Entry 261

Root Towns as a trait

worldbuild

Filescrates/piney-world/src/town.rscrates/piney-world/src/town01.rscrates/piney-world/src/town02.rscrates/piney-world/src/town03.rscrates/piney-world/src/town04.rscrates/piney-world/src/town05.rs

The five Root Towns are now five types behind one trait. Before this, the port had grown one town at a time from Mac Anu:

  • Town held the shared fields and an enum Class with a variant per town.
  • Town::select and Town::draw matched on the variant and called free functions in town02.rs .. town05.rs. Each of those took the whole Town and got its own state back with let Class::X(d) = &t.class else { return }, so a check at run time stood in for a type the compiler could have known.
  • There were five Option accessors (mac_anu(), dun_loireag(), ...).
  • One shared Piece enum carried every town's variants: the airship, the glows, the flares. Every town's draw had to ignore the others' variants.
  • Mac Anu's own code sat in town.rs beside the shared code.

The owner pointed out that this was not how Rust is written. The shape now follows the game's own classes, where each ROOTTOWN0N overrides a virtual Draw:

typewhat it is
town::BaseWhat every constructor builds from its Spec (number, tables, light animation, fog, clear colour): the file, the static models and objects, the lights, the fog, the hits, the events. The shared helpers are its methods: rows(pass), step_objects(pass, eye), row_draw, object_draw, clump_draw.
town::RootTownThe trait: draw(&mut self, base, layers, to_screen, view), and water_time and water0 for the towns with water.
town01::MacAnu .. town05::LiaFailOne per class, each in its own module. Each has its own Piece enum and an inherent select(&mut self, base, view), which is Draw's choice of pieces, the thing the eemu harnesses compare.
town::Town{ pub base: Base, class: Box<dyn RootTown> }, with open(no), draw, and the generic class::<T>() and parts_mut::<T>() (downcasts through the trait's Any supertrait) in place of the five accessors.

Two other shapes were weighed and turned down:

  • Keep the enum and add a trait on each variant (enum_dispatch style). The set of towns is closed, so an enum would be fair. But the per-town piece types and the split borrow of base and class read more plainly as trait objects, and the game's own shape is virtual dispatch.
  • Keep the shared fields directly on Town. The class's methods need &mut of the shared part and of themselves together. With both in one struct, self.class.draw(self) cannot borrow. The other way round, taking the class out and putting it back, is a hack. So the shared part became its own Base. That costs about 45 outside reads going through .base (town.base.hits, town.base.file).

Nothing drawn changed:

  • The eemu comparisons of the game's own Draw against the port still match: tools/test_world_rs.py (test_town_draw, test_gate, test_merchants), tools/test_town02_rs.py and tools/test_town03_rs.py.
  • Each town's own tests pass (town01::tests::gate_plaza_pieces, town02::tests::first_draw, town03::tests::the_airship_flies_its_first_leg, town04::tests::first_draw, town05::tests::a_frame), and so does the whole workspace (705).