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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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61
src/evaluate.cpp
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@ -720,8 +720,12 @@ namespace {
void evaluate_king(const Position& pos, EvalInfo& ei) {
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);
int shelter = 0;
// King shelter
if (relative_rank(Us, s) <= RANK_4)
@ -738,53 +742,53 @@ namespace {
&& ei.kingAdjacentZoneAttacksCount[Them])
{
// 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
// apart from the king itself
Bitboard undefended =
ei.attacked_by(Them) & ~ei.attacked_by(Us, PAWN)
& ~ei.attacked_by(Us, KNIGHT) & ~ei.attacked_by(Us, BISHOP)
& ~ei.attacked_by(Us, ROOK) & ~ei.attacked_by(Us, QUEEN)
& ei.attacked_by(Us, KING);
Bitboard occ = pos.occupied_squares(), b, b2;
undefended = ei.attacked_by(Them) & ei.attacked_by(Us, KING);
undefended &= ~( ei.attacked_by(Us, PAWN) | ei.attacked_by(Us, KNIGHT)
| ei.attacked_by(Us, BISHOP) | ei.attacked_by(Us, ROOK)
| ei.attacked_by(Us, QUEEN));
// Initialize the 'attackUnits' variable, which is used later on as an
// index to the SafetyTable[] array. The initial value is based on the
// number and types of the attacking pieces, the number of attacked and
// undefended squares around the king, the square of the king, and the
// quality of the pawn shelter.
int attackUnits =
Min((ei.kingAttackersCount[Them] * ei.kingAttackersWeight[Them]) / 2, 25)
+ (ei.kingAdjacentZoneAttacksCount[Them] + count_1s_max_15<HasPopCnt>(undefended)) * 3
+ InitKingDanger[relative_square(Us, s)] - (shelter >> 5);
attackUnits = Min(25, (ei.kingAttackersCount[Them] * ei.kingAttackersWeight[Them]) / 2)
+ 3 * (ei.kingAdjacentZoneAttacksCount[Them] + count_1s_max_15<HasPopCnt>(undefended))
+ InitKingDanger[relative_square(Us, s)]
- (shelter >> 5);
// Analyse safe queen contact checks
b = undefended & ei.attacked_by(Them, QUEEN) & ~pos.pieces_of_color(Them);
if (b)
{
Bitboard attackedByOthers =
ei.attacked_by(Them, PAWN) | ei.attacked_by(Them, KNIGHT)
attackedByOthers = ei.attacked_by(Them, PAWN) | ei.attacked_by(Them, KNIGHT)
| ei.attacked_by(Them, BISHOP) | ei.attacked_by(Them, ROOK);
b &= attackedByOthers;
// Squares attacked by the queen and supported by another enemy piece and
// not defended by other pieces but our king.
if (b)
{
// The bitboard b now contains the squares available for safe queen
// contact checks.
int count = count_1s_max_15<HasPopCnt>(b);
count = count_1s_max_15<HasPopCnt>(b);
attackUnits += QueenContactCheckBonus * count * (sente ? 2 : 1);
// Is there a mate threat?
if (QueenContactMates && !pos.is_check())
{
Bitboard escapeSquares =
pos.attacks_from<KING>(s) & ~pos.pieces_of_color(Us) & ~attackedByOthers;
escapeSquares = pos.attacks_from<KING>(s) & ~pos.pieces_of_color(Us) & ~attackedByOthers;
occ = pos.occupied_squares();
while (b)
{
Square from, to = pop_1st_bit(&b);
to = pop_1st_bit(&b);
// 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
@ -797,6 +801,7 @@ namespace {
&& !(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);
}
}
@ -806,9 +811,11 @@ namespace {
}
// Analyse safe distance checks
safe = ~(pos.pieces_of_color(Them) | ei.attacked_by(Us));
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
b2 = b & ei.attacked_by(Them, QUEEN);
@ -822,7 +829,7 @@ namespace {
}
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
b2 = b & ei.attacked_by(Them, QUEEN);
@ -836,7 +843,7 @@ namespace {
}
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
b2 = b & ei.attacked_by(Them, KNIGHT);
@ -861,12 +868,8 @@ namespace {
attackUnits += MateThreatBonus;
// Ensure that attackUnits is between 0 and 99, in order to avoid array
// out of bounds errors:
if (attackUnits < 0)
attackUnits = 0;
if (attackUnits >= 100)
attackUnits = 99;
// out of bounds errors.
attackUnits = Min(99, Max(0, attackUnits));
// Finally, extract the king safety score from the SafetyTable[] array.
// Add the score to the evaluation, and also to ei.futilityMargin. The