/
pawns.cpp
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/
pawns.cpp
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/*
Stockfish, a UCI chess playing engine derived from Glaurung 2.1
Copyright (C) 2004-2008 Tord Romstad (Glaurung author)
Copyright (C) 2008-2014 Marco Costalba, Joona Kiiski, Tord Romstad
Stockfish is free software: you can redistribute it and/or modify
it under the terms of the GNU General Public License as published by
the Free Software Foundation, either version 3 of the License, or
(at your option) any later version.
Stockfish is distributed in the hope that it will be useful,
but WITHOUT ANY WARRANTY; without even the implied warranty of
MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
GNU General Public License for more details.
You should have received a copy of the GNU General Public License
along with this program. If not, see <http://www.gnu.org/licenses/>.
*/
#include <algorithm>
#include <cassert>
#include "bitboard.h"
#include "bitcount.h"
#include "pawns.h"
#include "position.h"
namespace {
#define V Value
#define S(mg, eg) make_score(mg, eg)
// Doubled pawn penalty by file
const Score Doubled[FILE_NB] = {
S(13, 43), S(20, 48), S(23, 48), S(23, 48),
S(23, 48), S(23, 48), S(20, 48), S(13, 43) };
// Isolated pawn penalty by opposed flag and file
const Score Isolated[2][FILE_NB] = {
{ S(37, 45), S(54, 52), S(60, 52), S(60, 52),
S(60, 52), S(60, 52), S(54, 52), S(37, 45) },
{ S(25, 30), S(36, 35), S(40, 35), S(40, 35),
S(40, 35), S(40, 35), S(36, 35), S(25, 30) } };
// Backward pawn penalty by opposed flag and file
const Score Backward[2][FILE_NB] = {
{ S(30, 42), S(43, 46), S(49, 46), S(49, 46),
S(49, 46), S(49, 46), S(43, 46), S(30, 42) },
{ S(20, 28), S(29, 31), S(33, 31), S(33, 31),
S(33, 31), S(33, 31), S(29, 31), S(20, 28) } };
// Connected pawn bonus by file and rank (initialized by formula)
Score Connected[FILE_NB][RANK_NB];
// Candidate passed pawn bonus by rank
const Score CandidatePassed[RANK_NB] = {
S( 0, 0), S( 6, 13), S(6,13), S(14,29),
S(34,68), S(83,166), S(0, 0), S( 0, 0) };
// Bonus for file distance of the two outermost pawns
const Score PawnsFileSpan = S(0, 15);
// Weakness of our pawn shelter in front of the king indexed by [rank]
const Value ShelterWeakness[RANK_NB] =
{ V(100), V(0), V(27), V(73), V(92), V(101), V(101) };
// Danger of enemy pawns moving toward our king indexed by
// [no friendly pawn | pawn unblocked | pawn blocked][rank of enemy pawn]
const Value StormDanger[3][RANK_NB] = {
{ V( 0), V(64), V(128), V(51), V(26) },
{ V(26), V(32), V( 96), V(38), V(20) },
{ V( 0), V( 0), V( 64), V(25), V(13) } };
// Max bonus for king safety. Corresponds to start position with all the pawns
// in front of the king and no enemy pawn on the horizon.
const Value MaxSafetyBonus = V(263);
#undef S
#undef V
template<Color Us>
Score evaluate(const Position& pos, Pawns::Entry* e) {
const Color Them = (Us == WHITE ? BLACK : WHITE);
const Square Up = (Us == WHITE ? DELTA_N : DELTA_S);
const Square Right = (Us == WHITE ? DELTA_NE : DELTA_SW);
const Square Left = (Us == WHITE ? DELTA_NW : DELTA_SE);
Bitboard b;
Square s;
File f;
bool passed, isolated, doubled, opposed, connected, backward, candidate;
Score value = SCORE_ZERO;
const Square* pl = pos.list<PAWN>(Us);
Bitboard ourPawns = pos.pieces(Us, PAWN);
Bitboard theirPawns = pos.pieces(Them, PAWN);
e->passedPawns[Us] = e->candidatePawns[Us] = 0;
e->kingSquares[Us] = SQ_NONE;
e->semiopenFiles[Us] = 0xFF;
e->pawnAttacks[Us] = shift_bb<Right>(ourPawns) | shift_bb<Left>(ourPawns);
e->pawnsOnSquares[Us][BLACK] = popcount<Max15>(ourPawns & DarkSquares);
e->pawnsOnSquares[Us][WHITE] = pos.count<PAWN>(Us) - e->pawnsOnSquares[Us][BLACK];
// Loop through all pawns of the current color and score each pawn
while ((s = *pl++) != SQ_NONE)
{
assert(pos.piece_on(s) == make_piece(Us, PAWN));
f = file_of(s);
// This file cannot be semi-open
e->semiopenFiles[Us] &= ~(1 << f);
// Our rank plus previous one
b = rank_bb(s) | rank_bb(s - pawn_push(Us));
// Flag the pawn as passed, isolated, doubled or
// connected (but not the backward one).
connected = ourPawns & adjacent_files_bb(f) & b;
isolated = !(ourPawns & adjacent_files_bb(f));
doubled = ourPawns & forward_bb(Us, s);
opposed = theirPawns & forward_bb(Us, s);
passed = !(theirPawns & passed_pawn_mask(Us, s));
// Test for backward pawn.
// If the pawn is passed, isolated, or connected it cannot be
// backward. If there are friendly pawns behind on adjacent files
// or if it can capture an enemy pawn it cannot be backward either.
if ( (passed | isolated | connected)
|| (ourPawns & pawn_attack_span(Them, s))
|| (pos.attacks_from<PAWN>(s, Us) & theirPawns))
backward = false;
else
{
// We now know that there are no friendly pawns beside or behind this
// pawn on adjacent files. We now check whether the pawn is
// backward by looking in the forward direction on the adjacent
// files, and picking the closest pawn there.
b = pawn_attack_span(Us, s) & (ourPawns | theirPawns);
b = pawn_attack_span(Us, s) & rank_bb(backmost_sq(Us, b));
// If we have an enemy pawn in the same or next rank, the pawn is
// backward because it cannot advance without being captured.
backward = (b | shift_bb<Up>(b)) & theirPawns;
}
assert(opposed | passed | (pawn_attack_span(Us, s) & theirPawns));
// A not-passed pawn is a candidate to become passed, if it is free to
// advance and if the number of friendly pawns beside or behind this
// pawn on adjacent files is higher than or equal to the number of
// enemy pawns in the forward direction on the adjacent files.
candidate = !(opposed | passed | backward | isolated)
&& (b = pawn_attack_span(Them, s + pawn_push(Us)) & ourPawns) != 0
&& popcount<Max15>(b) >= popcount<Max15>(pawn_attack_span(Us, s) & theirPawns);
// Passed pawns will be properly scored in evaluation because we need
// full attack info to evaluate passed pawns. Only the frontmost passed
// pawn on each file is considered a true passed pawn.
if (passed && !doubled)
e->passedPawns[Us] |= s;
// Score this pawn
if (isolated)
value -= Isolated[opposed][f];
if (doubled)
value -= Doubled[f];
if (backward)
value -= Backward[opposed][f];
if (connected)
value += Connected[f][relative_rank(Us, s)];
if (candidate)
{
value += CandidatePassed[relative_rank(Us, s)];
if (!doubled)
e->candidatePawns[Us] |= s;
}
}
// In endgame it's better to have pawns on both wings. So give a bonus according
// to file distance between left and right outermost pawns.
if (pos.count<PAWN>(Us) > 1)
{
b = e->semiopenFiles[Us] ^ 0xFF;
value += PawnsFileSpan * int(msb(b) - lsb(b));
}
return value;
}
} // namespace
namespace Pawns {
/// init() initializes some tables by formula instead of hard-coding their values
void init() {
const int bonusesByFile[8] = { 1, 3, 3, 4, 4, 3, 3, 1 };
int bonus;
for (Rank r = RANK_1; r < RANK_8; ++r)
for (File f = FILE_A; f <= FILE_H; ++f)
{
bonus = r * (r-1) * (r-2) + bonusesByFile[f] * (r/2 + 1);
Connected[f][r] = make_score(bonus, bonus);
}
}
/// probe() takes a position object as input, computes a Entry object, and returns
/// a pointer to it. The result is also stored in a hash table, so we don't have
/// to recompute everything when the same pawn structure occurs again.
Entry* probe(const Position& pos, Table& entries) {
Key key = pos.pawn_key();
Entry* e = entries[key];
if (e->key == key)
return e;
e->key = key;
e->value = evaluate<WHITE>(pos, e) - evaluate<BLACK>(pos, e);
return e;
}
/// Entry::shelter_storm() calculates shelter and storm penalties for the file
/// the king is on, as well as the two adjacent files.
template<Color Us>
Value Entry::shelter_storm(const Position& pos, Square ksq) {
const Color Them = (Us == WHITE ? BLACK : WHITE);
Value safety = MaxSafetyBonus;
Bitboard b = pos.pieces(PAWN) & (in_front_bb(Us, rank_of(ksq)) | rank_bb(ksq));
Bitboard ourPawns = b & pos.pieces(Us);
Bitboard theirPawns = b & pos.pieces(Them);
Rank rkUs, rkThem;
File kf = std::max(FILE_B, std::min(FILE_G, file_of(ksq)));
for (File f = kf - File(1); f <= kf + File(1); ++f)
{
b = ourPawns & file_bb(f);
rkUs = b ? relative_rank(Us, backmost_sq(Us, b)) : RANK_1;
safety -= ShelterWeakness[rkUs];
b = theirPawns & file_bb(f);
rkThem = b ? relative_rank(Us, frontmost_sq(Them, b)) : RANK_1;
safety -= StormDanger[rkUs == RANK_1 ? 0 : rkThem == rkUs + 1 ? 2 : 1][rkThem];
}
return safety;
}
/// Entry::update_safety() calculates and caches a bonus for king safety.
/// It is called only when king square changes, which is about 20% of total
/// king_safety() calls.
template<Color Us>
Score Entry::update_safety(const Position& pos, Square ksq) {
kingSquares[Us] = ksq;
castlingFlags[Us] = pos.can_castle(Us);
minKPdistance[Us] = 0;
Bitboard pawns = pos.pieces(Us, PAWN);
if (pawns)
while (!(DistanceRingsBB[ksq][minKPdistance[Us]++] & pawns)) {}
if (relative_rank(Us, ksq) > RANK_4)
return kingSafety[Us] = make_score(0, -16 * minKPdistance[Us]);
Value bonus = shelter_storm<Us>(pos, ksq);
// If we can castle use the bonus after the castling if it is bigger
if (pos.can_castle(make_castling_flag(Us, KING_SIDE)))
bonus = std::max(bonus, shelter_storm<Us>(pos, relative_square(Us, SQ_G1)));
if (pos.can_castle(make_castling_flag(Us, QUEEN_SIDE)))
bonus = std::max(bonus, shelter_storm<Us>(pos, relative_square(Us, SQ_C1)));
return kingSafety[Us] = make_score(bonus, -16 * minKPdistance[Us]);
}
// Explicit template instantiation
template Score Entry::update_safety<WHITE>(const Position& pos, Square ksq);
template Score Entry::update_safety<BLACK>(const Position& pos, Square ksq);
} // namespace Pawns