hierarchical operations on cell ids: parents

xdggs/accessor.py

@@ -174,6 +174,26 @@ def cell_boundaries(self):
             boundaries, coords={self._name: self.cell_ids}, dims=self.cell_ids.dims
         )
 
+    def parents(self, level: int) -> xr.DataArray:
+        """determine the parent cell ids of the cells
+
+        Parameters
+        ----------
+        level : int
+            The parent resolution level. Must be smaller than the current resolution.
+
+        Returns
+        -------
+        parents : DataArray
+            The parent cell ids, one for each input cell.
+        """
+        data = self.index.parents(level)
+
+        params = self.grid_info.to_dict()
+        params["resolution"] = level
+
+        return self.coord.copy(data=data).assign_attrs(**params).rename("parents")
+
     def explore(self, *, cmap="viridis", center=None, alpha=None):
         """interactively explore the data using `lonboard`
 

xdggs/h3.py

@@ -201,6 +201,14 @@ def cell_boundaries(self, cell_ids, backend="shapely"):
             raise ValueError("invalid backend: {backend!r}")
         return backend_func(wkb)
 
+    def parents(self, cell_ids, resolution):
+        if resolution >= self.resolution:
+            raise ValueError(
+                f"resolution is not a parent: {resolution} >= {self.resolution}"
+            )
+
+        return change_resolution(cell_ids, resolution)
+
 
 @register_dggs("h3")
 class H3Index(DGGSIndex):

xdggs/healpix.py

@@ -308,6 +308,18 @@ def cell_boundaries(self, cell_ids: Any, backend="shapely") -> np.ndarray:
 
         return backend_func(vertices)
 
+    def parents(self, cell_ids, level):
+        if level >= self.level:
+            raise ValueError(f"given level is not a parent: {level} >= {self.level}")
+
+        # TODO: reimplement using `cdshealpix`
+        offset = self.level - level
+        x, y, f = healpy.pix2xyf(self.nside, cell_ids, nest=self.nest)
+        x_ = x >> offset
+        y_ = y >> offset
+
+        return healpy.xyf2pix(self.nside, x_, y_, f, nest=self.nest)
+
 
 @register_dggs("healpix")
 class HealpixIndex(DGGSIndex):

xdggs/index.py

@@ -95,6 +95,9 @@ def cell_centers(self) -> tuple[np.ndarray, np.ndarray]:
     def cell_boundaries(self) -> np.ndarray:
         return self.grid_info.cell_boundaries(self._pd_index.index.values)
 
+    def parents(self, resolution: int) -> np.ndarray:
+        return self._grid.parents(self._pd_index.index.values, resolution=resolution)
+
     @property
     def grid_info(self) -> DGGSInfo:
         return self._grid