From 35a0558a7f7434e52ba0933ce043314cc0c253eb Mon Sep 17 00:00:00 2001
From: Panu Lahtinen <pnuu+git@iki.fi>
Date: Thu, 7 May 2020 14:05:07 +0300
Subject: [PATCH] Check the area slices, add tests

---
 pyresample/geometry.py        |  9 ++++++---
 pyresample/test/test_utils.py | 29 +++++++++++++++++++++++++++++
 pyresample/utils/__init__.py  | 10 ++++++++++
 3 files changed, 45 insertions(+), 3 deletions(-)

diff --git a/pyresample/geometry.py b/pyresample/geometry.py
index b294f197a..28d43bb9b 100644
--- a/pyresample/geometry.py
+++ b/pyresample/geometry.py
@@ -37,7 +37,8 @@
 from pyresample._spatial_mp import Cartesian, Cartesian_MP, Proj, Proj_MP
 from pyresample.boundary import AreaDefBoundary, Boundary, SimpleBoundary
 from pyresample.utils import (proj4_str_to_dict, proj4_dict_to_str,
-                              convert_proj_floats, proj4_radius_parameters)
+                              convert_proj_floats, proj4_radius_parameters,
+                              check_slice_orientation)
 from pyresample.area_config import create_area_def
 
 try:
@@ -1964,7 +1965,8 @@ def get_area_slices(self, area_to_cover, shape_divisible_by=None):
             xstop = self.width if x[1] is np.ma.masked else x[1] + 1
             ystop = self.height if y[0] is np.ma.masked else y[0] + 1
 
-            return slice(xstart, xstop), slice(ystart, ystop)
+            return (check_slice_orientation(slice(xstart, xstop)),
+                    check_slice_orientation(slice(ystart, ystop)))
 
         if self.proj_dict.get('proj') != 'geos':
             raise NotImplementedError("Source projection must be 'geos' if "
@@ -1995,7 +1997,8 @@ def get_area_slices(self, area_to_cover, shape_divisible_by=None):
             y_slice = _make_slice_divisible(y_slice, self.height,
                                             factor=shape_divisible_by)
 
-        return (x_slice, y_slice)
+        return (check_slice_orientation(x_slice),
+                check_slice_orientation(y_slice))
 
     def crop_around(self, other_area):
         """Crop this area around `other_area`."""
diff --git a/pyresample/test/test_utils.py b/pyresample/test/test_utils.py
index e7675da6d..80008ed21 100644
--- a/pyresample/test/test_utils.py
+++ b/pyresample/test/test_utils.py
@@ -645,3 +645,32 @@ def test_norotation_yaml(self):
         test_area = load_area(f.name, 'regionB')
         self.assertEqual(test_area.rotation, 0)
         os.remove(f.name)
+
+
+def test_check_slice_orientation():
+    """Test that slicing fix is doing what it should."""
+    from pyresample.utils import check_slice_orientation
+
+    # Forward slicing should not be changed
+    start, stop, step = 0, 10, None
+    slice_in = slice(start, stop, step)
+    res = check_slice_orientation(slice_in)
+    assert res is slice_in
+
+    # Reverse slicing should not be changed if the step is negative
+    start, stop, step = 10, 0, -1
+    slice_in = slice(start, stop, step)
+    res = check_slice_orientation(slice_in)
+    assert res is slice_in
+
+    # Reverse slicing should be fixed if step is positive
+    start, stop, step = 10, 0, 2
+    slice_in = slice(start, stop, step)
+    res = check_slice_orientation(slice_in)
+    assert res == slice(start, stop, -step)
+
+    # Reverse slicing should be fixed if step is None
+    start, stop, step = 10, 0, None
+    slice_in = slice(start, stop, step)
+    res = check_slice_orientation(slice_in)
+    assert res == slice(start, stop, -1)
diff --git a/pyresample/utils/__init__.py b/pyresample/utils/__init__.py
index 70c9c3801..d9324f90a 100644
--- a/pyresample/utils/__init__.py
+++ b/pyresample/utils/__init__.py
@@ -243,3 +243,13 @@ def recursive_dict_update(d, u):
 def is_pyproj2():
     """Determine whether the current pyproj version is >= 2.0"""
     return pyproj.__version__ >= '2'
+
+
+def check_slice_orientation(sli):
+    """Check that the slice is slicing the right way."""
+    if sli.start > sli.stop:
+        if sli.step is None or sli.step > 0:
+            step = -(sli.step or 1)
+            sli = slice(sli.start, sli.stop, step)
+
+    return sli