text: CREATE OR REPLACE FUNCTION brunnel(is_bridge BOOL, is_tunnel BOOL, is_ford BOOL) RETURNS TEXT AS $$
      SELECT CASE
          WHEN is_bridge THEN 'bridge'
          WHEN is_tunnel THEN 'tunnel'
          WHEN is_ford THEN 'ford'
          ELSE NULL
      END;
  $$ LANGUAGE SQL IMMUTABLE STRICT;
  
  -- The classes for highways are derived from the classes used in ClearTables
  -- https://github.com/ClearTables/ClearTables/blob/master/transportation.lua
  CREATE OR REPLACE FUNCTION highway_class(highway TEXT, public_transport TEXT, construction TEXT) RETURNS TEXT AS $$
      SELECT CASE
          WHEN highway IN ('motorway', 'motorway_link') THEN 'motorway'
          WHEN highway IN ('trunk', 'trunk_link') THEN 'trunk'
          WHEN highway IN ('primary', 'primary_link') THEN 'primary'
          WHEN highway IN ('secondary', 'secondary_link') THEN 'secondary'
          WHEN highway IN ('tertiary', 'tertiary_link') THEN 'tertiary'
          WHEN highway IN ('unclassified', 'residential', 'living_street', 'road') THEN 'minor'
          WHEN highway IN ('pedestrian', 'path', 'footway', 'cycleway', 'steps', 'bridleway', 'corridor') OR public_transport IN ('platform') THEN 'path'
          WHEN highway IN ('service', 'track', 'raceway') THEN highway
          WHEN highway = 'construction' THEN CASE
            WHEN construction IN ('motorway', 'motorway_link') THEN 'motorway_construction'
            WHEN construction IN ('trunk', 'trunk_link') THEN 'trunk_construction'
            WHEN construction IN ('primary', 'primary_link') THEN 'primary_construction'
            WHEN construction IN ('secondary', 'secondary_link') THEN 'secondary_construction'
            WHEN construction IN ('tertiary', 'tertiary_link') THEN 'tertiary_construction'
            WHEN construction = '' OR construction IN ('unclassified', 'residential', 'living_street', 'road') THEN 'minor_construction'
            WHEN construction IN ('pedestrian', 'path', 'footway', 'cycleway', 'steps', 'bridleway', 'corridor') OR public_transport IN ('platform') THEN 'path_construction'
            WHEN construction IN ('service', 'track', 'raceway') THEN CONCAT(highway, '_construction')
            ELSE NULL
          END
          ELSE NULL
      END;
  $$ LANGUAGE SQL IMMUTABLE;
  
  -- The classes for railways are derived from the classes used in ClearTables
  -- https://github.com/ClearTables/ClearTables/blob/master/transportation.lua
  CREATE OR REPLACE FUNCTION railway_class(railway TEXT) RETURNS TEXT AS $$
      SELECT CASE
          WHEN railway IN ('rail', 'narrow_gauge', 'preserved', 'funicular') THEN 'rail'
          WHEN railway IN ('subway', 'light_rail', 'monorail', 'tram') THEN 'transit'
          ELSE NULL
      END;
  $$ LANGUAGE SQL IMMUTABLE STRICT;
  
  -- Limit service to only the most important values to ensure
  -- we always know the values of service
  CREATE OR REPLACE FUNCTION service_value(service TEXT) RETURNS TEXT AS $$
      SELECT CASE
          WHEN service IN ('spur', 'yard', 'siding', 'crossover', 'driveway', 'alley', 'parking_aisle') THEN service
          ELSE NULL
      END;
  $$ LANGUAGE SQL IMMUTABLE STRICT;
  
  -- Limit surface to only the most important values to ensure
  -- we always know the values of surface
  CREATE OR REPLACE FUNCTION surface_value(surface TEXT) RETURNS TEXT AS $$
      SELECT CASE
          WHEN surface IN ('paved', 'asphalt', 'cobblestone', 'concrete', 'concrete:lanes', 'concrete:plates', 'metal', 'paving_stones', 'sett', 'unhewn_cobblestone', 'wood') THEN 'paved'
          WHEN surface IN ('unpaved', 'compacted', 'dirt', 'earth', 'fine_gravel', 'grass', 'grass_paver', 'gravel', 'gravel_turf', 'ground', 'ice', 'mud', 'pebblestone', 'salt', 'sand', 'snow', 'woodchips') THEN 'unpaved'
          ELSE NULL
      END;
  $$ LANGUAGE SQL IMMUTABLE STRICT;
  