What causes "impossible to start from a non-sane situation." in a-b-street/abstreet?

Ensure all persons are initialized to the sane initial state (event plus sampled time) before calling start.

impossible to start from a non-sane situation.

Error message

impossible to start from a non-sane situation.

What it means

The pandemic model's start() initializes each person's state by pattern-matching on an expected initial variant; only the sane initial state (with its event/time) can begin the simulation. Any other variant at startup — meaning the model was seeded with an impossible or uninitialized person state — bails with this fixed message. It is an internal invariant check on the SIR-state machine's starting conditions.

Solutions

Ensure all persons are initialized to the sane initial state (event plus sampled time) before calling start.
Check initialization code that constructs the initial person states for gaps/uninitialized variants.
If a new initial state was added, extend the match in start() to handle it.
Try a different RNG seed to confirm whether the failure is seed-specific or structural.

Example fix

// before: person starts in an exotic variant
let init = Self::Infected; // not handled
// after
let init = Self::Sane((initial_event, now));

Defensive patterns

Strategy: try-catch

Type guard

fn starts_sane(s: &PersonState) -> bool {
    matches!(s, PersonState::Sane(_))
}

Try / catch

match sim.start(now, &mut rng) {
    Err(e) if e.to_string().contains("non-sane situation") => {
        // re-initialize all persons to Sane((event, time)) and retry
    }
    other => other?,
}

Prevention

Always seed every person with the sane initial state before start().
Extend start()'s match when adding new initial-state variants.
Cover initialization across several RNG seeds in tests to catch uncovered branches.

When it happens

Trigger: Calling Simulator::start (sim/src/pandemic/mod.rs:327, invoked by initialize) when a person's initial state is anything other than the expected Sane((event, time)) variant — e.g. the RNG/seed produced a state outside the handled match arms, or initialization constructed persons incorrectly.

Common situations: Custom initial-condition modifications to the pandemic model, mismatched state enum after refactoring, or seeded RNG paths not covered by the initialization logic.

Understand the failure class

Background: "This is a bug, please report it": internal invariant violations, unreachable panics, and SNH errors explained — this error's family across 47 libraries.

Related errors

AI-assisted analysis of a-b-street/abstreet@0964f29315 (2026-09-13). Data as JSON: /api/errors/1bc9654d6b2f9469. Report an issue: GitHub.

Appendix: source

Thrown at sim/src/pandemic/mod.rs:327

                }
            }
            Self::Recovered(t) => Some(Self::Recovered(t)),
            Self::Dead(t) => Some(Self::Dead(t)),
        }
    }

    // TODO: not sure if we want an option here... I guess here we want because we could have
    pub fn start(self, now: AnyTime, overlap: Duration, rng: &mut XorShiftRng) -> Result<Self> {
        // rewrite this part with it
        match self {
            Self::Sane((ev, t)) => {
                if overlap >= Self::get_time_exp(State::R_0 / State::T_INF, rng) {
                    Ok(ev.next(now, rng))
                } else {
                    Ok(Self::Sane((ev, t)))
                }
            }
            _ => bail!("impossible to start from a non-sane situation.",),
        }
    }
}

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