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Add Internal Iterative Reduction #4

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9b7b00b
Add conservative internal iterative reduction
KnightStackSys Oct 9, 2026
db8aaf4
Apply IIR on PV nodes
KnightStackSys Oct 9, 2026
426cfe2
Merge pull request #12 from KnightStackSys/main
KnightStackSys Oct 10, 2026
7618222
Allow non-pv nodes
KnightStackSys Oct 10, 2026
e84518b
Tune IIR to PV-only at depth 6
KnightStackSys Oct 10, 2026
8c2bc49
Merge latest main into internal-iterative-reduction
KnightStackSys Oct 13, 2026
3045b90
Normalize IIR
KnightStackSys Oct 13, 2026
a01dc68
Add cut/all node search state
KnightStackSys Oct 13, 2026
0fe2db5
Propagate cut and all node types
KnightStackSys Oct 13, 2026
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7 changes: 5 additions & 2 deletions include/ironphoenix/search.hpp
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Original file line number Diff line number Diff line change
Expand Up @@ -87,7 +87,10 @@ namespace ironphoenix {
void run(Position position, std::ostream& out);
void runMultiPV(Position position, int multiPV, std::ostream& out);

int negamax(Position& pos, int depth, int alpha, int beta, int ply, bool pvNode, bool allowNull = true);
// cutNode marks a non-PV node expected to fail high. allNode is derived
// inside negamax as the complementary non-PV node type.
int negamax(Position& pos, int depth, int alpha, int beta, int ply,
bool pvNode, bool allowNull = true, bool cutNode = false);
int qsearch(Position& pos, int alpha, int beta, int ply, bool pvNode);
int evaluate(const Position& pos) const noexcept;
int moveScore(const Position& pos, Move move, Move ttMove, int ply, bool givesCheck) const noexcept;
Expand Down Expand Up @@ -132,4 +135,4 @@ namespace ironphoenix {
void emitBestMove(std::ostream& out, Move move);
};

}
}
150 changes: 112 additions & 38 deletions src/search.cpp
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Original file line number Diff line number Diff line change
Expand Up @@ -24,6 +24,8 @@ namespace ironphoenix {
constexpr int HISTORY_MOVE_PRUNE_MAX_DEPTH = 4;
constexpr int HISTORY_MOVE_PRUNE_MARGIN_PER_DEPTH = 1'500;

constexpr int IIR_MIN_DEPTH = 6;

constexpr int NULL_MOVE_MIN_DEPTH = 2;
constexpr int NULL_MOVE_DIVISOR = 190;
constexpr std::size_t CHECK_BIT_WORDS = (MAX_MOVES + 63u) / 64u;
Expand Down Expand Up @@ -473,7 +475,8 @@ namespace ironphoenix {
return bestScore;
}

int SearchEngine::negamax(Position& pos, int depth, int alpha, int beta, int ply, bool pvNode, bool allowNull) {
int SearchEngine::negamax(Position& pos, int depth, int alpha, int beta, int ply,
bool pvNode, bool allowNull, bool cutNode) {
if (ply >= MAX_PLY - 1)
return evaluate(pos);
if (shouldStop())
Expand All @@ -482,6 +485,11 @@ namespace ironphoenix {
if (depth <= 0)
return qsearch(pos, alpha, beta, ply, pvNode);

// Node-type classification. PV nodes are never cut nodes. A non-PV node
// that is not expected to fail high is an all node.
cutNode = !pvNode && cutNode;
const bool allNode = !pvNode && !cutNode;

++nodes_;
selDepth_ = std::max(selDepth_, ply);
pvLength_[ply] = ply;
Expand Down Expand Up @@ -513,9 +521,16 @@ namespace ironphoenix {
return ttProbe.score;
}

const int staticEval = evaluate(pos);

const bool inCheckNode = pos.inCheck();
const auto& tuning = Tuning::search();

const Color us = pos.sideToMove();

const Color partner = static_cast<Color>(static_cast<unsigned>(us) ^ 2u);
const bool partnerChecked = pos.inCheck(partner);

const bool mateWindow = !(alpha > -MATE_THRESHOLD && beta < MATE_THRESHOLD);

// Null Move Pruning (NMP): keep expensive partner-check detection and
Expand All @@ -524,42 +539,50 @@ namespace ironphoenix {
&& !pvNode
&& ply > 0
&& depth >= NULL_MOVE_MIN_DEPTH
&& staticEval >= beta
&& !inCheckNode
&& !partnerChecked
&& !mateWindow
&& hasNonPawnMaterial(pos, us)) {

const Color partner = static_cast<Color>(static_cast<unsigned>(us) ^ 2u);
if (!pos.inCheck(partner)) {
const int staticEval = evaluate(pos);

if (staticEval >= beta) {
StateInfo nullState;
pos.makeNullMove(nullState);
searchStack_[ply + 1] = HistoryContext{};

// Dynamic null-move reduction: reduce the depth more aggressively
// at deeper nodes and for higher static evaluations.
const int reduction = 4 + depth / 5 + std::min(3, (staticEval - beta) / NULL_MOVE_DIVISOR);
const int nullScore = -negamax(
pos,
depth - reduction,
-beta,
-beta + 1,
ply + 1,
false,
false
);

pos.undoNullMove(nullState);

if (shouldStop())
return 0;
StateInfo nullState;
pos.makeNullMove(nullState);
searchStack_[ply + 1] = HistoryContext{};

// Dynamic null-move reduction: reduce the depth more aggressively
// at deeper nodes and for higher static evaluations.
const int reduction = 4 + depth / 5 + std::min(3, (staticEval - beta) / NULL_MOVE_DIVISOR);
const int nullScore = -negamax(
pos,
depth - reduction,
-beta,
-beta + 1,
ply + 1,
false,
false,
false
);

pos.undoNullMove(nullState);

if (shouldStop())
return 0;

// Never manufacture a mate score from a synthetic pass.
if (nullScore >= beta && nullScore < MATE_THRESHOLD)
return nullScore;
}

// Never manufacture a mate score from a synthetic pass.
if (nullScore >= beta && nullScore < MATE_THRESHOLD)
return nullScore;
}
}
// Internal Iterative Reduction (IIR): keep this deliberately conservative
// in 4PC. Only reduce deep PV nodes when move ordering has no TT move to
// guide the search. Non-PV nodes retain their full depth.
if (pvNode
&& ply > 0
&& depth >= IIR_MIN_DEPTH
&& !inCheckNode
&& !partnerChecked
&& !ttMove) {
depth -= 1;
}

MoveList moves;
Expand All @@ -580,7 +603,7 @@ namespace ironphoenix {
givesCheckBits.data());
}
else {
// At cut nodes avoid sorting the entire move list and avoid eager
// At non-PV nodes avoid sorting the entire move list and avoid eager
// givesCheck() calls. Score cheaply, then select the best few moves
// only as they are actually requested by the search loop.
for (std::size_t i = 0; i < moves.size; ++i)
Expand Down Expand Up @@ -696,7 +719,19 @@ namespace ironphoenix {

++legalMoves;
if (legalMoves == 1) {
score = -negamax(pos, depth - 1, -beta, -alpha, ply + 1, pvNode);
// PV nodes keep a PV child. For non-PV nodes, an all node
// expects its first child to be a cut node, while a cut node
// expects its first child to be an all node.
score = -negamax(
pos,
depth - 1,
-beta,
-alpha,
ply + 1,
pvNode,
true,
allNode
);
}
else {

Expand All @@ -712,19 +747,58 @@ namespace ironphoenix {
reduction = std::clamp(reduction, 0, depth - 2);
}

// PVS scout searches are cut nodes: from the child point of
// view we expect a fail-high against the null window when the
// move is not good enough to raise the parent's alpha.
if (reduction > 0) {
const int reducedDepth = depth - 1 - reduction;
score = -negamax(pos, reducedDepth, -alpha - 1, -alpha, ply + 1, false);
score = -negamax(
pos,
reducedDepth,
-alpha - 1,
-alpha,
ply + 1,
false,
true,
true
);

if (!shouldStop() && score > alpha)
score = -negamax(pos, depth - 1, -alpha - 1, -alpha, ply + 1, false);
score = -negamax(
pos,
depth - 1,
-alpha - 1,
-alpha,
ply + 1,
false,
true,
true
);
}
else {
score = -negamax(pos, depth - 1, -alpha - 1, -alpha, ply + 1, false);
score = -negamax(
pos,
depth - 1,
-alpha - 1,
-alpha,
ply + 1,
false,
true,
true
);
}

if (!shouldStop() && score > alpha && score < beta)
score = -negamax(pos, depth - 1, -beta, -alpha, ply + 1, pvNode);
score = -negamax(
pos,
depth - 1,
-beta,
-alpha,
ply + 1,
pvNode,
true,
false
);
}
}
pos.undoMove(move, st);
Expand Down
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