Running ticks

A built Model advances one tick at a time. A tick has no fixed meaning; the framework defines the order in which the parts of a tick run.

Advancing the model

model.tick()?;        // advance one tick
model.run(50)?;       // advance 50 ticks
let done = model.tick_count();

tick runs one tick; run runs a fixed number. Both return an ECSResult. A failure in any system stops the tick and returns the error; the world is not left partially updated.

What happens in a tick

Each tick runs, in order:

1. Sub-schedulers, in the order they were added. A sub-scheduler shares the model's world and has its own systems and stages.
2. Nested models, each of which completes its own tick and then runs its bridge to write parent-facing effects.
3. The root scheduler, which runs the model's own systems.

Within a scheduler, systems are grouped into stages. Systems in the same stage have non-conflicting access and run in parallel; stages run in sequence. See scheduling.

Determinism across ticks

Each system's run context carries the model seed and the tick counter. Draws taken through DetRng::from_context depend on (seed, tick, system_id, salt), so they do not vary with the worker-thread count. The same model and seed produce the same trajectory on one core or many. See reproducibility.