CREATE OR REPLACE FUNCTION brunnel(is_bridge BOOL, is_tunnel BOOL, is_ford BOOL) RETURNS TEXT AS $$
    SELECT CASE
        WHEN is_bridge THEN 'bridge'
        WHEN is_tunnel THEN 'tunnel'
        WHEN is_ford THEN 'ford'
    END;
$$ 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 $$
    SELECT CASE
        %%FIELD_MAPPING: class %%
    END;
$$ 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 $$
    SELECT CASE
        WHEN railway IN ('rail', 'narrow_gauge', 'preserved', 'funicular') THEN 'rail'
        WHEN railway IN ('subway', 'light_rail', 'monorail', 'tram') THEN 'transit'
    END;
$$ 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 $$
    SELECT CASE
        WHEN service IN ('spur', 'yard', 'siding', 'crossover', 'driveway', 'alley', 'parking_aisle') THEN service
    END;
$$ 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 $$
    SELECT CASE
        WHEN surface IN ('paved', 'asphalt', 'cobblestone', 'concrete', 'concrete:lanes', 'concrete:plates', 'metal', 'paving_stones', 'sett', 'unhewn_cobblestone', 'wood') THEN 'paved'
        WHEN surface IN ('unpaved', 'compacted', 'dirt', 'earth', 'fine_gravel', 'grass', 'grass_paver', 'gravel', 'gravel_turf', 'ground', 'ice', 'mud', 'pebblestone', 'salt', 'sand', 'snow', 'woodchips') THEN 'unpaved'
    END;
$$ LANGUAGE SQL IMMUTABLE STRICT;