The S2 curve is based on the Hilbert curveThe Hilbert curve is a function from the unit interval [0,1] to the unit square [0,1]×[0,1] that is space-filling, meaning that it visits every point in the unit square S2 CellsThe S2 library defines a framework for decomposing the unit sphere into a hierarchy of cells. Each cell is a quadrilateral bounded by four geodesics. The top level of the hierarchy is obtained by projecting the six faces of a cube onto the unit sphere, and lower levels are obtained by subdividing each cell into four children recursively.https://s2geometry.io/devguide/s2cell_hierarchy.html#s2cellid-numbering
