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NMP! #10

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Jul 8, 2024
Merged

NMP! #10

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27 changes: 26 additions & 1 deletion src/board/position.cpp
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Original file line number Diff line number Diff line change
Expand Up @@ -238,12 +238,28 @@ void position::make_move(const moves::move move) {
m_stm = ~m_stm;
m_key ^= utils::zobrist::get_side_key(m_stm);

m_checkers_bb = attacks_to_king(king_square(m_stm), m_stm);
m_checkers_bb = attacks_to_king(king_square(m_stm), m_stm);
m_last_move_was_null = false;
}

template void position::make_move<true>(moves::move move);
template void position::make_move<false>(moves::move move);

void position::make_null_move() {
++m_half_move_clock;

m_hash_history.push_back(m_key);

m_key ^= utils::zobrist::get_side_key(m_stm);
m_key ^= utils::zobrist::get_en_passant_key(m_ep_sq);

m_stm = ~m_stm;
m_key ^= utils::zobrist::get_side_key(m_stm);

m_checkers_bb = attacks_to_king(king_square(m_stm), m_stm);
m_last_move_was_null = true;
}

void position::reset_to_start_pos() {
m_hash_history.clear();
m_pieces.fill(piece::none);
Expand Down Expand Up @@ -296,6 +312,15 @@ void position::reset_to_start_pos() {
m_pieces[std::to_underlying(square::e8)] = piece::b_king;
}

template <color C>
bool position::has_no_pawns() const {
return piece_count<C>(piece_type::pawn) == 0;
}

bool position::has_no_pawns(const color c) const {
return c == color::white ? has_no_pawns<color::white>() : has_no_pawns<color::black>();
}

bool position::is_square_attacked_by(const square sq, const color c) const {
const auto& our_pieces = occupancies(c);
const auto& our_pawns = piece_type_bb(piece_type::pawn) & our_pieces;
Expand Down
24 changes: 16 additions & 8 deletions src/board/position.hpp
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Expand Up @@ -139,14 +139,14 @@ class position {
return copy;
}

[[nodiscard]] bitboards::bitboard checkers() const { return m_checkers_bb; }
[[nodiscard]] color side_to_move() const { return m_stm; }
[[nodiscard]] square ep_square() const { return m_ep_sq; }
[[nodiscard]] castling_rights castling() const { return m_castling; }
[[nodiscard]] u8 fifty_move_rule() const { return m_half_move_clock; }
[[nodiscard]] u16 full_moves() const { return m_full_move_number; }
[[nodiscard]] zobrist_key key() const { return m_key; }
[[nodiscard]] std::vector<zobrist_key> hash_history() const { return m_hash_history; }
[[nodiscard]] bitboards::bitboard checkers() const { return m_checkers_bb; }
[[nodiscard]] color side_to_move() const { return m_stm; }
[[nodiscard]] square ep_square() const { return m_ep_sq; }
[[nodiscard]] castling_rights castling() const { return m_castling; }
[[nodiscard]] u8 fifty_move_rule() const { return m_half_move_clock; }
[[nodiscard]] u16 full_moves() const { return m_full_move_number; }
[[nodiscard]] zobrist_key key() const { return m_key; }
[[nodiscard]] bool last_move_was_null() const { return m_last_move_was_null; }

[[nodiscard]] piece piece_on(const square sq) const {
return m_pieces[std::to_underlying(sq)];
Expand Down Expand Up @@ -182,8 +182,12 @@ class position {
template <bool SaveHashHistory>
void make_move(moves::move move);

void make_null_move();

void reset_to_start_pos();

[[nodiscard]] bool has_no_pawns(color c) const;

[[nodiscard]] bool is_square_attacked_by(square sq, color c) const;

[[nodiscard]] bool is_valid() const;
Expand All @@ -208,6 +212,9 @@ class position {
};
// clang-format on

template <color C>
[[nodiscard]] bool has_no_pawns() const;

std::vector<zobrist_key> m_hash_history;
std::array<piece, constants::num_squares> m_pieces;
std::array<bitboards::bitboard, constants::num_piece_types> m_piece_bb;
Expand All @@ -219,6 +226,7 @@ class position {
square m_ep_sq;
castling_rights m_castling;
u8 m_half_move_clock;
bool m_last_move_was_null{false};
};

namespace util {
Expand Down
2 changes: 1 addition & 1 deletion src/main.cpp
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Original file line number Diff line number Diff line change
Expand Up @@ -8,7 +8,7 @@ int main(const int argc, const char *argv[]) {
board::bitboards::attacks::init();

if (argc > 1 && !strcmp(argv[1], "bench")) {
constexpr int bench_depth = 6;
constexpr int bench_depth = 7;

search::searcher searcher;
search::bench::run(searcher, bench_depth);
Expand Down
30 changes: 25 additions & 5 deletions src/search/search.cpp
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Original file line number Diff line number Diff line change
Expand Up @@ -13,6 +13,8 @@ namespace search {
constexpr int rfp_depth_limit = 6;
constexpr int rfp_margin = 70;

constexpr int nmp_base_reduction = 3;

void searcher::reset_info() {
m_info.stopped = false;
m_info.searched_nodes = 0ULL;
Expand Down Expand Up @@ -123,7 +125,7 @@ score searcher::qsearch(const board::position& pos, score alpha, const score bet
for (usize i = 0; i < move_list.size(); i++) {
const auto current_move = move_list.move_at(i);

board::position copy = pos;
auto copy = pos;
copy.make_move<true>(current_move);

if (!copy.was_legal())
Expand Down Expand Up @@ -193,21 +195,39 @@ score searcher::negamax(const board::position& pos,
&& entry.can_use_score(alpha, beta))
return tt_score;

pv_line child_pv{};

const score static_eval = eval::evaluate(pos);
const bool in_check = pos.checkers().bit_count() > 0;

if (!in_check && !pv_node) {
// Reverse Futility Pruning
if (depth <= rfp_depth_limit && static_eval - rfp_margin * depth >= beta)
return static_eval;

// Null Move Pruning: If after making a null move (forfeiting the side to move) we still
// have a strong enough position to produce a cutoff, we cut the search.
// See https://en.wikipedia.org/wiki/Null-move_heuristic for reference

if (!pos.last_move_was_null() && !pos.has_no_pawns(pos.side_to_move())
&& static_eval >= beta) {
const int r = nmp_base_reduction + depth / nmp_base_reduction;

auto copy = pos;
copy.make_null_move();

const score null_move_score =
-negamax<false>(copy, -beta, -beta - 1, depth - r, ply + 1, child_pv);

if (null_move_score >= beta)
return null_move_score;
}
}

if (ply >= constants::max_ply)
return static_eval;

u16 legal_moves{};
pv_line child_pv{};

u16 legal_moves{};
score best_score = -constants::score_infinite;
auto best_move = moves::move::null();
const score original_alpha = alpha;
Expand All @@ -220,7 +240,7 @@ score searcher::negamax(const board::position& pos,
for (usize i = 0; i < move_list.size(); ++i) {
const auto current_move = move_list.move_at(i);

board::position copy = pos;
auto copy = pos;
copy.make_move<true>(current_move);

if (!copy.was_legal())
Expand Down