# Module 4 reference engine (Rust)

The course's own engine as of the end of Module 4. It adds auras as card data and one routine that decides how many times a queued trigger activates, so "Your minions trigger their Deathrattles twice." is a card, not a change to Deathrattle. It also adds playing a minion from hand, and Battlecries. Module 3's reference was the starting point, so a diff between the two shows what this module adds. Module 5's reference starts from this one.

- `src/engine/`: generic machinery. Names no cards.
  - `aura.rs`: the Aura Update (Other), which snapshots the auras of every minion in play, and `activations`, the one routine that reads the snapshot and returns how many times a trigger of a given kind activates for a given player. Stacking policy (`Max` or `Additive`) is a field of each multiplier.
  - `queue.rs`: actions, the context an effect sees, and the resolver. The end of the outermost Phase runs the Death Creation Step, then the Aura Update (Other). Summoning a minion also runs the Aura Update; playing one does not. Each queued trigger asks `activations` once, as it starts, and then activates that many times back to back. A `Play` action moves a minion from hand to board and raises a `Played` event, which the minion's Battlecry answers.
  - `death.rs`: the Death Creation Step, unchanged from Module 3.
  - `world.rs`: entities, zones, order of play, heroes, stats, Armor, the pending-destroy flag, the aura snapshot and the log.
  - `card.rs`: card definitions (stats, triggers, auras, a spell's effect) and the library.
  - `event.rs`: the engine's events. `Played` is engine-only: `src/harness.rs` leaves it out of the course's `GameEvent` log, where the played minion's `Summoned` already shows it.
- `src/content/mod.rs`: card definitions. Baron Rivendare is one aura entry: `Aura::TriggerMultiplier` on `TriggerOn::Deathrattle`, `times` 2, `Stacking::Max`. Twisting Nether (the Module 3 Expansion Drop) is here because the Module 4 tests use it.
- `src/harness.rs`: the adapter the course tests call. `summon` goes through the engine's summon, so it runs an Aura Update. `play` (the Module 1 Expansion Drop) runs one Sequence with a `Play` action.

Run it with the module's tests: `node tools/verify-references.mjs 04` from the course repository.