Support aggregating local authority clusters

This applies some heuristics to try and keep the overall list of
areas short when many are selected in the same wider area.

Currently we only have relationship information between upper and
lower tier local authorities, so we can't / won't aggregate up to
Greater London (it's own special thing) or whole countries.

app/broadcast_areas/utils.py

@@ -1,9 +1,12 @@
+from collections import defaultdict
+
 from app.broadcast_areas.models import CustomBroadcastArea
 
 
 def aggregate_areas(areas):
     areas = _convert_custom_areas_to_wards(areas)
     areas = _aggregate_wards_by_local_authority(areas)
+    areas = _aggregate_lower_tier_authorities(areas)
     return areas
 
 
@@ -24,3 +27,39 @@ def _aggregate_wards_by_local_authority(areas):
         area.parent if area.id.startswith('wd20-')
         else area for area in areas
     }
+
+
+def _aggregate_lower_tier_authorities(areas):
+    results = set()
+    clusters = _cluster_lower_tier_authorities(areas)
+
+    for cluster in clusters:
+        # always keep lone area as itself
+        if len(cluster) == 1:
+            results |= set(cluster)
+        # aggregate multi-area cluster
+        elif len(cluster) > 3:
+            results |= {cluster[0].parent}
+        # aggregate many small clusters
+        elif len(clusters) > 1:
+            area = cluster[0]
+            results |= {area.parent or area}
+        # keep one small cluster in full
+        else:
+            results |= set(cluster)
+
+    return results
+
+
+def _cluster_lower_tier_authorities(areas):
+    result = defaultdict(lambda: [])
+
+    for area in areas:
+        # group lower tier authorities by "county"
+        if area.id.startswith('lad20-') and area.parent:
+            result[area.parent] += [area]
+        # leave countries, unitary authorities as-is
+        else:
+            result[area] = [area]
+
+    return result.values()