ensure consistent country assignments

When OSM data has areas with overlapping countries, use the country
assignments from the pre-defined country grid for tie-breaking.
If that fails, fall back to the country with the smaller partition
number.

lib-sql/functions/utils.sql

@@ -164,18 +164,44 @@ CREATE OR REPLACE FUNCTION get_country_code(place geometry)
 DECLARE
   place_centre GEOMETRY;
   nearcountry RECORD;
+  countries TEXT[];
 BEGIN
   place_centre := ST_PointOnSurface(place);
 
 -- RAISE WARNING 'get_country_code, start: %', ST_AsText(place_centre);
 
   -- Try for a OSM polygon
-  FOR nearcountry IN
-    SELECT country_code from location_area_country
-    WHERE country_code is not null and st_covers(geometry, place_centre) limit 1
-  LOOP
-    RETURN nearcountry.country_code;
-  END LOOP;
+  SELECT array_agg(country_code) FROM location_area_country
+    WHERE country_code is not null and st_covers(geometry, place_centre)
+    INTO countries;
+
+  IF array_length(countries, 1) = 1 THEN
+    RETURN countries[1];
+  END IF;
+
+  IF array_length(countries, 1) > 1 THEN
+    -- more than one country found, confirm against the fallback data what to choose
+    FOR nearcountry IN
+        SELECT country_code FROM country_osm_grid
+          WHERE ST_Covers(geometry, place_centre) AND country_code = ANY(countries)
+          ORDER BY area ASC
+    LOOP
+        RETURN nearcountry.country_code;
+    END LOOP;
+    -- Still nothing? Choose the country code with the smallest partition number.
+    -- And failing that, just go by the alphabet.
+    FOR nearcountry IN
+        SELECT cc,
+               (SELECT partition FROM country_name WHERE country_code = cc) as partition
+        FROM unnest(countries) cc
+        ORDER BY partition, cc
+    LOOP
+        RETURN nearcountry.cc;
+    END LOOP;
+
+    -- Should never be reached.
+    RETURN countries[1];
+  END IF;
 
 -- RAISE WARNING 'osm fallback: %', ST_AsText(place_centre);