From 3bdf9b0ab8dfbafb52285873410bf743e4df0102 Mon Sep 17 00:00:00 2001
From: Justus Magin <keewis@posteo.de>
Date: Fri, 26 Jul 2024 16:30:13 +0200
Subject: [PATCH 1/4] index and accessor implementations for `parents`

---
 xdggs/accessor.py | 15 +++++++++++++++
 xdggs/index.py    |  3 +++
 2 files changed, 18 insertions(+)

diff --git a/xdggs/accessor.py b/xdggs/accessor.py
index f8b67cc..6ebd9fa 100644
--- a/xdggs/accessor.py
+++ b/xdggs/accessor.py
@@ -104,3 +104,18 @@ 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.
+        """
+        return self.index.parents(resolution)
diff --git a/xdggs/index.py b/xdggs/index.py
index 2c3a7c2..c6a94bc 100644
--- a/xdggs/index.py
+++ b/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

From a387db3e59da16e273f9337062004c769061a9af Mon Sep 17 00:00:00 2001
From: Justus Magin <keewis@posteo.de>
Date: Fri, 26 Jul 2024 16:50:50 +0200
Subject: [PATCH 2/4] return the parents as a `DataArray`

---
 xdggs/accessor.py | 7 ++++++-
 1 file changed, 6 insertions(+), 1 deletion(-)

diff --git a/xdggs/accessor.py b/xdggs/accessor.py
index 6ebd9fa..97fbc12 100644
--- a/xdggs/accessor.py
+++ b/xdggs/accessor.py
@@ -118,4 +118,9 @@ def parents(self, resolution: int) -> xr.DataArray:
         parents : DataArray
             The parent cell ids, one for each input cell.
         """
-        return self.index.parents(resolution)
+        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")

From cf066e55d917d7a69ffe37b831307f0a805f8359 Mon Sep 17 00:00:00 2001
From: Justus Magin <keewis@posteo.de>
Date: Fri, 26 Jul 2024 16:51:19 +0200
Subject: [PATCH 3/4] implement the healpix version of the `parents`
 computation

---
 xdggs/healpix.py | 13 +++++++++++++
 1 file changed, 13 insertions(+)

diff --git a/xdggs/healpix.py b/xdggs/healpix.py
index bb76e32..b5e6579 100644
--- a/xdggs/healpix.py
+++ b/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):

From 6531f599b523ea27dc2bdf868f5cccb03025394b Mon Sep 17 00:00:00 2001
From: Justus Magin <keewis@posteo.de>
Date: Fri, 26 Jul 2024 16:57:34 +0200
Subject: [PATCH 4/4] same for h3

---
 xdggs/h3.py | 9 +++++++++
 1 file changed, 9 insertions(+)

diff --git a/xdggs/h3.py b/xdggs/h3.py
index 17db4ed..56266a9 100644
--- a/xdggs/h3.py
+++ b/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):