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Better document king safety evaluation

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No functional change.

Signed-off-by: Marco Costalba <mcostalba@gmail.com>
This commit is contained in:
Marco Costalba 2009-11-12 19:01:44 +01:00
parent 764229a2e2
commit 7d0e0ff95e
1 changed files with 58 additions and 55 deletions
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113
src/evaluate.cpp
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@ -720,8 +720,12 @@ namespace {
void evaluate_king(const Position& pos, EvalInfo& ei) { void evaluate_king(const Position& pos, EvalInfo& ei) {
const Color Them = (Us == WHITE ? BLACK : WHITE); const Color Them = (Us == WHITE ? BLACK : WHITE);
Bitboard undefended, attackedByOthers, escapeSquares, occ, b, b2, safe;
Square from, to;
bool sente;
int attackUnits, count, shelter = 0;
const Square s = pos.king_square(Us); const Square s = pos.king_square(Us);
int shelter = 0;
// King shelter // King shelter
if (relative_rank(Us, s) <= RANK_4) if (relative_rank(Us, s) <= RANK_4)
@ -738,77 +742,80 @@ namespace {
&& ei.kingAdjacentZoneAttacksCount[Them]) && ei.kingAdjacentZoneAttacksCount[Them])
{ {
// Is it the attackers turn to move? // Is it the attackers turn to move?
bool sente = (Them == pos.side_to_move()); sente = (Them == pos.side_to_move());
// Find the attacked squares around the king which has no defenders // Find the attacked squares around the king which has no defenders
// apart from the king itself // apart from the king itself
Bitboard undefended = undefended = ei.attacked_by(Them) & ei.attacked_by(Us, KING);
ei.attacked_by(Them) & ~ei.attacked_by(Us, PAWN) undefended &= ~( ei.attacked_by(Us, PAWN) | ei.attacked_by(Us, KNIGHT)
& ~ei.attacked_by(Us, KNIGHT) & ~ei.attacked_by(Us, BISHOP) | ei.attacked_by(Us, BISHOP) | ei.attacked_by(Us, ROOK)
& ~ei.attacked_by(Us, ROOK) & ~ei.attacked_by(Us, QUEEN) | ei.attacked_by(Us, QUEEN));
& ei.attacked_by(Us, KING);
Bitboard occ = pos.occupied_squares(), b, b2;
// Initialize the 'attackUnits' variable, which is used later on as an // Initialize the 'attackUnits' variable, which is used later on as an
// index to the SafetyTable[] array. The initial value is based on the // index to the SafetyTable[] array. The initial value is based on the
// number and types of the attacking pieces, the number of attacked and // number and types of the attacking pieces, the number of attacked and
// undefended squares around the king, the square of the king, and the // undefended squares around the king, the square of the king, and the
// quality of the pawn shelter. // quality of the pawn shelter.
int attackUnits = attackUnits = Min(25, (ei.kingAttackersCount[Them] * ei.kingAttackersWeight[Them]) / 2)
Min((ei.kingAttackersCount[Them] * ei.kingAttackersWeight[Them]) / 2, 25) + 3 * (ei.kingAdjacentZoneAttacksCount[Them] + count_1s_max_15<HasPopCnt>(undefended))
+ (ei.kingAdjacentZoneAttacksCount[Them] + count_1s_max_15<HasPopCnt>(undefended)) * 3 + InitKingDanger[relative_square(Us, s)]
+ InitKingDanger[relative_square(Us, s)] - (shelter >> 5); - (shelter >> 5);
// Analyse safe queen contact checks // Analyse safe queen contact checks
b = undefended & ei.attacked_by(Them, QUEEN) & ~pos.pieces_of_color(Them); b = undefended & ei.attacked_by(Them, QUEEN) & ~pos.pieces_of_color(Them);
if (b) if (b)
{ {
Bitboard attackedByOthers = attackedByOthers = ei.attacked_by(Them, PAWN) | ei.attacked_by(Them, KNIGHT)
ei.attacked_by(Them, PAWN) | ei.attacked_by(Them, KNIGHT) | ei.attacked_by(Them, BISHOP) | ei.attacked_by(Them, ROOK);
| ei.attacked_by(Them, BISHOP) | ei.attacked_by(Them, ROOK);
b &= attackedByOthers; b &= attackedByOthers;
// Squares attacked by the queen and supported by another enemy piece and
// not defended by other pieces but our king.
if (b) if (b)
{ {
// The bitboard b now contains the squares available for safe queen // The bitboard b now contains the squares available for safe queen
// contact checks. // contact checks.
int count = count_1s_max_15<HasPopCnt>(b); count = count_1s_max_15<HasPopCnt>(b);
attackUnits += QueenContactCheckBonus * count * (sente ? 2 : 1); attackUnits += QueenContactCheckBonus * count * (sente ? 2 : 1);
// Is there a mate threat? // Is there a mate threat?
if (QueenContactMates && !pos.is_check()) if (QueenContactMates && !pos.is_check())
{
Bitboard escapeSquares =
pos.attacks_from<KING>(s) & ~pos.pieces_of_color(Us) & ~attackedByOthers;
while (b)
{ {
Square from, to = pop_1st_bit(&b); escapeSquares = pos.attacks_from<KING>(s) & ~pos.pieces_of_color(Us) & ~attackedByOthers;
if (!(escapeSquares & ~queen_attacks_bb(to, occ & ClearMaskBB[s]))) occ = pos.occupied_squares();
while (b)
{ {
// We have a mate, unless the queen is pinned or there to = pop_1st_bit(&b);
// is an X-ray attack through the queen.
for (int i = 0; i < pos.piece_count(Them, QUEEN); i++)
{
from = pos.piece_list(Them, QUEEN, i);
if ( bit_is_set(pos.attacks_from<QUEEN>(from), to)
&& !bit_is_set(pos.pinned_pieces(Them), from)
&& !(rook_attacks_bb(to, occ & ClearMaskBB[from]) & pos.pieces(ROOK, QUEEN, Us))
&& !(bishop_attacks_bb(to, occ & ClearMaskBB[from]) & pos.pieces(BISHOP, QUEEN, Us)))
ei.mateThreat[Them] = make_move(from, to); // Do we have escape squares from queen contact check attack ?
if (!(escapeSquares & ~queen_attacks_bb(to, occ & ClearMaskBB[s])))
{
// We have a mate, unless the queen is pinned or there
// is an X-ray attack through the queen.
for (int i = 0; i < pos.piece_count(Them, QUEEN); i++)
{
from = pos.piece_list(Them, QUEEN, i);
if ( bit_is_set(pos.attacks_from<QUEEN>(from), to)
&& !bit_is_set(pos.pinned_pieces(Them), from)
&& !(rook_attacks_bb(to, occ & ClearMaskBB[from]) & pos.pieces(ROOK, QUEEN, Us))
&& !(bishop_attacks_bb(to, occ & ClearMaskBB[from]) & pos.pieces(BISHOP, QUEEN, Us)))
// Set the mate threat move
ei.mateThreat[Them] = make_move(from, to);
}
} }
} }
} }
}
} }
} }
// Analyse safe distance checks // Analyse safe distance checks
safe = ~(pos.pieces_of_color(Them) | ei.attacked_by(Us));
if (QueenCheckBonus > 0 || RookCheckBonus > 0) if (QueenCheckBonus > 0 || RookCheckBonus > 0)
{ {
b = pos.attacks_from<ROOK>(s) & ~pos.pieces_of_color(Them) & ~ei.attacked_by(Us); b = pos.attacks_from<ROOK>(s) & safe;
// Queen checks // Queen checks
b2 = b & ei.attacked_by(Them, QUEEN); b2 = b & ei.attacked_by(Them, QUEEN);
@ -822,7 +829,7 @@ namespace {
} }
if (QueenCheckBonus > 0 || BishopCheckBonus > 0) if (QueenCheckBonus > 0 || BishopCheckBonus > 0)
{ {
b = pos.attacks_from<BISHOP>(s) & ~pos.pieces_of_color(Them) & ~ei.attacked_by(Us); b = pos.attacks_from<BISHOP>(s) & safe;
// Queen checks // Queen checks
b2 = b & ei.attacked_by(Them, QUEEN); b2 = b & ei.attacked_by(Them, QUEEN);
@ -836,7 +843,7 @@ namespace {
} }
if (KnightCheckBonus > 0) if (KnightCheckBonus > 0)
{ {
b = pos.attacks_from<KNIGHT>(s) & ~pos.pieces_of_color(Them) & ~ei.attacked_by(Us); b = pos.attacks_from<KNIGHT>(s) & safe;
// Knight checks // Knight checks
b2 = b & ei.attacked_by(Them, KNIGHT); b2 = b & ei.attacked_by(Them, KNIGHT);
@ -848,12 +855,12 @@ namespace {
// adding pawns later). // adding pawns later).
if (DiscoveredCheckBonus) if (DiscoveredCheckBonus)
{ {
b = pos.discovered_check_candidates(Them) & ~pos.pieces(PAWN); b = pos.discovered_check_candidates(Them) & ~pos.pieces(PAWN);
if (b) if (b)
attackUnits += DiscoveredCheckBonus * count_1s_max_15<HasPopCnt>(b) * (sente ? 2 : 1); attackUnits += DiscoveredCheckBonus * count_1s_max_15<HasPopCnt>(b) * (sente ? 2 : 1);
} }
// Has a mate threat been found? We don't do anything here if the // Has a mate threat been found? We don't do anything here if the
// side with the mating move is the side to move, because in that // side with the mating move is the side to move, because in that
// case the mating side will get a huge bonus at the end of the main // case the mating side will get a huge bonus at the end of the main
// evaluation function instead. // evaluation function instead.
@ -861,15 +868,11 @@ namespace {
attackUnits += MateThreatBonus; attackUnits += MateThreatBonus;
// Ensure that attackUnits is between 0 and 99, in order to avoid array // Ensure that attackUnits is between 0 and 99, in order to avoid array
// out of bounds errors: // out of bounds errors.
if (attackUnits < 0) attackUnits = Min(99, Max(0, attackUnits));
attackUnits = 0;
if (attackUnits >= 100)
attackUnits = 99;
// Finally, extract the king safety score from the SafetyTable[] array. // Finally, extract the king safety score from the SafetyTable[] array.
// Add the score to the evaluation, and also to ei.futilityMargin. The // Add the score to the evaluation, and also to ei.futilityMargin. The
// reason for adding the king safety score to the futility margin is // reason for adding the king safety score to the futility margin is
// that the king safety scores can sometimes be very big, and that // that the king safety scores can sometimes be very big, and that
// capturing a single attacking piece can therefore result in a score // capturing a single attacking piece can therefore result in a score