From 3b4f765e03c4b0ed5e44750eab575012ad88f3e6 Mon Sep 17 00:00:00 2001
From: Stephen Waits <steve@waits.net>
Date: Thu, 14 May 2026 06:41:33 -0600
Subject: [PATCH] perf(operators): make PMX and OX crossovers O(n) with
 value-indexed lookups

Both crossovers had an O(n) inner scan making them O(n^2): PMX located the
value to swap with `child.iter().position`, OX tested segment membership
with `segment.contains`. Both operate on strict permutations of 0..n, so a
value-indexed table (PMX: position kept in sync across swaps; OX: a static
membership bitmap) gives O(1) lookups. debug_asserts document the range
assumption, consistent with ERX and CX.

pmx_crossover_vary n=100: 21_507 -> 5_900 (-73%, 3.65x); n=30 -18%.
order_crossover_vary n=100: 18_265 -> 7_389 (-60%, 2.47x); n=30 -29%.
All four permutation crossovers are now O(n). Bit-identical for valid
permutations -- all 606 tests pass.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>
---
 src/operators/permutation.rs | 31 ++++++++++++++++++++++++++-----
 1 file changed, 26 insertions(+), 5 deletions(-)

diff --git a/src/operators/permutation.rs b/src/operators/permutation.rs
index 31eb652..9a261d1 100644
--- a/src/operators/permutation.rs
+++ b/src/operators/permutation.rs
@@ -372,12 +372,20 @@ fn ox_child(donor: &[usize], filler: &[usize], lo: usize, hi: usize) -> Vec<usiz
     let mut child = vec![0_usize; n];
     let segment = &donor[lo..hi];
     child[lo..hi].copy_from_slice(segment);
+    // Membership bitmap for the donor segment. OX operates on strict
+    // permutations of 0..n, so a value-indexed bitmap replaces the
+    // O(segment) `contains` scan inside the fill loop with an O(1) check.
+    let mut in_segment = vec![false; n];
+    for &v in segment {
+        debug_assert!(v < n, "OrderCrossover requires strict permutations of 0..n");
+        in_segment[v] = true;
+    }
     let mut fill_pos = hi % n;
     let mut filler_pos = hi % n;
     let mut placed = hi - lo;
     while placed < n {
         let v = filler[filler_pos];
-        if !segment.contains(&v) {
+        if !in_segment[v] {
             child[fill_pos] = v;
             fill_pos = (fill_pos + 1) % n;
             placed += 1;
@@ -452,16 +460,29 @@ fn pmx_child(donor: &[usize], base: &[usize], lo: usize, hi: usize) -> Vec<usize
     // Start from a copy of `base`; for each position k in [lo, hi), swap so
     // that child[k] == donor[k]. Each swap preserves the permutation.
     let mut child = base.to_vec();
+    let n = child.len();
+    // pos[value] = current index of that value in `child`. PMX operates on
+    // strict permutations of 0..n, so this value-indexed table replaces the
+    // O(n) `position` scan with an O(1) lookup, kept in sync across swaps.
+    let mut pos = vec![0_usize; n];
+    for (i, &v) in child.iter().enumerate() {
+        debug_assert!(
+            v < n,
+            "PartiallyMappedCrossover requires strict permutations of 0..n",
+        );
+        pos[v] = i;
+    }
     for k in lo..hi {
         let v = donor[k];
         if child[k] == v {
             continue;
         }
-        let cur = child
-            .iter()
-            .position(|&x| x == v)
-            .expect("permutation invariant: every value must appear");
+        let cur = pos[v];
+        let displaced = child[k];
         child.swap(k, cur);
+        // child[k] is now `v`; child[cur] is now `displaced`.
+        pos[v] = k;
+        pos[displaced] = cur;
     }
     child
 }