FixedKSwapMutation

Mutation operator that preserves subset cardinality K by swapping one 1 → 0 and one 0 → 1.

Intent:

• Pick one index from the 1‑pool (selected topics) and set it to false (swap OUT).

• Pick one index from the 0‑pool (unselected topics) and set it to true (swap IN).

Robustness:

• jMetal may occasionally pass DefaultBinarySolution instances. This operator lifts any BinarySolution into a BestSubsetSolution view before mutating (cloning bitset/objectives and synthesizing labels as needed).

Guarantees:

• If both pools are non‑empty, K is preserved exactly.

• Degenerate masks are repaired safely:

• all‑zeros → flip one random bit to true (K becomes 1)

• all‑ones → flip one random bit to false (K becomes N‑1)

• Operates in‑place on the (adapted) BestSubsetSolution.

Delta‑evaluation hook:

• On a true swap or repair, sets:

• BestSubsetSolution.lastSwapOutIndex

• BestSubsetSolution.lastSwapInIndex

• BestSubsetSolution.lastMutationWasFixedKSwap = true so that BestSubsetProblem.evaluate(...) can update cached per‑system sums in O(S).

Determinism:

• All randomness goes through jMetal’s singleton RNG, so external seeding works.

Complexity:

• O(N) to collect indices + O(1) updates.

Parameters