hierarchical operations on cell ids: parents

xdggs/accessor.py

@@ -104,3 +104,23 @@ def cell_centers(self):
                 "longitude": (self.index._dim, lon_data),
             }
         )
+
+    def parents(self, resolution: int) -> xr.DataArray:
+        """determine the parent cell ids of the cells
+
+        Parameters
+        ----------
+        resolution : int
+            The parent resolution. Must be smaller than the current resolution.
+
+        Returns
+        -------
+        parents : DataArray
+            The parent cell ids, one for each input cell.
+        """
+        data = self.index.parents(resolution)
+
+        params = self.grid_info.to_dict()
+        params["resolution"] = resolution
+
+        return self.coord.copy(data=data).assign_attrs(**params).rename("parents")

xdggs/h3.py

@@ -9,6 +9,7 @@
 
 import numpy as np
 import xarray as xr
+from h3ronpy.arrow import change_resolution
 from h3ronpy.arrow.vector import cells_to_coordinates, coordinates_to_cells
 from xarray.indexes import PandasIndex
 
@@ -45,6 +46,14 @@ def cell_ids2geographic(
     def geographic2cell_ids(self, lon, lat):
         return coordinates_to_cells(lat, lon, self.resolution, radians=False)
 
+    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

@@ -135,6 +135,19 @@ def cell_ids2geographic(self, cell_ids):
     def geographic2cell_ids(self, lon, lat):
         return healpy.ang2pix(self.nside, lon, lat, lonlat=True, nest=self.nest)
 
+    def parents(self, cell_ids, resolution):
+        if resolution >= self.resolution:
+            raise ValueError(
+                f"resolution is not a parent: {resolution} >= {self.resolution}"
+            )
+
+        offset = self.resolution - resolution
+        x, y, f = healpy.pix2xyf(self.nside, cell_ids, nest=self.nest)
+        x_ = x >> offset
+        y_ = y >> offset
+
+        return healpy.xyf2pix(2**resolution, x_, y_, f, nest=self.nest)
+
 
 @register_dggs("healpix")
 class HealpixIndex(DGGSIndex):

xdggs/index.py

@@ -72,6 +72,9 @@ def _replace(self, new_pd_index: PandasIndex):
     def cell_centers(self) -> tuple[np.ndarray, np.ndarray]:
         return self._grid.cell_ids2geographic(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