Pass polygons through if they’re small already

If a polygon is smaller than the largest polygon in our dataset of
simplified polygons then we’re only throwing away useful detail by
simplifying it.

We should still simplify larger polygons as a fallback, to avoid sending
anything to the CBC that we’re not sure it will like.

The thresholds here are low: we can raise them as we test and experiment
more.

Here’s some data about the Flood Warning Service polygons

Percentile | 80% | 90%   | 95%    | 98%     | 99%     | 99.9%
-----------|-----|-------|--------|---------|---------|---------
Point count| 226 | 401.9 | 640.45 | 1015.38 | 1389.07 | 3008.609

Percentile    | 80% | 90%   | 95%    | 98%     | 99%     | 99.9%
--------------|-----|-------|--------|---------|---------|---------
Polygon count |2----|3------|5-------|8--------|10-------|40.469

app/broadcast_message/translators.py

@@ -29,5 +29,5 @@ def cap_xml_polygon_to_list(polygon_string):
         [
             float(coordinate) for coordinate in pair.split(',')
         ]
-        for pair in polygon_string.split(' ')
+        for pair in polygon_string.strip().split(' ')
     ]

app/v2/broadcast/post_broadcast.py

@@ -44,9 +44,17 @@ def create_broadcast():
     _validate_template(broadcast_json)
 
     polygons = Polygons(list(chain.from_iterable((
-        area['polygons'] for area in broadcast_json['areas']
+        [
+            [[y, x] for x, y in polygon]
+            for polygon in area['polygons']
+        ] for area in broadcast_json['areas']
     ))))
 
+    if len(polygons) > 12 or polygons.point_count > 250:
+        simple_polygons = polygons.smooth.simplify
+    else:
+        simple_polygons = polygons
+
     broadcast_message = BroadcastMessage(
         service_id=authenticated_service.id,
         content=broadcast_json['content'],
@@ -56,7 +64,7 @@ def create_broadcast():
             'names': [
                 area['name'] for area in broadcast_json['areas']
             ],
-            'simple_polygons': polygons.smooth.simplify.as_coordinate_pairs_long_lat,
+            'simple_polygons': simple_polygons.as_coordinate_pairs_lat_long,
         },
         status=BroadcastStatusType.PENDING_APPROVAL,
         api_key_id=api_user.id,

requirements.in

@@ -36,7 +36,7 @@ notifications-python-client==6.0.2
 # PaaS
 awscli-cwlogs==1.4.6
 
-git+https://github.com/alphagov/notifications-utils.git@48.1.0#egg=notifications-utils==48.1.0
+git+https://github.com/alphagov/notifications-utils.git@48.0.0#egg=notifications-utils==49.0.0
 
 # gds-metrics requires prometheseus 0.2.0, override that requirement as 0.7.1 brings significant performance gains
 prometheus-client==0.10.1

tests/app/v2/broadcast/test_post_broadcast.py

@@ -104,10 +104,10 @@ def test_valid_post_cap_xml_broadcast_returns_201(
     assert response_json['personalisation'] is None
     assert response_json['service_id'] == str(sample_broadcast_service.id)
 
-    assert len(response_json['areas']['simple_polygons']) == 1
-    assert len(response_json['areas']['simple_polygons'][0]) == 22
-    assert response_json['areas']['simple_polygons'][0][0] == [53.10562, 0.244127]
-    assert response_json['areas']['simple_polygons'][0][-1] == [53.10562, 0.244127]
+    assert len(response_json['simple_polygons']) == 1
+    assert len(response_json['simple_polygons'][0]) == 29
+    assert response_json['simple_polygons'][0][0] == [53.10569, 0.24453]
+    assert response_json['simple_polygons'][0][-1] == [53.10569, 0.24453]
     assert response_json['areas']['names'] == ['River Steeping in Wainfleet All Saints']
     assert 'ids' not in response_json['areas']  # only for broadcasts created in Admin
 
@@ -119,6 +119,27 @@ def test_valid_post_cap_xml_broadcast_returns_201(
     assert response_json['updated_at'] is None
 
 
+def test_large_polygon_is_simplified(
+    client,
+    sample_broadcast_service,
+):
+    auth_header = create_service_authorization_header(service_id=sample_broadcast_service.id)
+    response = client.post(
+        path='/v2/broadcast',
+        data=sample_cap_xml_documents.WINDEMERE,
+        headers=[('Content-Type', 'application/cap+xml'), auth_header],
+    )
+    assert response.status_code == 201
+
+    response_json = json.loads(response.get_data(as_text=True))
+
+    assert len(response_json['simple_polygons']) == 1
+    assert len(response_json['simple_polygons'][0]) == 110
+
+    assert response_json['simple_polygons'][0][0] == [54.419546, -2.988521]
+    assert response_json['simple_polygons'][0][-1] == [54.419546, -2.988521]
+
+
 @pytest.mark.parametrize("training_mode_service", [True, False])
 def test_valid_post_cap_xml_broadcast_sets_stubbed_to_true_for_training_mode_services(
     client,