use line interpolation to create centroid for lines

ST_PointOnSurface always returns one of the vertices of a line.
This means that a two-point line will have the centroid at
one of the ends, which is less then ideal.

lib-sql/functions/interpolation.sql

@@ -176,8 +176,8 @@ BEGIN
   END IF;
 
   NEW.parent_place_id := get_interpolation_parent(NEW.token_info, NEW.partition,
-                                                 ST_PointOnSurface(NEW.linegeo),
+                                                  get_center_point(NEW.linegeo),
-                                                 NEW.linegeo);
+                                                  NEW.linegeo);
 
   -- Cannot find a parent street. We will not be able to display a reliable
   -- address, so drop entire interpolation.

lib-sql/functions/placex_triggers.sql

@@ -668,7 +668,7 @@ BEGIN
   NEW.place_id := nextval('seq_place');
   NEW.indexed_status := 1; --STATUS_NEW
 
-  NEW.centroid := ST_PointOnSurface(NEW.geometry);
+  NEW.centroid := get_center_point(NEW.geometry);
   NEW.country_code := lower(get_country_code(NEW.centroid));
 
   NEW.partition := get_partition(NEW.country_code);
@@ -870,7 +870,7 @@ BEGIN
   END IF;
 
   -- Compute a preliminary centroid.
-  NEW.centroid := ST_PointOnSurface(NEW.geometry);
+  NEW.centroid := get_center_point(NEW.geometry);
 
     -- recalculate country and partition
   IF NEW.rank_search = 4 AND NEW.address is not NULL AND NEW.address ? 'country' THEN

lib-sql/functions/utils.sql

@@ -7,6 +7,26 @@
 
 -- Assorted helper functions for the triggers.
 
+CREATE OR REPLACE FUNCTION get_center_point(place GEOMETRY)
+  RETURNS GEOMETRY
+  AS $$
+DECLARE
+  geom_type TEXT;
+BEGIN
+  geom_type := ST_GeometryType(place);
+  IF geom_type = ' ST_Point' THEN
+    RETURN place;
+  END IF;
+  IF geom_type = 'ST_LineString' THEN
+    RETURN ST_LineInterpolatePoint(place, 0.5);
+  END IF;
+
+  RETURN ST_PointOnSurface(place);
+END;
+$$
+LANGUAGE plpgsql IMMUTABLE;
+
+
 CREATE OR REPLACE FUNCTION geometry_sector(partition INTEGER, place geometry)
   RETURNS INTEGER
   AS $$
@@ -21,6 +41,7 @@ $$
 LANGUAGE plpgsql IMMUTABLE;
 
 
+
 CREATE OR REPLACE FUNCTION array_merge(a INTEGER[], b INTEGER[])
   RETURNS INTEGER[]
   AS $$