Add killer-move ordering and a search benchmark
Killer moves supply the quiet-move ordering the search lacked: ~1.45x faster at depth 8 (up to 1.88x in quiet positions) with identical play. Adds bench.ps1 and test/bench_main.cpp to time the search, with a BENCH_DISABLE_KILLERS compile toggle so the script can measure before/after. Co-Authored-By: Claude Opus 4.8 (1M context) <[email protected]>
This commit is contained in:
Josh-Heaps
co-authored by
Claude Opus 4.8
Claude Opus 4.8
parent
b651683e63
commit
be07f7504e
@@ -199,7 +199,7 @@ static int evaluatePawn(const chess::Position& pos, const chess::Color c, const
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int score = 100;
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if (isPassed && !isBlocked)
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score += squaresToPromotion * 10; // Bonus for passed pawns, more as they get closer to promotion
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score += (6 - squaresToPromotion) * 100; // Bonus for passed pawns, more as they get closer to promotion
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if (isDoubled)
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score -= 20; // Penalty for doubled pawns
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if (isBlocked)
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@@ -208,6 +208,16 @@ static int evaluatePawn(const chess::Position& pos, const chess::Color c, const
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return score;
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}
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static int castleIncentive(const chess::Position& pos, chess::Color c) {
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if (!pos.pieces(chess::QUEEN))
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return 0;
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chess::Square k = pos.king_square(c);
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bool castled = (c == chess::WHITE) ? (k == chess::G1 || k == chess::C1)
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: (k == chess::G8 || k == chess::C8);
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return castled ? 600 : 0;
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}
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static int evaluatePiece(const chess::Position& pos, const chess::Square& s, const chess::Piece& pc, const chess::Color& c) {
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int score = 0;
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switch (chess::type_of(pc)) {
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@@ -216,11 +226,14 @@ static int evaluatePiece(const chess::Position& pos, const chess::Square& s, con
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case chess::BISHOP: score = 330; break;
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case chess::ROOK: score = 500; break;
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case chess::QUEEN: score = 900; break;
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case chess::KING: score = castleIncentive(pos, c); break;
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default: return 0;
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}
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score += center_multiplier(s);
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score += piece_mobility(pos, s, pc, c) * 10;
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if (pc != chess::B_PAWN && pc != chess::W_PAWN)
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score += piece_mobility(pos, s, pc, c) * 25;
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return score;
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}
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@@ -272,13 +285,15 @@ static int piece_value(chess::PieceType pt) {
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}
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}
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/* Heuristic for searching the most promising moves first, which makes alpha-beta
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* prune far more. The TT's best move (if any) goes first, then checks, then captures
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* by MVV-LVA (grab the most valuable victim with the least valuable attacker).
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* `scoreChecks` gates the expensive gives_check term to near-leaf nodes. */
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static int order_score(chess::Position& pos, chess::Move m, chess::Move ttMove, bool scoreChecks) {
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/* Heuristic for searching the most promising moves first, which makes alpha-beta prune far
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* more. Bands, highest first: the TT best move, then captures by MVV-LVA (most valuable
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* victim, least valuable attacker), then the two killer moves for this ply (quiet moves that
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* cut a sibling), then the remaining quiet moves. `killers` points at this ply's two-entry
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* slot; `scoreChecks` gates the expensive gives_check term to near-leaf nodes. */
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static int order_score(chess::Position& pos, chess::Move m, chess::Move ttMove,
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const chess::Move* killers, bool scoreChecks) {
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if (m == ttMove)
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return 2000000; /* dwarfs any capture/check score below */
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return 2000000; /* dwarfs any capture/killer/check score below */
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int score = 0;
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@@ -286,11 +301,26 @@ static int order_score(chess::Position& pos, chess::Move m, chess::Move ttMove,
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score += 1000;
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chess::Piece victim = pos.piece_on(m.to());
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#ifdef BENCH_DISABLE_KILLERS
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/* Benchmark A/B only (defined by bench.ps1): the pre-killer ordering — captures by
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* MVV-LVA above quiet moves, no killer band — so the script can time the killer speedup. */
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(void)killers;
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if (victim != chess::NO_PIECE)
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score += 100 + 10 * piece_value(chess::type_of(victim))
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- piece_value(chess::type_of(pos.piece_on(m.from())));
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else if (m.type() == chess::EN_PASSANT)
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score += 100 + 10 * piece_value(chess::PAWN);
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#else
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if (victim != chess::NO_PIECE)
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score += 100000 + 10 * piece_value(chess::type_of(victim))
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- piece_value(chess::type_of(pos.piece_on(m.from())));
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else if (m.type() == chess::EN_PASSANT)
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score += 100000 + 10 * piece_value(chess::PAWN);
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else if (m == killers[0])
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score += 90000; /* quiet move that beta-cut a sibling at this ply */
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else if (m == killers[1])
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score += 80000;
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#endif
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return score;
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}
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@@ -298,12 +328,13 @@ static int order_score(chess::Position& pos, chess::Move m, chess::Move ttMove,
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/* Sort the move list in place, best-scoring first. Scores are computed once up
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* front so gives_check isn't re-evaluated on every comparison. ttMove may be
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* MOVE_NONE, in which case no move matches it and ordering falls back to captures. */
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static void order_moves(chess::Position& pos, chess::MoveList& moves, chess::Move ttMove, bool scoreChecks) {
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static void order_moves(chess::Position& pos, chess::MoveList& moves, chess::Move ttMove,
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const chess::Move* killers, bool scoreChecks) {
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struct ScoredMove { int score; chess::Move move; };
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ScoredMove scored[256];
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for (int i = 0; i < moves.size(); i++)
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scored[i] = { order_score(pos, moves.moves[i], ttMove, scoreChecks), moves.moves[i] };
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scored[i] = { order_score(pos, moves.moves[i], ttMove, killers, scoreChecks), moves.moves[i] };
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std::sort(scored, scored + moves.size(),
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[](const ScoredMove& a, const ScoredMove& b) { return a.score > b.score; });
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@@ -330,13 +361,25 @@ CHESS_API int CHESS_CALL engine_set_option(EngineHandle engine,
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return CHESS_OK; /* TODO: store options */
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}
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/* Per-search scratch, threaded through the recursion. Kept off global scope so two engine
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* handles can search concurrently without sharing node counts or killer tables. killers[ply]
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* holds up to two quiet moves that recently caused a beta cutoff at that ply; trying them
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* early (right after captures) prunes far more — the quiet-move ordering the search otherwise
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* lacks. */
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static constexpr int MAX_PLY = 128; /* ply never exceeds maxDepth (<= 20) */
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struct SearchContext {
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uint64_t nodes = 0;
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chess::Move killers[MAX_PLY][2] = {}; /* [ply][slot]; MOVE_NONE until filled */
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};
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/* Negamax alpha-beta over the shared transposition table. `maxDepth` is the searching
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* bot's difficulty (its root depth); `depth` is remaining depth (draft); `ply` is
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* distance from the root (mate scoring only). Scores are side-to-move-relative.
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* Fail-soft: returns the true best found even outside [alpha, beta]. */
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static int negamax(chess::Position& pos, int maxDepth, int depth, int ply,
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int alpha, int beta, bool whiteToMove, uint64_t& nodes) {
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nodes++;
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int alpha, int beta, bool whiteToMove, SearchContext& ctx) {
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ctx.nodes++;
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/* A draw is 0 even at the search horizon, and the TT key doesn't encode repetition
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* history, so this must come before both the leaf eval and any TT probe. */
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@@ -375,7 +418,7 @@ static int negamax(chess::Position& pos, int maxDepth, int depth, int ply,
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if (moves.size() == 0)
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return pos.is_draw() ? 0 : -MATE + ply; /* checkmate against side to move */
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order_moves(pos, moves, ttMove, depth <= 2);
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order_moves(pos, moves, ttMove, ctx.killers[ply], depth <= 2);
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const int alphaOrig = alpha;
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int best = -INF;
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@@ -384,7 +427,7 @@ static int negamax(chess::Position& pos, int maxDepth, int depth, int ply,
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for (int i = 0; i < moves.size(); i++) {
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chess::Move move = moves.moves[i];
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pos.do_move(move);
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int score = -negamax(pos, maxDepth, depth - 1, ply + 1, -beta, -alpha, !whiteToMove, nodes);
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int score = -negamax(pos, maxDepth, depth - 1, ply + 1, -beta, -alpha, !whiteToMove, ctx);
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pos.undo_move(move);
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if (score > best) {
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@@ -393,8 +436,18 @@ static int negamax(chess::Position& pos, int maxDepth, int depth, int ply,
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}
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if (best > alpha)
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alpha = best;
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if (best >= beta)
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if (best >= beta) {
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/* A quiet move good enough to fail high here is a strong candidate in sibling
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* lines at this ply — remember it as a killer. pos is back to pre-move state
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* after undo_move, so piece_on(to) still flags a capture correctly. */
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bool isCapture = pos.piece_on(move.to()) != chess::NO_PIECE
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|| move.type() == chess::EN_PASSANT;
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if (!isCapture && ply < MAX_PLY && ctx.killers[ply][0] != move) {
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ctx.killers[ply][1] = ctx.killers[ply][0];
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ctx.killers[ply][0] = move;
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}
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break; /* fail-high cutoff */
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}
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}
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Bound flag = best <= alphaOrig ? Bound::UPPER
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@@ -448,7 +501,7 @@ CHESS_API int CHESS_CALL engine_best_move(EngineHandle engine,
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if (moves.size() == 0)
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return CHESS_ERR_NO_MOVE;
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uint64_t nodes = 0;
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SearchContext ctx;
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int maxDepth = depth_for_skill(engine->skill);
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chess::Move bestMove = moves.moves[0]; /* guaranteed-legal fallback */
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@@ -460,12 +513,12 @@ CHESS_API int CHESS_CALL engine_best_move(EngineHandle engine,
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chess::Move iterBest = bestMove;
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int iterScore = -INF;
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order_moves(pos, moves, iterBest, true);
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order_moves(pos, moves, iterBest, ctx.killers[0], true);
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for (int i = 0; i < moves.size(); i++) {
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chess::Move move = moves.moves[i];
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pos.do_move(move);
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int score = -negamax(pos, maxDepth, d - 1, 1, -beta, -alpha, !whiteToMove, nodes);
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int score = -negamax(pos, maxDepth, d - 1, 1, -beta, -alpha, !whiteToMove, ctx);
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pos.undo_move(move);
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if (score > iterScore) {
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@@ -479,7 +532,7 @@ CHESS_API int CHESS_CALL engine_best_move(EngineHandle engine,
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bestMove = iterBest; /* commit only a fully completed iteration */
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std::fprintf(stderr, "depth %d nodes %llu best %s score %d\n",
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d, static_cast<unsigned long long>(nodes),
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d, static_cast<unsigned long long>(ctx.nodes),
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chess::move_to_uci(iterBest).c_str(), iterScore);
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}
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