Support reversing with isel (#48)

Author: dcherian

src/rasterix/raster_index.py

@@ -201,14 +201,10 @@ def generate_coords(self, dims: tuple[str, ...] | None = None) -> dict[Hashable,
         return self.forward({self.dim: np.arange(self.size)})
 
     def slice(self, slice: slice) -> AxisAffineTransform:
-        start = max(slice.start or 0, 0)
-        stop = min(slice.stop or self.size, self.size)
-        step = slice.step or 1
-
-        # TODO: support reverse transform (i.e., start > stop)?
-        assert start < stop
-
-        size = (stop - start) // step
+        newrange = range(self.size)[slice]
+        start = newrange.start
+        step = newrange.step or 1
+        size = len(newrange)
         scale = float(step)
 
         if self.is_xaxis:

tests/test_raster_index.py

@@ -69,6 +69,18 @@ def test_sel_slice():
     assert actual_transform == actual.rio.transform()
     assert actual_transform == (transform * Affine.translation(0, 3))
 
+    reverse = ds.isel(y=slice(None, None, -1))
+    assert_identical(reverse.y, ds.y[::-1])
+
+    reverse = ds.isel(y=slice(8, 5, -1))
+    assert_identical(reverse.y, ds.y[8:5:-1])
+
+    reverse = ds.isel(y=slice(8, None, -1))
+    assert_identical(reverse.y, ds.y[8::-1])
+
+    reverse = ds.isel(y=slice(None, 5, -1))
+    assert_identical(reverse.y, ds.y[:5:-1])
+
 
 def test_crs() -> None:
     index = RasterIndex.from_transform(Affine.identity(), width=12, height=10)