Refactor z12-14 transportation filter logic (openmaptiles#1207)

This PR is a refactor.  The purpose of this PR is to refactor the logic in the z12-z14 transportation layer to increase readability and reduce code complexity.

In addition, this fixes bugs in the original filter code:
* Create filter parity between `service` and `service_construction` highway classes.
* Removal of unintended `class` values `minor_construction`, `path_construction`, `service_construction`, and `track_construction` from zoom 12 filter by explicitly listing the highway classes that are included.

layers/transportation/class.sql

@@ -56,3 +56,37 @@ SELECT CASE
 $$ LANGUAGE SQL IMMUTABLE
                 STRICT
                 PARALLEL SAFE;
+
+-- Determine which transportation features are shown at zoom 12
+CREATE OR REPLACE FUNCTION transportation_filter_z12(highway text, construction text) RETURNS boolean AS
+$$
+SELECT CASE
+           WHEN highway IN ('unclassified', 'residential') THEN TRUE
+           WHEN highway_class(highway, '', construction) IN
+               (
+                'motorway', 'trunk', 'primary', 'secondary', 'tertiary', 'raceway',
+                'motorway_construction', 'trunk_construction', 'primary_construction',
+                'secondary_construction', 'tertiary_construction', 'raceway_construction'
+               ) THEN TRUE --includes ramps
+           ELSE FALSE
+       END
+$$ LANGUAGE SQL IMMUTABLE
+                STRICT
+                PARALLEL SAFE;
+
+-- Determine which transportation features are shown at zoom 13
+-- Assumes that piers have already been excluded
+CREATE OR REPLACE FUNCTION transportation_filter_z13(highway text,
+                                                     public_transport text,
+                                                     construction text,
+                                                     service text) RETURNS boolean AS
+$$
+SELECT CASE
+           WHEN transportation_filter_z12(highway, construction) THEN TRUE
+           WHEN highway = 'service' OR construction = 'service' THEN service NOT IN ('driveway', 'parking_aisle')
+           WHEN highway_class(highway, public_transport, construction) IN ('minor', 'minor_construction') THEN TRUE
+           ELSE FALSE
+       END
+$$ LANGUAGE SQL IMMUTABLE
+                STRICT
+                PARALLEL SAFE;

layers/transportation/transportation.sql

@@ -358,26 +358,17 @@ FROM (
                 z_order
          FROM osm_highway_linestring
          WHERE NOT is_area
-           AND (
-                     zoom_level = 12 AND (
-                             highway_class(highway, public_transport, construction) NOT IN ('track', 'path', 'minor', 'service')
-                         OR highway IN ('unclassified', 'residential')
-                     ) AND man_made <> 'pier'
-                 OR zoom_level = 13
-                         AND (
-                                    highway_class(highway, public_transport, construction) NOT IN ('track', 'path') AND
-                                    man_made <> 'pier'
-                            OR
-                                    man_made = 'pier' AND NOT ST_IsClosed(geometry)
-                        )
-                        AND service NOT IN ('driveway', 'parking_aisle')
-                 OR zoom_level >= 14
-                         AND (
-                            man_made <> 'pier'
-                            OR
-                            NOT ST_IsClosed(geometry)
-                        )
-             )
+           AND
+               CASE WHEN zoom_level = 12 THEN transportation_filter_z12(highway, construction)
+                    WHEN zoom_level = 13 THEN
+                         CASE WHEN man_made='pier' THEN NOT ST_IsClosed(geometry)
+                              ELSE transportation_filter_z13(highway, public_transport, construction, service)
+                         END
+                    WHEN zoom_level >= 14 THEN
+                         CASE WHEN man_made='pier' THEN NOT ST_IsClosed(geometry)
+                              ELSE TRUE
+                         END
+               END
          UNION ALL
 
          -- etldoc: osm_railway_linestring_gen_z8  ->  layer_transportation:z8

layers/transportation/transportation.yaml

@@ -22,7 +22,7 @@ layer:
           [`aerialway`](http://wiki.openstreetmap.org/wiki/Key:aerialway),
           [`route`](http://wiki.openstreetmap.org/wiki/Key:route) tag (for
           shipping ways), or
-          [`man_made`](http://wiki.openstreetmap.org/wiki/Key:route).
+          [`man_made`](http://wiki.openstreetmap.org/wiki/Key:man_made).
       values:
         motorway:
           highway: ['motorway', 'motorway_link']