From 214f07975ae72f925f161b8ef76cf778dd03b07d Mon Sep 17 00:00:00 2001
From: Stephen Waits <steve@waits.net>
Date: Tue, 5 May 2026 12:03:52 -0600
Subject: [PATCH] perf(non_dominated_sort): cache oriented values + inline
 pareto_compare
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The Deb fast non-dominated sort calls `pareto_compare` twice for
every (i, j) pair, and each `pareto_compare` call invokes
`ObjectiveSpace::as_minimization` twice — so for an N-point
population that's 4·N·(N-1) fresh `Vec<f64>` allocations per sort.
At N=100 with thousands of generations across the compare harness,
this dominated the per-generation cost of every Pareto-based MOEA.

Cache `as_minimization`/feasibility/violation once per individual
up front, then inline the dominance test against those cached
arrays. The output (per-pair dominance outcome and the per-i
`dominates` lists) is bit-identical to `pareto_compare`.

gungraun (instructions):
- non_dominated_sort_2d n=50:    852 317 →   198 574 (-77 %, 4.3×)
- non_dominated_sort_2d n=200: 13 513 271 → 2 601 813 (-81 %, 5.2×)

Wall-clock (compare harness, 10-seed mean):
- NSGA-II / ZDT1:        268 →  65 ms (4.1×)
- NSGA-II / ZDT3:        267 →  65 ms (4.1×)
- NSGA-II / DTLZ2:       344 → 106 ms (3.2×)
- NSGA-II / Rastrigin:   260 →  71 ms (3.7×)
- NSGA-III / DTLZ2:      318 → 122 ms (2.6×)
- NSGA-III / DTLZ1:      303 → 122 ms (2.5×)
- SMS-EMOA / DTLZ2:     1413 → 1369 ms (small additional win on top of HV)
- AGE-MOEA / DTLZ1:      430 → 229 ms (1.9×, on top of the AGE-MOEA caching)
- HypE / DTLZ2:           80 →  44 ms (1.8×)
---
 src/pareto/sort.rs | 72 ++++++++++++++++++++++++++++++++++++++++------
 1 file changed, 63 insertions(+), 9 deletions(-)

diff --git a/src/pareto/sort.rs b/src/pareto/sort.rs
index 19ea0e2..d030fed 100644
--- a/src/pareto/sort.rs
+++ b/src/pareto/sort.rs
@@ -2,7 +2,6 @@
 
 use crate::core::candidate::Candidate;
 use crate::core::objective::ObjectiveSpace;
-use crate::pareto::dominance::{Dominance, pareto_compare};
 
 /// Partition the population into Pareto fronts by dominance rank.
 ///
@@ -19,24 +18,79 @@ pub fn non_dominated_sort<D>(
         return Vec::new();
     }
 
+    // Precompute the per-individual feasibility, violation, and
+    // minimization-oriented objective vectors. The naïve formulation
+    // calls `pareto_compare` (and therefore `as_minimization`) twice for
+    // every pair, allocating two fresh Vec<f64>s per call; doing it once
+    // up front cuts that to one allocation per individual.
+    let feasible: Vec<bool> = population
+        .iter()
+        .map(|c| c.evaluation.is_feasible())
+        .collect();
+    let violation: Vec<f64> = population
+        .iter()
+        .map(|c| c.evaluation.constraint_violation)
+        .collect();
+    let oriented: Vec<Vec<f64>> = population
+        .iter()
+        .map(|c| objectives.as_minimization(&c.evaluation.objectives))
+        .collect();
+    let m = objectives.len();
+
     let mut dominates: Vec<Vec<usize>> = vec![Vec::new(); n];
     let mut dominated_by_count: Vec<usize> = vec![0; n];
     let mut fronts: Vec<Vec<usize>> = Vec::new();
     let mut first_front: Vec<usize> = Vec::new();
 
     for i in 0..n {
+        let ai_feasible = feasible[i];
+        let ai_violation = violation[i];
+        let ai = &oriented[i];
         for j in 0..n {
             if i == j {
                 continue;
             }
-            match pareto_compare(
-                &population[i].evaluation,
-                &population[j].evaluation,
-                objectives,
-            ) {
-                Dominance::Dominates => dominates[i].push(j),
-                Dominance::DominatedBy => dominated_by_count[i] += 1,
-                _ => {}
+            let bi_feasible = feasible[j];
+            let bi_violation = violation[j];
+            // Inline the body of `pareto_compare`. We only care about
+            // `Dominates` vs `DominatedBy`; `Equal` and `NonDominated`
+            // are no-ops here.
+            let dominates_outcome = match (ai_feasible, bi_feasible) {
+                (true, false) => Some(true),  // i dominates j
+                (false, true) => Some(false), // i is dominated
+                (false, false) => {
+                    if ai_violation < bi_violation {
+                        Some(true)
+                    } else if ai_violation > bi_violation {
+                        Some(false)
+                    } else {
+                        None
+                    }
+                }
+                (true, true) => {
+                    let bj = &oriented[j];
+                    let mut a_better_anywhere = false;
+                    let mut b_better_anywhere = false;
+                    for k in 0..m {
+                        let av = ai[k];
+                        let bv = bj[k];
+                        if av < bv {
+                            a_better_anywhere = true;
+                        } else if av > bv {
+                            b_better_anywhere = true;
+                        }
+                    }
+                    match (a_better_anywhere, b_better_anywhere) {
+                        (true, false) => Some(true),
+                        (false, true) => Some(false),
+                        _ => None,
+                    }
+                }
+            };
+            match dominates_outcome {
+                Some(true) => dominates[i].push(j),
+                Some(false) => dominated_by_count[i] += 1,
+                None => {}
             }
         }
         if dominated_by_count[i] == 0 {