S2CellUnion Expand for extract operation about osmexpress HOT 4 CLOSED

  // Expands the cell union by adding a buffer of cells at "expand_level"
  // around the union boundary.
  //
  // For each cell "c" in the union, we add all neighboring cells at level
  // "expand_level" that are adjacent to "c".  Note that there can be many
  // such cells if "c" is large compared to "expand_level".  If "c" is smaller
  // than "expand_level", we first add the parent of "c" at "expand_level" and
  // then add all the neighbors of that cell.
  //
  // Note that the size of the output is exponential in "expand_level".  For
  // example, if expand_level == 20 and the input has a cell at level 10,
  // there will be on the order of 4000 adjacent cells in the output.  For
  // most applications the Expand(min_radius, max_level_diff) method below is
  // easier to use.
  void Expand(int expand_level);

  // Expands the cell union such that it contains all points whose distance to
  // the cell union is at most "min_radius", but do not use cells that are
  // more than "max_level_diff" levels higher than the largest cell in the
  // input.  The second parameter controls the tradeoff between accuracy and
  // output size when a large region is being expanded by a small amount
  // (e.g. expanding Canada by 1km).  For example, if max_level_diff == 4 the
  // region will always be expanded by approximately 1/16 the width of its
  // largest cell.  Note that in the worst case, the number of cells in the
  // output can be up to 4 * (1 + 2 ** max_level_diff) times larger than the
  // number of cells in the input.
  void Expand(S1Angle min_radius, int max_level_diff);