Spherical padding and faster tests

src/xarray_regrid/methods/conservative.py

@@ -122,15 +122,13 @@ def conservative_regrid_dataset(
             weights = apply_spherical_correction(weights, latitude_coord)
 
         for array in data_vars.keys():
-            non_grid_dims = [d for d in data_vars[array].dims if d not in coords]
             if coord in data_vars[array].dims:
                 data_vars[array], valid_fracs[array] = apply_weights(
                     da=data_vars[array],
                     weights=weights,
                     coord=coord,
                     valid_frac=valid_fracs[array],
                     skipna=skipna,
-                    non_grid_dims=non_grid_dims,
                 )
                 # Mask out any regridded points outside the original domain
                 data_vars[array] = data_vars[array].where(covered_grid)
@@ -161,16 +159,13 @@ def apply_weights(
     coord: Hashable,
     valid_frac: xr.DataArray,
     skipna: bool,
-    non_grid_dims: list[Hashable],
 ) -> tuple[xr.DataArray, xr.DataArray]:
     """Apply the weights to convert data to the new coordinates."""
     coord_map = {f"target_{coord}": coord}
     weights_norm = weights.copy()
 
     if skipna:
         notnull = da.notnull()
-        if non_grid_dims:
-            notnull = notnull.any(non_grid_dims)
         # Renormalize the weights along this dim by the accumulated valid_frac
         # along previous dimensions
         if valid_frac.name != EMPTY_DA_NAME:

src/xarray_regrid/regrid.py

@@ -1,6 +1,7 @@
 import xarray as xr
 
 from xarray_regrid.methods import conservative, interp, most_common
+from xarray_regrid.utils import format_for_regrid
 
 
 @xr.register_dataarray_accessor("regrid")
@@ -34,7 +35,8 @@ def linear(
             Data regridded to the target dataset coordinates.
         """
         ds_target_grid = validate_input(self._obj, ds_target_grid, time_dim)
-        return interp.interp_regrid(self._obj, ds_target_grid, "linear")
+        ds_formatted = format_for_regrid(self._obj, ds_target_grid)
+        return interp.interp_regrid(ds_formatted, ds_target_grid, "linear")
 
     def nearest(
         self,
@@ -51,14 +53,14 @@ def nearest(
             Data regridded to the target dataset coordinates.
         """
         ds_target_grid = validate_input(self._obj, ds_target_grid, time_dim)
-        return interp.interp_regrid(self._obj, ds_target_grid, "nearest")
+        ds_formatted = format_for_regrid(self._obj, ds_target_grid)
+        return interp.interp_regrid(ds_formatted, ds_target_grid, "nearest")
 
     def cubic(
         self,
         ds_target_grid: xr.Dataset,
         time_dim: str = "time",
     ) -> xr.DataArray | xr.Dataset:
-        ds_target_grid = validate_input(self._obj, ds_target_grid, time_dim)
         """Regrid to the coords of the target dataset with cubic interpolation.
 
         Args:
@@ -68,7 +70,9 @@ def cubic(
         Returns:
             Data regridded to the target dataset coordinates.
         """
-        return interp.interp_regrid(self._obj, ds_target_grid, "cubic")
+        ds_target_grid = validate_input(self._obj, ds_target_grid, time_dim)
+        ds_formatted = format_for_regrid(self._obj, ds_target_grid)
+        return interp.interp_regrid(ds_formatted, ds_target_grid, "cubic")
 
     def conservative(
         self,
@@ -88,6 +92,9 @@ def conservative(
             time_dim: The name of the time dimension/coordinate.
             skipna: If True, enable handling for NaN values. This adds some overhead,
                 so can be disabled for optimal performance on data without any NaNs.
+                With `skipna=True, chunking is recommended in the non-grid dimensions,
+                otherwise the intermediate arrays that track the fraction of valid data
+                can become very large and consume excessive memory.
                 Warning: with `skipna=False`, isolated NaNs will propagate throughout
                 the dataset due to the sequential regridding scheme over each dimension.
             nan_threshold: Threshold value that will retain any output points
@@ -104,8 +111,9 @@ def conservative(
             raise ValueError(msg)
 
         ds_target_grid = validate_input(self._obj, ds_target_grid, time_dim)
+        ds_formatted = format_for_regrid(self._obj, ds_target_grid)
         return conservative.conservative_regrid(
-            self._obj, ds_target_grid, latitude_coord, skipna, nan_threshold
+            ds_formatted, ds_target_grid, latitude_coord, skipna, nan_threshold
         )
 
     def most_common(
@@ -134,8 +142,9 @@ def most_common(
             Regridded data.
         """
         ds_target_grid = validate_input(self._obj, ds_target_grid, time_dim)
+        ds_formatted = format_for_regrid(self._obj, ds_target_grid)
         return most_common.most_common_wrapper(
-            self._obj, ds_target_grid, time_dim, max_mem
+            ds_formatted, ds_target_grid, time_dim, max_mem
         )
 
 

src/xarray_regrid/utils.py

@@ -1,6 +1,6 @@
-from collections.abc import Callable
+from collections.abc import Callable, Hashable
 from dataclasses import dataclass
-from typing import Any, overload
+from typing import Any, TypedDict, overload
 
 import numpy as np
 import pandas as pd
@@ -10,6 +10,11 @@
 class InvalidBoundsError(Exception): ...
 
 
+class CoordHandler(TypedDict):
+    names: list[str]
+    func: Callable
+
+
 @dataclass
 class Grid:
     """Object storing grid information."""
@@ -75,7 +80,7 @@ def create_lat_lon_coords(grid: Grid) -> tuple[np.ndarray, np.ndarray]:
             grid.south, grid.north + grid.resolution_lat, grid.resolution_lat
         )
 
-    if np.remainder((grid.north - grid.south), grid.resolution_lat) > 0:
+    if np.remainder((grid.east - grid.west), grid.resolution_lat) > 0:
         lon_coords = np.arange(grid.west, grid.east, grid.resolution_lon)
     else:
         lon_coords = np.arange(
@@ -235,3 +240,151 @@ def call_on_dataset(
         return next(iter(result.data_vars.values())).rename(obj.name)
 
     return result
+
+
+def format_for_regrid(
+    obj: xr.DataArray | xr.Dataset, target: xr.Dataset
+) -> xr.DataArray | xr.Dataset:
+    """Apply any pre-formatting to the input dataset to prepare for regridding.
+    Currently handles padding of spherical geometry if lat/lon coordinates can
+    be inferred and the domain size requires boundary padding.
+    """
+    orig_chunksizes = obj.chunksizes
+
+    # Special-cased coordinates with accepted names and formatting function
+    coord_handlers: dict[str, CoordHandler] = {
+        "lat": {"names": ["lat", "latitude"], "func": format_lat},
+        "lon": {"names": ["lon", "longitude"], "func": format_lon},
+    }
+    # Identify coordinates that need to be formatted
+    formatted_coords = {}
+    for coord_type, handler in coord_handlers.items():
+        for coord in obj.coords.keys():
+            if str(coord).lower() in handler["names"]:
+                formatted_coords[coord_type] = str(coord)
+
+    # Apply formatting
+    for coord_type, coord in formatted_coords.items():
+        # Make sure formatted coords are sorted
+        obj = obj.sortby(coord)
+        target = target.sortby(coord)
+        obj = coord_handlers[coord_type]["func"](obj, target, formatted_coords)
+        # Coerce back to a single chunk if that's what was passed
+        if len(orig_chunksizes.get(coord, [])) == 1:
+            obj = obj.chunk({coord: -1})
+
+    return obj
+
+
+def format_lat(
+    obj: xr.DataArray | xr.Dataset,
+    target: xr.Dataset,  # noqa ARG001
+    formatted_coords: dict[str, str],
+) -> xr.DataArray | xr.Dataset:
+    """If the latitude coordinate is inferred to be global, defined as having
+    a value within one grid spacing of the poles, and the grid does not natively
+    have values at -90 and 90, add a single value at each pole computed as the
+    mean of the first and last latitude bands. This should be roughly equivalent
+    to the `Pole="all"` option in `ESMF`.
+
+    For example, with a grid spacing of 1 degree, and a source grid ranging from
+    -89.5 to 89.5, the poles would be padded with values at -90 and 90. A grid ranging
+    from -88 to 88 would not be padded because coverage does not extend all the way
+    to the poles. A grid ranging from -90 to 90 would also not be padded because the
+    poles will already be covered in the regridding weights.
+    """
+    lat_coord = formatted_coords["lat"]
+    lon_coord = formatted_coords.get("lon")
+
+    # Concat a padded value representing the mean of the first/last lat bands
+    # This should match the Pole="all" option of ESMF
+    # TODO: with cos(90) = 0 weighting, these weights might be 0?
+
+    polar_lat = 90
+    dy = obj.coords[lat_coord].diff(lat_coord).max().values.item()
+
+    # Only pad if global but don't have edge values directly at poles
+    # NOTE: could use xr.pad here instead of xr.concat, but none of the
+    # modes are an exact fit for this scheme
+    # South pole
+    if dy - polar_lat >= obj.coords[lat_coord].values[0] > -polar_lat:
+        south_pole = obj.isel({lat_coord: 0})
+        if lon_coord is not None:
+            south_pole = south_pole.mean(lon_coord)
+        obj = xr.concat([south_pole, obj], dim=lat_coord)  # type: ignore
+        obj.coords[lat_coord].values[0] = -polar_lat
+
+    # North pole
+    if polar_lat - dy <= obj.coords[lat_coord].values[-1] < polar_lat:
+        north_pole = obj.isel({lat_coord: -1})
+        if lon_coord is not None:
+            north_pole = north_pole.mean(lon_coord)
+        obj = xr.concat([obj, north_pole], dim=lat_coord)  # type: ignore
+        obj.coords[lat_coord].values[-1] = polar_lat
+
+    return obj
+
+
+def format_lon(
+    obj: xr.DataArray | xr.Dataset, target: xr.Dataset, formatted_coords: dict[str, str]
+) -> xr.DataArray | xr.Dataset:
+    """Format the longitude coordinate by shifting the source grid to line up with
+    the target anywhere in the range of -360 to 360, and then add a single wraparound
+    padding column if the domain is inferred to be global and the east or west edges
+    of the target lie outside the source grid centers.
+
+    For example, with a source grid ranging from 0.5 to 359.5 and a target grid ranging
+    from -180 to 180, the source grid would be shifted to -179.5 to 179.5 and then
+    padded on both the left and right with wraparound values at -180.5 and 180.5 to
+    provide full coverage for the target edge cells at -180 and 180.
+    """
+    lon_coord = formatted_coords["lon"]
+
+    # Find a wrap point outside of the left and right bounds of the target
+    # This ensures we have coverage on the target and handles global > regional
+    source_vals = obj.coords[lon_coord].values
+    target_vals = target.coords[lon_coord].values
+    wrap_point = (target_vals[-1] + target_vals[0] + 360) / 2
+    source_vals = np.where(
+        source_vals < wrap_point - 360, source_vals + 360, source_vals
+    )
+    source_vals = np.where(source_vals > wrap_point, source_vals - 360, source_vals)
+    obj.coords[lon_coord].values[:] = source_vals
+
+    # Shift operations can produce duplicates
+    # Simplest solution is to drop them and add back when padding
+    obj = obj.sortby(lon_coord).drop_duplicates(lon_coord)
+
+    # Only pad if domain is global in lon
+    source_lon = obj.coords[lon_coord]
+    target_lon = target.coords[lon_coord]
+    dx_s = source_lon.diff(lon_coord).max().values.item()
+    dx_t = target_lon.diff(lon_coord).max().values.item()
+    is_global_lon = source_lon.max().values - source_lon.min().values >= 360 - dx_s
+
+    if is_global_lon:
+        left_pad = (source_lon.values[0] - target_lon.values[0] + dx_t / 2) / dx_s
+        right_pad = (target_lon.values[-1] - source_lon.values[-1] + dx_t / 2) / dx_s
+        left_pad = int(np.ceil(np.max([left_pad, 0])))
+        right_pad = int(np.ceil(np.max([right_pad, 0])))
+        obj = obj.pad({lon_coord: (left_pad, right_pad)}, mode="wrap", keep_attrs=True)
+        if left_pad:
+            obj.coords[lon_coord].values[:left_pad] = (
+                source_lon.values[-left_pad:] - 360
+            )
+        if right_pad:
+            obj.coords[lon_coord].values[-right_pad:] = (
+                source_lon.values[:right_pad] + 360
+            )
+
+    return obj
+
+
+def coord_is_covered(
+    obj: xr.DataArray | xr.Dataset, target: xr.Dataset, coord: Hashable
+) -> bool:
+    """Check if the source coord fully covers the target coord."""
+    pad = target[coord].diff(coord).max().values
+    left_covered = obj[coord].min() <= target[coord].min() - pad
+    right_covered = obj[coord].max() >= target[coord].max() + pad
+    return bool(left_covered.item() and right_covered.item())