Facón 1.4 - Hoja, new version chess engine

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 1.4 - what's new?

+430 Elo over 1.3 (Ordo ~2330, gauntlet 1040 games at 2min+1sec). NPS: +53% (Linux), +92% (Windows).

Search

• Check extension: when in check, extend search depth by 1. Check evasions are now resolved at full depth instead of dropping into quiescence. +30 Elo.
• Static Exchange Evaluation (SEE): full exchange simulation with x-ray discovery. Losing captures pruned in quiescence search. Shortcut: skip SEE for captures where the victim is worth >= the attacker.
• Reverse futility pruning: at depth 1-3, if the static evaluation already exceeds beta by a margin, prune the node without searching. Like NMP without the null search cost.
• Move-level futility pruning: at depth 1-2, skip quiet moves when the static evaluation plus a margin is still below alpha.
• Make/unmake legality: the per-move board copy in is_legal() (~700 bytes per pseudo-legal move) replaced with make, check, unmake. NPS: +130%.
• Move scores computed once: parallel scoring array eliminates O(n^2) move_score() calls during selection sort. NPS: +9%.
• LMR table precalculated: startup-initialized LMR_table[MAX_PLY][MAX_MOVES] replaces per-node log() calls.
• Quiet queen promotions in qsearch: pawn pushing to the 8th rank without capturing is now visible to quiescence search.
• Forced move instant-play: only one legal move = play immediately with zero search time.

Evaluation

• Piece mobility: pseudo-legal squares per piece, excluding own pieces and enemy pawn attacks. Knight 4cp, bishop 5cp, rook 2cp, queen 1cp per square.
• Open/semi-open files: rook bonuses for files with no pawns (+20cp) or no friendly pawns (+10cp).
• Rook on 7th rank: +20cp bonus.
• Bishop pair: +30cp when both bishops present.
• Knight outposts: +20cp for knights on ranks 4-6 that cannot be attacked by enemy pawns.

Transposition Table

• Depth-preferred replacement: shallow entries for different positions no longer evict deeper entries.
• TTEntry::depth int8_t to uint8_t: fixes overflow at MAX_PLY (128).

Time Management

• Base time budget increased (MOVES_TO_GO 30 to 25).
• Progressive easy-move reduction (x0.95 per stable iteration, cumulative, cancellable).
• AW fail-high time extension (proportional to fail-high count, capped at x1.50).
• Time cap relaxed (remaining/3 to remaining*2/5).
• Mate reduction only when winning (no longer triggers when being mated).
• TM extensions now always run before checking soft_stop().
• go depth X without clock no longer activates TM.
• movestogo UCI parameter now parsed and used.

Infrastructure

• bench command: 10 positions, depth 15 default. Quiet by default, bench verbose for full output.
• FACON_DEBUG build mode: diagnostic counters compiled only with -DFACON_DEBUG=ON.

Facón 1.4 - Hoja download