fix(database): choose island migrants before moving any

openevolve/database.py

@@ -2078,12 +2078,18 @@ def migrate_programs(self) -> None:
             self._validate_migration_results()
             return
 
+        # Choose every island's migrants from the population before this round. Choosing them
+        # while migrating lets an island's top slots go to copies it has just received (which
+        # _migrate_one skips), and lets those copies displace its own programs from their cells
+        # before they get the chance to migrate.
+        planned_moves = []
         for i, island in enumerate(self.islands):
-            if len(island) == 0:
-                continue
-
-            # Select top programs from this island for migration
-            island_programs = [self.programs[pid] for pid in island if pid in self.programs]
+            # Migrant copies are never migrated again, so they must not take migration slots
+            island_programs = [
+                self.programs[pid]
+                for pid in island
+                if pid in self.programs and not self.programs[pid].metadata.get("migrant", False)
+            ]
             if not island_programs:
                 continue
 
@@ -2102,7 +2108,10 @@ def migrate_programs(self) -> None:
 
             for migrant in migrants:
                 for target_island in target_islands:
-                    self._migrate_one(migrant.id, target_island)
+                    planned_moves.append((migrant.id, target_island))
+
+        for program_id, target_island in planned_moves:
+            self._migrate_one(program_id, target_island)
 
         # Update last migration generation
         self.last_migration_generation = max(self.island_generations)

tests/test_island_migration.py

@@ -114,6 +114,21 @@ def test_migration_ring_topology(self):
         # This is a known limitation of the current implementation that processes islands
         # sequentially while modifying them, causing interference between migration rounds.
 
+    def test_every_island_migrates_its_own_best(self):
+        """Migrants are chosen before any are moved, so an island's own best still migrates
+        even when a better migrant has just arrived from the previous island in the ring."""
+        # Different code lengths put the programs in different MAP-Elites cells
+        for pid, score, island, pad in [("a", 0.9, 0, 0), ("b", 0.5, 1, 400), ("c", 0.3, 2, 800)]:
+            program = self._create_test_program(pid, score, island)
+            program.code += "#" * (pad + 1)
+            self.db.add(program, target_island=island)
+
+        self.db.migrate_programs()
+
+        for island in self.db.islands:
+            scores = sorted(self.db.programs[pid].metrics["score"] for pid in island)
+            self.assertEqual(scores, [0.3, 0.5, 0.9])
+
     def test_migration_rate_respected(self):
         """Test that migration rate is properly applied"""
         # Add multiple programs to island 0