Perf/solver optimizations#28
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The optimalValue in BestNodeSearch was initialized to 0 instead of the guess parameter from the previous iteration. In MTD(f) iterative deepening the guess carries forward the best value found at shallower depths, so discarding it reset the search bounds unnecessarily.
Data structure changes: - Transposition<TStep>: class -> struct (eliminates heap allocations) - CacheTables: Dictionary -> array-based cache with power-of-2 bitmask indexing and KeyMarker XOR for empty slot disambiguation (TT: 2M slots, Eval: 8M slots) Search algorithm improvements: - Buffer-based move ordering: per-depth reusable List<TStep>[] replaces yield-return iterator state machines and per-node List allocations - TT step legality verification: transposition table optimal step is only used if confirmed present in LegalStepsAt output (prevents crashes from Zobrist hash collisions when Position.Identifier excludes LastStep) - IsGoal fast-path at depth 0: replaces expensive LegalStepsAt().Any() with O(1) IsGoal() check, reducing LegalStepsCalls by ~89%% - HeuristicValue simplified: terminal branch removed (handled by caller) Infrastructure: - History heuristic recording with depth-squared weighting (IHeuristicTables.GetHistoryScore, not yet used for move sorting) Benchmark results vs main (SixMaking depth-6): Test #1: 626ms -> 600ms (-4%%) Test #3: 1457ms -> 1266ms (-13%%) Test #4: CRASH -> 1030ms (fixed) Test #5: 16562ms -> 11218ms (-32%%)
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