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Add guarded qsearch delta pruning #16

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Add guarded qsearch delta pruning #16

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30 changes: 29 additions & 1 deletion src/search.cpp
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Original file line number Diff line number Diff line change
Expand Up @@ -17,6 +17,7 @@ namespace ironphoenix {
namespace {

constexpr int LMP_HISTORY_THRESHOLD = 2'000;
constexpr int DELTA_MARGIN = 200;

IRONPHOENIX_FORCE_INLINE Bitboard teamOccupancy(const Position& pos, Color c) noexcept {
const Color partner = static_cast<Color>(static_cast<unsigned>(c) ^ 2u);
Expand Down Expand Up @@ -281,10 +282,12 @@ namespace ironphoenix {
}

int bestScore = -INF;
int staticEval = -INF;
Move bestMove{};

if (!inCheckNode) {
bestScore = evaluate(pos);
staticEval = evaluate(pos);
bestScore = staticEval;

if (bestScore >= beta) {
tt_.store(pos.key(), 0, bestScore, TTBound::Lower, Move{}, pvNode, ply);
Expand Down Expand Up @@ -319,6 +322,31 @@ namespace ironphoenix {

const bool terminalKingCapture = isTerminalKingCapture(pos, move);
const bool givesCheckMove = !terminalKingCapture && pos.givesCheck(move);
const bool normalWindow = alpha > -MATE_THRESHOLD && beta < MATE_THRESHOLD;

// Delta pruning: in quiet qsearch positions, skip a non-checking
// capture when even the captured material plus a safety margin cannot
// raise alpha. Promotions and terminal king captures are never pruned.
// Keep PV qsearch exact and avoid all mate-window applications.
if (!pvNode
&& !inCheckNode
&& normalWindow
&& move.isCapture()
&& !move.isPromotion()
&& !terminalKingCapture
&& !givesCheckMove) {
Piece captured = move.captured();
if (!isPiece(captured)) {
const Square capSq = pos.capturedSquare(move, us);
if (capSq != SQ_NONE)
captured = pos.pieceAt(capSq);
}

if (isPiece(captured)
&& staticEval + pieceValue(captured) + DELTA_MARGIN <= alpha) {
continue;
}
}

int score = -INF;
bool legal = true;
Expand Down
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