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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) 2008-2009 Marco Costalba
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/>.
*/
////
//// Includes
////
#include <cassert>
#include <cstring>
#include "bitcount.h"
#include "pawns.h"
#include "position.h"
////
//// Local definitions
////
namespace {
/// Constants and variables
// Doubled pawn penalty by file, middle game
const Value DoubledPawnMidgamePenalty[8] = {
Value(13), Value(20), Value(23), Value(23),
Value(23), Value(23), Value(20), Value(13)
};
// Doubled pawn penalty by file, endgame
const Value DoubledPawnEndgamePenalty[8] = {
Value(43), Value(48), Value(48), Value(48),
Value(48), Value(48), Value(48), Value(43)
};
// Isolated pawn penalty by file, middle game
const Value IsolatedPawnMidgamePenalty[8] = {
Value(25), Value(36), Value(40), Value(40),
Value(40), Value(40), Value(36), Value(25)
};
// Isolated pawn penalty by file, endgame
const Value IsolatedPawnEndgamePenalty[8] = {
Value(30), Value(35), Value(35), Value(35),
Value(35), Value(35), Value(35), Value(30)
};
// Backward pawn penalty by file, middle game
const Value BackwardPawnMidgamePenalty[8] = {
Value(20), Value(29), Value(33), Value(33),
Value(33), Value(33), Value(29), Value(20)
};
// Backward pawn penalty by file, endgame
const Value BackwardPawnEndgamePenalty[8] = {
Value(28), Value(31), Value(31), Value(31),
Value(31), Value(31), Value(31), Value(28)
};
// Pawn chain membership bonus by file, middle game
const Value ChainMidgameBonus[8] = {
Value(11), Value(13), Value(13), Value(14),
Value(14), Value(13), Value(13), Value(11)
};
// Pawn chain membership bonus by file, endgame
const Value ChainEndgameBonus[8] = {
Value(-1), Value(-1), Value(-1), Value(-1),
Value(-1), Value(-1), Value(-1), Value(-1)
};
// Candidate passed pawn bonus by rank, middle game
const Value CandidateMidgameBonus[8] = {
Value( 0), Value( 6), Value(6), Value(14),
Value(34), Value(83), Value(0), Value( 0)
};
// Candidate passed pawn bonus by rank, endgame
const Value CandidateEndgameBonus[8] = {
Value( 0), Value( 13), Value(13), Value(29),
Value(68), Value(166), Value( 0), Value( 0)
};
// Pawn storm tables for positions with opposite castling
const int QStormTable[64] = {
0, 0, 0, 0, 0, 0, 0, 0,
-22,-22,-22,-14,-6, 0, 0, 0,
-6,-10,-10,-10,-6, 0, 0, 0,
4, 12, 16, 12, 4, 0, 0, 0,
16, 23, 23, 16, 0, 0, 0, 0,
23, 31, 31, 23, 0, 0, 0, 0,
23, 31, 31, 23, 0, 0, 0, 0,
0, 0, 0, 0, 0, 0, 0, 0
};
const int KStormTable[64] = {
0, 0, 0, 0, 0, 0, 0, 0,
0, 0, 0,-10,-19,-28,-33,-33,
0, 0, 0,-10,-15,-19,-24,-24,
0, 0, 0, 0, 1, 1, 1, 1,
0, 0, 0, 0, 1, 10, 19, 19,
0, 0, 0, 0, 1, 19, 31, 27,
0, 0, 0, 0, 0, 22, 31, 22,
0, 0, 0, 0, 0, 0, 0, 0
};
// Pawn storm open file bonuses by file
const int16_t KStormOpenFileBonus[8] = { 31, 31, 18, 0, 0, 0, 0, 0 };
const int16_t QStormOpenFileBonus[8] = { 0, 0, 0, 0, 0, 26, 42, 26 };
// Pawn storm lever bonuses by file
const int StormLeverBonus[8] = { -8, -8, -13, 0, 0, -13, -8, -8 };
}
////
//// Functions
////
/// Constructor
PawnInfoTable::PawnInfoTable(unsigned numOfEntries) {
size = numOfEntries;
entries = new PawnInfo[size];
if (!entries)
{
std::cerr << "Failed to allocate " << (numOfEntries * sizeof(PawnInfo))
<< " bytes for pawn hash table." << std::endl;
Application::exit_with_failure();
}
}
/// Destructor
PawnInfoTable::~PawnInfoTable() {
delete [] entries;
}
/// PawnInfo::clear() resets to zero the PawnInfo entry. Note that
/// kingSquares[] is initialized to SQ_NONE instead.
void PawnInfo::clear() {
memset(this, 0, sizeof(PawnInfo));
kingSquares[WHITE] = kingSquares[BLACK] = SQ_NONE;
}
/// PawnInfoTable::get_pawn_info() takes a position object as input, computes
/// a PawnInfo 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.
PawnInfo* PawnInfoTable::get_pawn_info(const Position& pos) {
assert(pos.is_ok());
Key key = pos.get_pawn_key();
int index = int(key & (size - 1));
PawnInfo* pi = entries + index;
// If pi->key matches the position's pawn hash key, it means that we
// have analysed this pawn structure before, and we can simply return
// the information we found the last time instead of recomputing it.
if (pi->key == key)
return pi;
// Clear the PawnInfo object, and set the key
pi->clear();
pi->key = key;
// Calculate pawn attacks
Bitboard whitePawns = pos.pieces(PAWN, WHITE);
Bitboard blackPawns = pos.pieces(PAWN, BLACK);
pi->pawnAttacks[WHITE] = ((whitePawns << 9) & ~FileABB) | ((whitePawns << 7) & ~FileHBB);
pi->pawnAttacks[BLACK] = ((blackPawns >> 7) & ~FileABB) | ((blackPawns >> 9) & ~FileHBB);
// Evaluate pawns for both colors
Values whiteValues = evaluate_pawns<WHITE>(pos, whitePawns, blackPawns, pi);
Values blackValues = evaluate_pawns<BLACK>(pos, blackPawns, whitePawns, pi);
pi->mgValue = int16_t(whiteValues.first - blackValues.first);
pi->egValue = int16_t(whiteValues.second - blackValues.second);
return pi;
}
/// PawnInfoTable::evaluate_pawns() evaluates each pawn of the given color
template<Color Us>
PawnInfoTable::Values PawnInfoTable::evaluate_pawns(const Position& pos, Bitboard ourPawns,
Bitboard theirPawns, PawnInfo* pi) {
Square s;
File f;
Rank r;
bool passed, isolated, doubled, chain, backward, candidate;
int bonus;
Value mgValue = Value(0);
Value egValue = Value(0);
const Square* ptr = pos.piece_list_begin(Us, PAWN);
// Initialize pawn storm scores by giving bonuses for open files
for (File f = FILE_A; f <= FILE_H; f++)
if (!(ourPawns & file_bb(f)))
{
pi->ksStormValue[Us] += KStormOpenFileBonus[f];
pi->qsStormValue[Us] += QStormOpenFileBonus[f];
pi->halfOpenFiles[Us] |= (1 << f);
}
// Loop through all pawns of the current color and score each pawn
while ((s = *ptr++) != SQ_NONE)
{
f = square_file(s);
r = square_rank(s);
assert(pos.piece_on(s) == piece_of_color_and_type(Us, PAWN));
// Passed, isolated or doubled pawn?
passed = Position::pawn_is_passed(theirPawns, Us, s);
isolated = Position::pawn_is_isolated(ourPawns, s);
doubled = Position::pawn_is_doubled(ourPawns, Us, s);
// We calculate kingside and queenside pawn storm
// scores for both colors. These are used when evaluating
// middle game positions with opposite side castling.
//
// Each pawn is given a base score given by a piece square table
// (KStormTable[] or QStormTable[]). Pawns which seem to have good
// chances of creating an open file by exchanging itself against an
// enemy pawn on an adjacent file gets an additional bonus.
// Kingside pawn storms
bonus = KStormTable[relative_square(Us, s)];
if (f >= FILE_F)
{
Bitboard b = outpost_mask(Us, s) & theirPawns & (FileFBB | FileGBB | FileHBB);
while (b)
{
Square s2 = pop_1st_bit(&b);
if (!(theirPawns & neighboring_files_bb(s2) & rank_bb(s2)))
{
// The enemy pawn has no pawn beside itself, which makes it
// particularly vulnerable. Big bonus, especially against a
// weakness on the rook file.
if (square_file(s2) == FILE_H)
bonus += 4*StormLeverBonus[f] - 8*square_distance(s, s2);
else
bonus += 2*StormLeverBonus[f] - 4*square_distance(s, s2);
} else
// There is at least one enemy pawn beside the enemy pawn we look
// at, which means that the pawn has somewhat better chances of
// defending itself by advancing. Smaller bonus.
bonus += StormLeverBonus[f] - 2*square_distance(s, s2);
}
}
pi->ksStormValue[Us] += bonus;
// Queenside pawn storms
bonus = QStormTable[relative_square(Us, s)];
if (f <= FILE_C)
{
Bitboard b = outpost_mask(Us, s) & theirPawns & (FileABB | FileBBB | FileCBB);
while (b)
{
Square s2 = pop_1st_bit(&b);
if (!(theirPawns & neighboring_files_bb(s2) & rank_bb(s2)))
{
// The enemy pawn has no pawn beside itself, which makes it
// particularly vulnerable. Big bonus, especially against a
// weakness on the rook file.
if (square_file(s2) == FILE_A)
bonus += 4*StormLeverBonus[f] - 16*square_distance(s, s2);
else
bonus += 2*StormLeverBonus[f] - 8*square_distance(s, s2);
} else
// There is at least one enemy pawn beside the enemy pawn we look
// at, which means that the pawn has somewhat better chances of
// defending itself by advancing. Smaller bonus.
bonus += StormLeverBonus[f] - 4*square_distance(s, s2);
}
}
pi->qsStormValue[Us] += bonus;
// Member of a pawn chain (but not the backward one)? We could speed up
// the test a little by introducing an array of masks indexed by color
// and square for doing the test, but because everything is hashed,
// it probably won't make any noticable difference.
chain = ourPawns
& neighboring_files_bb(f)
& (rank_bb(r) | rank_bb(r - (Us == WHITE ? 1 : -1)));
// Test for backward pawn
//
// If the pawn is passed, isolated, or member of a pawn chain
// it cannot be backward. If can capture an enemy pawn or if
// there are friendly pawns behind on neighboring files it cannot
// be backward either.
if ( (passed | isolated | chain)
|| (ourPawns & behind_bb(Us, r) & neighboring_files_bb(f))
|| (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 neighboring files. We now check whether the pawn is
// backward by looking in the forward direction on the neighboring
// files, and seeing whether we meet a friendly or an enemy pawn first.
Bitboard b = pos.attacks_from<PAWN>(s, Us);
// Note that we are sure to find something because pawn is not passed
// nor isolated, so loop is potentially infinite, but it isn't.
while (!(b & (ourPawns | theirPawns)))
Us == WHITE ? b <<= 8 : b >>= 8;
// The friendly pawn needs to be at least two ranks closer than the enemy
// pawn in order to help the potentially backward pawn advance.
backward = (b | (Us == WHITE ? b << 8 : b >> 8)) & theirPawns;
}
// Test for candidate passed pawn
candidate = !passed
&& !(theirPawns & file_bb(f))
&& ( count_1s_max_15(neighboring_files_bb(f) & (behind_bb(Us, r) | rank_bb(r)) & ourPawns)
- count_1s_max_15(neighboring_files_bb(f) & in_front_bb(Us, r) & theirPawns)
>= 0);
// In order to prevent doubled passed pawns from receiving a too big
// bonus, only the frontmost passed pawn on each file is considered as
// a true passed pawn.
if (passed && (ourPawns & squares_in_front_of(Us, s)))
passed = false;
// Score this pawn
if (passed)
set_bit(&(pi->passedPawns), s);
if (isolated)
{
mgValue -= IsolatedPawnMidgamePenalty[f];
egValue -= IsolatedPawnEndgamePenalty[f];
if (!(theirPawns & file_bb(f)))
{
mgValue -= IsolatedPawnMidgamePenalty[f] / 2;
egValue -= IsolatedPawnEndgamePenalty[f] / 2;
}
}
if (doubled)
{
mgValue -= DoubledPawnMidgamePenalty[f];
egValue -= DoubledPawnEndgamePenalty[f];
}
if (backward)
{
mgValue -= BackwardPawnMidgamePenalty[f];
egValue -= BackwardPawnEndgamePenalty[f];
if (!(theirPawns & file_bb(f)))
{
mgValue -= BackwardPawnMidgamePenalty[f] / 2;
egValue -= BackwardPawnEndgamePenalty[f] / 2;
}
}
if (chain)
{
mgValue += ChainMidgameBonus[f];
egValue += ChainEndgameBonus[f];
}
if (candidate)
{
mgValue += CandidateMidgameBonus[relative_rank(Us, s)];
egValue += CandidateEndgameBonus[relative_rank(Us, s)];
}
}
return Values(mgValue, egValue);
}
/// PawnInfo::updateShelter calculates and caches king shelter. It is called
/// only when king square changes, about 20% of total get_king_shelter() calls.
int PawnInfo::updateShelter(const Position& pos, Color c, Square ksq) {
unsigned shelter = 0;
Bitboard pawns = pos.pieces(PAWN, c) & this_and_neighboring_files_bb(ksq);
unsigned r = ksq & (7 << 3);
for (int i = 1, k = (c ? -8 : 8); i < 4; i++)
{
r += k;
shelter += BitCount8Bit[(pawns >> r) & 0xFF] * (128 >> i);
}
kingSquares[c] = ksq;
kingShelters[c] = shelter;
return shelter;
}