCoordinates, offset, endss, stride) {
  const flatCenters = [];
  let extent = createEmpty();
  for (let i = 0, ii = endss.length; i < ii; ++i) {
    const ends = endss[i];
    extent = createOrUpdateFromFlatCoordinates(
      flatCoordinates,
      offset,
      ends[0],
      stride,
    );
    flatCenters.push((extent[0] + extent[2]) / 2, (extent[1] + extent[3]) / 2);
    offset = ends[ends.length - 1];
  }
  return flatCenters;
}
@param {Array<number>} flatCoordinates Flat coordinates.
@param {number} offset Offset.
@param {Array<Array<number>>} endss Endss.
@param {number} stride Stride.
@return {Array<number>} Flat centers.