Fix CrossoverRejuvenate by replacing bottom parents with top parents

Author: basvanwesting

CHANGELOG.md

@@ -54,12 +54,16 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 ### Added
 * Add `best_chromosome_indices()` on `Population`, used to implement
   `elitism_rate` without the need for a full sort
-* Add `CrossoverRejuvenate` as new implementation for the old `CrossoverClone`
-  behaviour. No copying, just turn parents into offspring in-place. The
+* Add `CrossoverRejuvenate` as new limited cloning implementation for the old
+  `CrossoverClone` behaviour, which had limited cloning. The new
   `CrossoverClone` now adds actual clones as offspring without removing the
-  parents. However `CrossoverRejuvenate` works remarkably bad, as the old
-  `CrossoverClone` had the pre-v0.20 selection-rate which still copied the best
-  parent chromosomes to repopulate after selection.
+  parents, which is much more cloning than the original. For
+  `CrossoverRejuvenate`: drop non-selected parents, then clone top parents to
+  repopulate, then rejuvenate selected parents to children in place. No copying
+  of chromosomes for creating the offspring itself, only for repopulating the
+  dropped non-selected parents (smaller fraction). However the cloning of
+  top-parents is crucial for driving the Evolve process as experimenting with
+  the evolve_nqueens example demonstrated.
 
 ## [0.19.4] - 2025-05-05
 

examples/evolve_nqueens.rs

@@ -51,8 +51,8 @@ fn main() {
         .with_target_fitness_score(0)
         // .with_replace_on_equal_fitness(true) // not crucial for this problem
         .with_mutate(MutateSingleGene::new(0.2))
-        // .with_crossover(CrossoverRejuvenate::new(0.9)) // works remarkably bad, extremely interesting why, probably the best_genes are lost all the time?
-        .with_crossover(CrossoverClone::new(0.9))
+        .with_crossover(CrossoverRejuvenate::new(0.9))
+        // .with_crossover(CrossoverClone::new(0.9))
         .with_select(SelectElite::new(0.5, 0.5))
         .with_reporter(EvolveReporterSimple::new(100))
         .build()

src/crossover/clone.rs

@@ -24,7 +24,7 @@ impl Crossover for Clone {
         let now = Instant::now();
         let existing_population_size = state.population.chromosomes.len();
         let selected_population_size =
-            (state.population.size() as f32 * self.selection_rate).ceil() as usize;
+            (existing_population_size as f32 * self.selection_rate).ceil() as usize;
         genotype
             .chromosome_cloner_expand(&mut state.population.chromosomes, selected_population_size);
         state

src/crossover/rejuvenate.rs

@@ -6,7 +6,9 @@ use crate::strategy::{StrategyAction, StrategyReporter, StrategyState};
 use rand::Rng;
 use std::time::Instant;
 
-/// Rejuvenate parents to children in place, no copying of chromosomes
+/// Drop non-selected parents, then clone top parents to repopulate, then rejuvenate selected
+/// parents to children in place. No copying of chromosomes for creating the offspring itself, only
+/// for repopulating the dropped non-selected parents (smaller fraction)
 /// Allowed for unique genotypes.
 #[derive(Clone, Debug)]
 pub struct Rejuvenate {
@@ -15,15 +17,25 @@ pub struct Rejuvenate {
 impl Crossover for Rejuvenate {
     fn call<G: EvolveGenotype, R: Rng, SR: StrategyReporter<Genotype = G>>(
         &mut self,
-        _genotype: &mut G,
+        genotype: &mut G,
         state: &mut EvolveState<G>,
         _config: &EvolveConfig,
         _reporter: &mut SR,
         _rng: &mut R,
     ) {
         let now = Instant::now();
+        let existing_population_size = state.population.chromosomes.len();
         let selected_population_size =
-            (state.population.size() as f32 * self.selection_rate).ceil() as usize;
+            (existing_population_size as f32 * self.selection_rate).ceil() as usize;
+        let dropped_population_size = (existing_population_size - selected_population_size).max(0);
+
+        genotype.chromosome_destructor_truncate(
+            &mut state.population.chromosomes,
+            selected_population_size,
+        );
+        genotype
+            .chromosome_cloner_expand(&mut state.population.chromosomes, dropped_population_size);
+
         state
             .population
             .chromosomes

tests/crossover/rejuvenate_test.rs

@@ -16,7 +16,7 @@ fn standard() {
     let population: Population<BinaryChromosome> = build::population_with_age(vec![
         (vec![true, true, true], 1),
         (vec![false, false, false], 2),
-        (vec![true, true, true], 1),
+        (vec![true, false, false], 2),
     ]);
 
     let mut state = EvolveState::new(&genotype);