On a converged population the boost was permanently on, so there was nothing
left for it to intervene in. Its trigger, DIVERSITY_BOOST_THRESHOLD at 0.35, sat
above SELECTION_DIVERSITY_FLOOR at 0.30 - the floor the selection itself
guarantees - so `diversity < threshold` was true in every generation after
convergence. The threshold now sits below the floor.
The immigrants the boost injects are by construction the worst individuals in
the pool, and `_select_diverse` ran a plain tournament over parents and
offspring together, so they were removed in the very generation that created
them and their genes never recombined. A bounded share of seats
(IMMIGRANT_PROTECTION_FRACTION) is now reserved for them for
IMMIGRANT_PROTECTION_GENERATIONS selections. The incumbent is protected by
genome key, so no immigrant can evict the best solution or an equal-genome twin,
and offspring do not inherit the protection.
The log line was edge-triggered on `diversity_boost_active`, which was also
cleared on every fitness improvement, so a running boost re-announced itself
with "stagnation 0" while a boost that never stopped looked like several short
ones. It now tracks the boost alone, and the end of a boost is logged too.
Measured on a converged population: without protection 0 of 12 immigrants
survive the selection, with it all 12 do, and the incumbent is kept either way.
Andreas
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