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Merged
kevlu8 merged 1 commit into from
Jun 21, 2025
Merged

Add futility pruning #53

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18 changes: 17 additions & 1 deletion engine/search.cpp
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Original file line number Diff line number Diff line change
Expand Up @@ -273,6 +273,9 @@ Value __recurse(Board &board, int depth, Value alpha = -VALUE_INFINITE, Value be
if (cutoff)
return cutoff->eval;

Value cur_eval = 0;
if (!in_check) cur_eval = eval(board) * side;

// Reverse futility pruning
if (!in_check && !pv && depth <= 3) {
/**
Expand All @@ -282,7 +285,6 @@ Value __recurse(Board &board, int depth, Value alpha = -VALUE_INFINITE, Value be
* We need to make sure that we aren't in check (since we might get mated) and that the
* TT entry exists (so that the current position is actually good).
*/
int cur_eval = eval(board) * side; // TODO: Use the TT entry instead of the eval function?
int margin = RFP_THRESHOLD * depth;
if (cur_eval >= beta + margin)
return cur_eval - margin;
Expand Down Expand Up @@ -326,6 +328,20 @@ Value __recurse(Board &board, int depth, Value alpha = -VALUE_INFINITE, Value be
for (int i = 0; i < moves.size(); i++) {
Move &move = scores[i].first;
line[ply] = move;

bool capt = (board.piece_boards[OPPOCC(board.side)] & square_bits(move.dst()));
bool promo = (move.type() == PROMOTION);

if (depth == 1 && i > 0 && !in_check && !capt && !promo && abs(alpha) < VALUE_MATE_MAX_PLY && abs(beta) < VALUE_MATE_MAX_PLY) {
/**
* Futility pruning
*
* If we are at the leaf of the search, we can prune moves that are
* probably not going to be better than alpha.
*/
if (cur_eval + FUTILITY_THRESHOLD < alpha) continue;
}

board.make_move(move);

Value score;
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4 changes: 4 additions & 0 deletions engine/search.hpp
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Original file line number Diff line number Diff line change
Expand Up @@ -28,6 +28,10 @@
// This is the threshold for delta pruning (in centipawns)
#define DELTA_THRESHOLD (300 * CP_SCALE_FACTOR)

// Futility pruning threshold
// This is the threshold for futility pruning (in centipawns)
#define FUTILITY_THRESHOLD (300 * CP_SCALE_FACTOR)

extern uint64_t nodes;

std::pair<Move, Value> search(Board &board, int64_t time = 1e9, bool quiet = false);
Expand Down