test(fuzz): add cargo-fuzz harness for Pareto and operator hot paths

fuzz/fuzz_targets/pareto_archive.rs

@@ -0,0 +1,58 @@
+#![no_main]
+//! Fuzz `ParetoArchive` for the non-domination invariant under arbitrary
+//! insertion/truncation sequences.
+
+use arbitrary::Arbitrary;
+use libfuzzer_sys::fuzz_target;
+
+use heuropt::core::candidate::Candidate;
+use heuropt::core::evaluation::Evaluation;
+use heuropt::core::objective::{Objective, ObjectiveSpace};
+use heuropt::pareto::archive::ParetoArchive;
+use heuropt::pareto::dominance::{Dominance, pareto_compare};
+
+#[derive(Arbitrary, Debug)]
+enum Op {
+    Insert(f64, f64),
+    Truncate(u8),
+}
+
+#[derive(Arbitrary, Debug)]
+struct Input {
+    ops: Vec<Op>,
+}
+
+fuzz_target!(|input: Input| {
+    if input.ops.len() > 64 {
+        return;
+    }
+    let space = ObjectiveSpace::new(vec![Objective::minimize("f1"), Objective::minimize("f2")]);
+    let mut archive: ParetoArchive<()> = ParetoArchive::new(space.clone());
+
+    for op in input.ops {
+        match op {
+            Op::Insert(a, b) => {
+                let cand = Candidate::new((), Evaluation::new(vec![a, b]));
+                archive.insert(cand);
+            }
+            Op::Truncate(n) => archive.truncate(n as usize),
+        }
+    }
+
+    // Members must be pairwise non-dominated.
+    let m = archive.members();
+    for i in 0..m.len() {
+        for j in 0..m.len() {
+            if i == j {
+                continue;
+            }
+            let r = pareto_compare(&m[i].evaluation, &m[j].evaluation, &space);
+            assert!(
+                !matches!(r, Dominance::DominatedBy),
+                "archive member {i} dominated by {j}: {:?} vs {:?}",
+                m[i].evaluation.objectives,
+                m[j].evaluation.objectives,
+            );
+        }
+    }
+});