Add Pixel.northeast/southwest properties

- the properties are needed for the drawing functionalitites

src/urban_meal_delivery/db/pixels.py

@@ -1,9 +1,11 @@
 """Provide the ORM's `Pixel` model."""
 
 import sqlalchemy as sa
+import utm
 from sqlalchemy import orm
 
 from urban_meal_delivery.db import meta
+from urban_meal_delivery.db import utils
 
 
 class Pixel(meta.Base):
@@ -58,3 +60,48 @@ class Pixel(meta.Base):
     def area(self) -> float:
         """The area of a pixel in square kilometers."""
         return self.grid.pixel_area
+
+    @property
+    def northeast(self) -> utils.Location:
+        """The pixel's northeast corner, relative to `.grid.city.southwest`.
+
+        Implementation detail: This property is cached as none of the
+        underlying attributes to calculate the value are to be changed.
+        """
+        if not hasattr(self, '_northeast'):  # noqa:WPS421  note:d334120e
+            # The origin is the southwest corner of the `.grid.city`'s viewport.
+            easting_origin = self.grid.city.southwest.easting
+            northing_origin = self.grid.city.southwest.northing
+
+            # `+1` as otherwise we get the pixel's `.southwest` corner.
+            easting = easting_origin + ((self.n_x + 1) * self.side_length)
+            northing = northing_origin + ((self.n_y + 1) * self.side_length)
+            zone, band = self.grid.city.southwest.zone_details
+            latitude, longitude = utm.to_latlon(easting, northing, zone, band)
+
+            self._northeast = utils.Location(latitude, longitude)
+            self._northeast.relate_to(self.grid.city.southwest)
+
+        return self._northeast
+
+    @property
+    def southwest(self) -> utils.Location:
+        """The pixel's northeast corner, relative to `.grid.city.southwest`.
+
+        Implementation detail: This property is cached as none of the
+        underlying attributes to calculate the value are to be changed.
+        """
+        if not hasattr(self, '_southwest'):  # noqa:WPS421  note:d334120e
+            # The origin is the southwest corner of the `.grid.city`'s viewport.
+            easting_origin = self.grid.city.southwest.easting
+            northing_origin = self.grid.city.southwest.northing
+
+            easting = easting_origin + (self.n_x * self.side_length)
+            northing = northing_origin + (self.n_y * self.side_length)
+            zone, band = self.grid.city.southwest.zone_details
+            latitude, longitude = utm.to_latlon(easting, northing, zone, band)
+
+            self._southwest = utils.Location(latitude, longitude)
+            self._southwest.relate_to(self.grid.city.southwest)
+
+        return self._southwest

src/urban_meal_delivery/db/utils/locations.py

@@ -2,7 +2,7 @@
 
 from __future__ import annotations
 
-from typing import Optional
+from typing import Optional, Tuple
 
 import utm
 
@@ -82,6 +82,11 @@ class Location:
         """The UTM zone of the location."""
         return f'{self._zone}{self._band}'
 
+    @property
+    def zone_details(self) -> Tuple[int, str]:
+        """The UTM zone of the location as the zone number and the band."""
+        return (self._zone, self._band)
+
     def __eq__(self, other: object) -> bool:
         """Check if two `Location` objects are the same location."""
         if not isinstance(other, Location):

tests/db/test_grids.py

@@ -205,7 +205,7 @@ class TestGridification:
     @pytest.mark.db
     @pytest.mark.no_cover
     @pytest.mark.parametrize('side_length', [250, 500, 1_000, 2_000, 4_000, 8_000])
-    def test_make_random_grids(  # noqa:WPS211
+    def test_make_random_grids(  # noqa:WPS211,WPS218
         self, db_session, city, make_address, make_restaurant, make_order, side_length,
     ):
         """With 100 random `Address` objects, a grid must have ...
@@ -228,5 +228,12 @@ class TestGridification:
         assert isinstance(result, db.Grid)
         assert 1 <= len(result.pixels) <= n_pixels_x * n_pixels_y
 
+        # Sanity checks for `Pixel.southwest` and `Pixel.northeast`.
+        for pixel in result.pixels:
+            assert abs(pixel.southwest.x - pixel.n_x * side_length) < 2
+            assert abs(pixel.southwest.y - pixel.n_y * side_length) < 2
+            assert abs(pixel.northeast.x - (pixel.n_x + 1) * side_length) < 2
+            assert abs(pixel.northeast.y - (pixel.n_y + 1) * side_length) < 2
+
         db_session.add(result)
         db_session.commit()

tests/db/test_pixels.py

@@ -87,3 +87,31 @@ class TestProperties:
         result = pixel.area
 
         assert result == 1.0
+
+    def test_northeast(self, pixel, city):
+        """Test `Pixel.northeast` property."""
+        result = pixel.northeast
+
+        assert abs(result.x - pixel.side_length) < 2
+        assert abs(result.y - pixel.side_length) < 2
+
+    def test_northeast_is_cached(self, pixel):
+        """Test `Pixel.northeast` property."""
+        result1 = pixel.northeast
+        result2 = pixel.northeast
+
+        assert result1 is result2
+
+    def test_southwest(self, pixel, city):
+        """Test `Pixel.southwest` property."""
+        result = pixel.southwest
+
+        assert abs(result.x) < 2
+        assert abs(result.y) < 2
+
+    def test_southwest_is_cached(self, pixel):
+        """Test `Pixel.southwest` property."""
+        result1 = pixel.southwest
+        result2 = pixel.southwest
+
+        assert result1 is result2

tests/db/utils/test_locations.py

@@ -140,6 +140,13 @@ class TestProperties:
 
         assert result == ZONE
 
+    def test_zone_details(self, location):
+        """Test `Location.zone_details` property."""
+        result = location.zone_details
+
+        zone, band = result
+        assert ZONE == f'{zone}{band}'
+
 
 class TestRelateTo:
     """Test the `Location.relate_to()` method and the `.x` and `.y` properties."""