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PV Search! #8

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merged 1 commit into from
Jul 8, 2024
Merged

PV Search! #8

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35 changes: 25 additions & 10 deletions src/search/search.cpp
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Original file line number Diff line number Diff line change
Expand Up @@ -55,8 +55,9 @@ void searcher::main_search(const board::position& pos) {

// Iterative deepening loop
for (int current_depth = 1; current_depth <= m_limits.depth_limit; ++current_depth) {
const score best_score = negamax(pos, -constants::score_infinite, constants::score_infinite,
current_depth, 0, m_info.pv);
const score best_score =
negamax<true>(pos, -constants::score_infinite, constants::score_infinite, current_depth,
0, m_info.pv);

if (m_info.stopped) {
// If search stopped too early and we don't have a best move, we update it in order to
Expand All @@ -76,6 +77,7 @@ void searcher::main_search(const board::position& pos) {
std::cout << std::format("bestmove {}", best_move.to_string()) << std::endl;
}

template <bool pv_node>
score searcher::qsearch(const board::position& pos, score alpha, const score beta, const int ply) {
++m_info.searched_nodes;

Expand All @@ -93,7 +95,7 @@ score searcher::qsearch(const board::position& pos, score alpha, const score bet
const auto tt_score = tt_hit ? tt::score_from_tt(entry.value(), ply) : constants::score_none;
const auto tt_move = tt_hit ? entry.move() : moves::move::null();

if (tt_score != constants::score_none && entry.can_use_score(alpha, beta))
if (!pv_node && tt_score != constants::score_none && entry.can_use_score(alpha, beta))
return tt_score;

const score static_eval = eval::evaluate(pos);
Expand Down Expand Up @@ -124,10 +126,7 @@ score searcher::qsearch(const board::position& pos, score alpha, const score bet
if (!copy.was_legal())
continue;

const score current_score = -qsearch(copy, -beta, -alpha, ply + 1);

if (m_info.stopped)
return 0;
const score current_score = -qsearch<pv_node>(copy, -beta, -alpha, ply + 1);

if (current_score > best_score) {
best_score = current_score;
Expand All @@ -140,6 +139,9 @@ score searcher::qsearch(const board::position& pos, score alpha, const score bet
break;
}
}

if (m_info.stopped)
return 0;
}

const auto tt_flag = best_score >= beta ? tt::tt_entry::tt_flag::lower_bound
Expand All @@ -151,6 +153,7 @@ score searcher::qsearch(const board::position& pos, score alpha, const score bet
return best_score;
}

template <bool pv_node>
score searcher::negamax(const board::position& pos,
score alpha,
const score beta,
Expand All @@ -169,7 +172,7 @@ score searcher::negamax(const board::position& pos,
}

if (depth <= 0)
return qsearch(pos, alpha, beta, ply);
return qsearch<pv_node>(pos, alpha, beta, ply);

const bool root_node = ply == 0;

Expand All @@ -186,7 +189,7 @@ score searcher::negamax(const board::position& pos,
const auto tt_move = tt_hit ? entry.move() : moves::move::null();
const u8 tt_depth = entry.depth();

if (!root_node && tt_score != constants::score_none && tt_depth >= depth
if (!pv_node && tt_score != constants::score_none && tt_depth >= depth
&& entry.can_use_score(alpha, beta))
return tt_score;

Expand All @@ -213,7 +216,19 @@ score searcher::negamax(const board::position& pos,

++legal_moves;

const score current_score = -negamax(copy, -beta, -alpha, depth - 1, ply + 1, child_pv);
score current_score;

if (i == 0)
// Search the first move with a full window
current_score = -negamax<pv_node>(copy, -beta, -alpha, depth - 1, ply + 1, child_pv);
else {
// Do a null window search to see if we find a better move
current_score = -negamax<false>(copy, -alpha - 1, -alpha, depth - 1, ply + 1, child_pv);

if (current_score > alpha && pv_node)
// If we found a better move, do a full re-search
current_score = -negamax<true>(copy, -beta, -alpha, depth - 1, ply + 1, child_pv);
}

if (current_score > best_score) {
best_score = current_score;
Expand Down
4 changes: 4 additions & 0 deletions src/search/search.hpp
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Original file line number Diff line number Diff line change
Expand Up @@ -69,15 +69,18 @@ class searcher {
time_manager m_timer{};

/// @brief Quiescence search, to get rid of the horizon effect
/// @tparam pv_node Indicates if the current node is from the principal variation
/// @param pos Position to search from
/// @param alpha Best score for the maximizing player
/// @param beta Best score for the minimizing player
/// @param ply Internal depth of the search tree (seldepth)
/// @returns The best score found
/// @note See https://en.wikipedia.org/wiki/Quiescence_search for reference
template <bool pv_node>
score qsearch(const board::position& pos, score alpha, score beta, int ply);

/// @brief Fail-soft negamax algorithm with alpha-beta pruning
/// @tparam pv_node Indicates if the current node is from the principal variation
/// @param pos Position to search from
/// @param alpha Best score for the maximizing player
/// @param beta Best score for the minimizing player
Expand All @@ -86,6 +89,7 @@ class searcher {
/// @param pv PV-List on the stack, to keep track of the principal variation
/// @returns The best score found
/// @note See https://en.wikipedia.org/wiki/Negamax for reference
template <bool pv_node>
score negamax(
const board::position& pos, score alpha, score beta, int depth, int ply, pv_line& pv);

Expand Down