Allow inverting lake generation

To create archipelago-style maps.

Author: ehughsbaird

doc/JSON/REGION_SETTINGS.md

@@ -241,6 +241,7 @@ are interpreted.
 | `lake_depth`                               | Depth of lakes, expressed in Z-levels (e.g. -1 to -10).                     |
 | `shore_extendable_overmap_terrain`         | List of overmap terrains that can be extended to the shore if adjacent.     |
 | `shore_extendable_overmap_terrain_aliases` | Overmap terrains to treat as different overmap terrain for extending shore. |
+| `invert_lakes`                             | Invert drawing of lakes. What would be a lake is land, what would be land is a lake. |
 
 ### Example
 

src/overmap_water.cpp

@@ -261,8 +261,14 @@ void overmap::place_lakes( const std::vector<const overmap *> &neighbor_overmaps
     double noise_threshold = settings_lake.noise_threshold_lake;
     const int lake_depth = settings_lake.lake_depth;
 
+    // For cases where lakes take up most or all of the map, we need to stop the flood-fill
+    // from being unbounded. However, the bounds cannot be simply the overmap edges, or the
+    // shorelines will not be generated properly.
+    inclusive_rectangle<point_om_omt> considered_bounds( point_om_omt( -5, -5 ),
+            point_om_omt( OMAPX + 5, OMAPY + 5 ) );
     const auto is_lake = [&]( const point_om_omt & p ) {
-        return omt_lake_noise_threshold( origin, p, noise_threshold );
+        return considered_bounds.contains( p ) &&
+               settings_lake.invert_lakes ^ omt_lake_noise_threshold( origin, p, noise_threshold );
     };
 
     const oter_id lake_surface( "lake_surface" );
@@ -281,7 +287,7 @@ void overmap::place_lakes( const std::vector<const overmap *> &neighbor_overmaps
             }
 
             // It's a lake if it exceeds the noise threshold defined in the region settings.
-            if( !omt_lake_noise_threshold( origin, seed_point, noise_threshold ) ) {
+            if( !is_lake( seed_point ) ) {
                 continue;
             }
 

src/regional_settings.cpp

@@ -526,6 +526,7 @@ void region_settings_lake::load( const JsonObject &jo, std::string_view )
               shore_extendable_overmap_terrain, sid_reader );
     optional( jo, was_loaded, "shore_extendable_overmap_terrain_aliases",
               shore_extendable_overmap_terrain_aliases );
+    optional( jo, was_loaded, "invert_lakes", invert_lakes, false );
 }
 
 void shore_extendable_overmap_terrain_alias::deserialize( const JsonObject &jo )

src/regional_settings.h

@@ -326,6 +326,7 @@ struct region_settings_lake {
     int lake_depth = -5;
     std::vector<oter_str_id> shore_extendable_overmap_terrain;
     std::vector<shore_extendable_overmap_terrain_alias> shore_extendable_overmap_terrain_aliases;
+    bool invert_lakes = false;
 
     bool was_loaded = false;
     void load( const JsonObject &jo, std::string_view );