Function dominates 

pub fn dominates(a: &[f64], b: &[f64], directions: &[Direction]) -> bool

Direction-aware Pareto dominance: a dominates b iff it is no worse in every objective and strictly better in at least one.

Each objective is compared through its own Direction, so a study that minimises loss while maximising return needs no sign flipping at the call site.

Degenerate and hostile inputs

Case	Answer
equal vectors	false both ways — nothing is strictly better
a single objective	ordinary ranking: a dominates b iff it is strictly better
a carries a NaN, or has the wrong arity	false — a NaN never dominates
b carries a NaN (and a does not)	true — well-formed beats rubbish
both carry a NaN	false both ways — they tie at the back
directions is empty	false — there is nothing to compare

±∞ is an ordinary float and compares normally; only NaN is special. The relation is irreflexive, antisymmetric and transitive on every input, including the NaN cases — which is what makes fast_non_dominated_sort terminate.

use atune_core::pareto::dominates;
use atune_core::study::Direction::{Maximize, Minimize};

let dirs = [Minimize, Maximize];
assert!(dominates(&[1.0, 5.0], &[2.0, 5.0], &dirs)); // better in objective 0
assert!(!dominates(&[1.0, 5.0], &[1.0, 5.0], &dirs)); // equal: no strict gain
assert!(!dominates(&[1.0, 4.0], &[2.0, 5.0], &dirs)); // a trade-off: neither wins
assert!(!dominates(&[f64::NAN, 9.0], &[2.0, 5.0], &dirs), "NaN never dominates");
assert!(dominates(&[2.0, 5.0], &[f64::NAN, 9.0], &dirs), "…and is dominated");