Solutions: 13 — A system is a function over tables

Exercise 1 — Identify the shape

The aggregate case is real but rare in this book — most systems run row-by-row. When you do need an aggregate, treat it as a system whose output is a single-row table.

Exercise 2 — Motion as a system

#![allow(unused)]
fn main() {
/// motion: advance each creature's position by its velocity over `dt` seconds.
///
/// Read-set:  vel, dt
/// Write-set: pos
fn motion(pos: &mut [(f32, f32)], vel: &[(f32, f32)], dt: f32) {
    assert_eq!(pos.len(), vel.len());
    for i in 0..pos.len() {
        pos[i].0 += vel[i].0 * dt;
        pos[i].1 += vel[i].1 * dt;
    }
}
}

The assert_eq! enforces alignment (§6). Without it, a mismatched-length call silently iterates over the shorter array.

Exercise 4 — Filter

#![allow(unused)]
fn main() {
fn starving(energy: &[f32]) -> Vec<usize> {
    let mut out = Vec::new();
    for i in 0..energy.len() {
        if energy[i] <= 0.0 {
            out.push(i);
        }
    }
    out
}
}

This is the read-only first half of apply_starve. The actual apply_starve would push these indices into to_remove. Splitting the query from the mutation lets you test the query in isolation.

Exercise 5 — Emission

#![allow(unused)]
fn main() {
fn reproduce(parent_energy: &[f32], threshold: f32) -> Vec<(usize, f32)> {
    let mut out = Vec::new();
    for i in 0..parent_energy.len() {
        if parent_energy[i] >= threshold {
            let half = parent_energy[i] / 2.0;
            out.push((i, half));
            out.push((i, half));
        }
    }
    out
}
}

For each parent above threshold, two output rows. A 1→2 emission. The pattern is clear: the output Vec has a variable length depending on how many parents qualified.

Exercises 3, 6, 7 — Sketches

Exercise 3. Doc comments listing read-set and write-set are the system’s contract in machine-readable form. A reader of the function knows exactly what can change.

Exercise 6. Anti-system patterns: fn update(world: &mut World) (no declared write-set), fn step() that touches a static mut (hidden state), fn motion(pos: &mut [(f32, f32)]) with an eprintln! inside (side effect — reduces parallelism, harms determinism, makes testing harder).

Exercise 7. A “test” is a system whose write-set is empty (or a small report table). Read pos/vel, output a list of suspicious creatures. Same code path as a debug inspector.