Solutions: 29 — The wall at 10K → 1M

Exercise 1-2 — Calibration and scale-up

Run the simulator at 10K for 1000 ticks: typical wall-clock ~1–3 s.

Run at 1M for 100 ticks (same total entity-ticks): expect ~10–30 s if the simulator is well-tuned, ~100–300 s if it has unaddressed walls.

The ratio is the diagnostic. Anything above ~15× indicates that constant-factor walls are binding.

Exercise 3 — Profile

cargo flamegraph produces a flamegraph.svg. The wide frames at the top of the graph are the hottest functions. Common offenders at the 1M boundary:

• <Vec as Extend>::extend — uncapped reallocations
• core::iter::any over a Vec<u32> — linear scan that should be an indexed lookup
• std::collections::HashMap::iter — non-deterministic, slow at scale
• core::fmt::Write — println! in the hot path

Exercise 4 — Pre-size to_insert

#![allow(unused)]
fn main() {
let estimated_max = creatures.len() / 50; // 2% reproduction rate, with margin
let to_insert: Vec<CreatureRow> = Vec::with_capacity(estimated_max);
}

Re-profile: the Vec::extend frames should shrink dramatically. A typical fix removes 5–15 % of total wall time.

Exercise 5 — Hot/cold split

Apply §26’s split. Re-profile. Cache-miss counters (visible in perf stat -e cache-misses) should drop by ~30–50 %. Wall-clock for motion drops by a similar fraction.

Exercise 6 — Index maps

Replace hungry.iter().any(|&id| id == target) with id_to_slot[target] != INVALID && hungry_membership[target]. A function that was O(N) per call is now O(1).

For a system that asks the question 100K times per tick at 1M creatures, this is the difference between 100 s and 0.005 s per tick.

Exercise 7 — Find one new wall

Open-ended. Common discoveries the first time a reader runs this exercise:

• A Vec<Box<T>> somewhere in the code, costing one allocation per element.
• A clone() inside a hot loop where a & would do.
• A String::from(...) in a logging path that runs millions of times.
• A HashMap::contains_key where a Vec<bool> mask would be O(1) and 100× faster.

In each case, the fix is a one-line change once the wall is found. The challenge is finding the wall, not removing it.