Measuring ported systems when an unported one draws first

What to know

2026-10-24, crates/sim/tests/recorded.rs, docs/sim.md ("Against the recorded runs").

The recordings give the random streams' state at one point of each step. A port that lacks some of the step's draws can be compared by counting how far it is short of the original on each step. But a ported system that draws after an unported one draws other numbers than the original's: the ambient systems (the flash, the fog, the drift) came out right in count and wrong in value, and their countdowns then fired on other steps.

When the unported system is the first of the step to draw (pilots_update here), its share can be found: for k from 0 up to the step's total, make k draws, run the ported step on a copy of the world, and keep the k that lands exactly on the original's state. Two details mattered:

• The search is ambiguous. A smaller k and a longer branch of a ported system can land on the same state (a change of course of 6 draws after 3, or of 5 after 4). The unported system's count changes rarely between steps, so trying last step's k first picked the right one; with it, every paused step of run4 came out k = 4, and the ported state equalled all five memory dumps.
• Know when the state is in sync. On a step the search cannot explain (the vehicles and weapons draw between the ported systems in flight), a ported system that draws takes other numbers, and from then on its state is not the original's until the next memory dump resets it. The test tracks that and requires exactness only while in sync; out of sync it re-runs the step with those systems silenced, so the rest of the world is still held to never drawing past the original.

For GameFrame: a general method for comparing a partial port against recordings of a single shared random stream (re-verify).