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Facón 1.3 - Yunque, new version chess engine

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Facón is a chess engine built from scratch in C++17, designed as a learning project and long-term development platform. The name comes from the facón — a traditional Argentine gaucho knife, forged by hand, raw and functional.
Each version carries a codename that follows the knife-making process: from rough rusty iron to a sharp, precise blade.
Author: Carlos M. Canavessi

Facón 1.3 - Yunque what's new?

+200 Elo over 1.2 (Ordo ~1900, gauntlet 1040 games at 2min+1sec)

Pre-work bug fixes

  • NMP depth floor (search.cpp): at depth == NMP_MIN_DEPTH (3), the NMP recursive call produced depth -1. With the corrected depth == 0 quiescence entry condition, this caused infinite recursion. Fixed: std::max(0, depth - 1 - NMP_REDUCTION).
  • depth <= 0 to depth == 0 (search.cpp): prerequisite for LMR. Negative depths from buggy reductions are now immediately detectable instead of silently entering quiescence.

Search

  • Late Move Reductions (LMR): quiet moves searched after the first 3 legal moves at depth >= 3 are searched at reduced depth. Formula: log(depth) * log(move_number) / 2.25, floored at 1. Re-searched at full depth if the reduced search raises alpha. Skipped for: captures, en passant, promotions, killer moves, in check.
  • History heuristic: history_[color][from][to] incremented by depth^2 on beta cutoffs. Replaces the flat ORDER_QUIET=0 score for quiet move ordering, improving LMR accuracy. Capped at 50,000 (below ORDER_KILLER2). Reset each search.
  • Aspiration windows: iterative deepening searches with a +/-50cp window around the previous score from depth 4+. On fail-low or fail-high, widens only in the failing direction and doubles delta. Full window used for depth < 4 and mate scores.

Evaluation

  • Pawn structure: five terms via bitboard operations -- isolated (-15cp), doubled (-15cp), backward (-12cp), passed (rank-scaled: 0/0/10/20/35/55/80/0cp), connected (+8cp). All computed symmetrically for both colors.
  • Mopup insufficient material guard: K+B vs K and K+N vs K are theoretical draws. Both exceed MOPUP_THRESHOLD (300cp) and previously activated corner-chasing. mopup_eval() now returns 0 when the strong side has exactly one minor piece.

Time Management

  • Quadratic extension scaling: extend_time() factors pre-scaled by (depth^2 / EXTENSION_FULL_DEPTH^2). PV changes at depth 2-9 have near-zero effect; extensions at the engine's operating depth (14+) apply the full factor. EXTENSION_FULL_DEPTH = 18.
  • accumulated_ext_ cap removed: the 2.0x cap consumed the budget at low depths before real extensions at depth 14+ could fire. The soft limit is now bounded only by the hard limit.
  • Easy move reduction (reduce_time): mate found (x0.05, one-shot), forced move (x0.1), PV+score stable >= 7 iterations at depth > 12 (x0.40, one-shot). Cancelled before extensions so they act on the full soft limit.
  • Emergency hard limit: when depth >= 25 and the extended soft would exceed hard, hard is raised to match (capped at 50% of raw remaining clock). Both limits rise together on subsequent extensions. Only triggered by real instability -- stable positions at depth 25+ would have already fired easy-move reduction.

Infrastructure

  • Centralized version system: PROJECT_VERSION and FACON_CODENAME in CMakeLists.txt control the binary name, startup banner, UCI id, and Windows version resource. To release: change codename from "dev" to "Yunque" and recompile.
  • perft command: perft N counts leaf nodes, perft divide N gives per-move breakdown. Bulk-counting at depth 1. Verified: startpos depth 5 = 4,865,609.
  • bitboard.cpp init message gated: suppressed when launched by GUI or automated tool.

Bug fixes

  • Aspiration window fail-low: the old handler set beta_asp = (alpha_asp + beta_asp) / 2, squeezing the upper bound. On re-search via TT/LMR interactions, this triggered artificial fail-highs (yo-yo effect). Fixed: widen only in the failing direction.
  • Mate reduction one-shot: is_mate_score() is true on every iteration after a mate is found. reduce_time(0.05) firing repeatedly collapsed the soft limit exponentially (x0.05^N). Fixed: mate_reduction_applied_ one-shot guard.
  • Race condition in cmd_ucinewgame(): TT.clear() could race with TT.probe()/TT.store() in the search thread. Fixed: join the search thread first.
  • Castling SAN check/mate: move_to_san() returned immediately for castling without checking if it delivers check or mate. Fixed: castling now falls through to the check/mate detection block.
  • seen[] guard mismatch: array enlarged to 1154 slots in 1.2, but the insertion guard still stopped at 1152. Updated to match.
Facón 1.3 - Yunque download from page


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