/*

Stockfish, a UCI chess playing engine derived from Glaurung 2.1

Copyright (C) 2004-2025 The Stockfish developers (see AUTHORS file)

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 "evaluate.h"

#include <algorithm>

#include <cassert>

#include <cmath>

#include <cstdlib>

#include <iomanip>

#include <iostream>

#include <memory>

#include <sstream>

#include <tuple>

#include "nnue/network.h"

#include "nnue/nnue_misc.h"

#include "position.h"

#include "types.h"

#include "uci.hpp"

#include "nnue/nnue_accumulator.h"

namespace Stockfish {

// Returns a static, purely materialistic evaluation of the position from

// the point of view of the side to move. It can be divided by PawnValue to get

// an approximation of the material advantage on the board in terms of pawns.

int Eval::simple_eval(const Position& pos) {

Color c = pos.side_to_move();

return PawnValue * (pos.count<PAWN>(c) - pos.count<PAWN>(~c))

+ (pos.non_pawn_material(c) - pos.non_pawn_material(~c));

}

bool Eval::use_smallnet(const Position& pos) { return std::abs(simple_eval(pos)) > 962; }

// Evaluate is the evaluator for the outer world. It returns a static evaluation

// of the position from the point of view of the side to move.

Value Eval::evaluate(const Eval::NNUE::Networks& networks,

const Position& pos,

Eval::NNUE::AccumulatorStack& accumulators,

Eval::NNUE::AccumulatorCaches& caches,

int optimism) {

assert(!pos.b.inCheck());

bool smallNet = use_smallnet(pos);

auto [psqt, positional] = smallNet ? networks.small.evaluate(pos, accumulators, &caches.small)

: networks.big.evaluate(pos, accumulators, &caches.big);

Value nnue = (125 * psqt + 131 * positional) / 128;

// Re-evaluate the position when higher eval accuracy is worth the time spent

if (smallNet && (std::abs(nnue) < 236))

{

std::tie(psqt, positional) = networks.big.evaluate(pos, accumulators, &caches.big);

nnue = (125 * psqt + 131 * positional) / 128;

smallNet = false;

}

// Blend optimism and eval with nnue complexity

int nnueComplexity = std::abs(psqt - positional);

optimism += optimism * nnueComplexity / 468;

nnue -= nnue * nnueComplexity / 18000;

int material = 535 * pos.count<PAWN>() + pos.non_pawn_material();

int v = (nnue * (77777 + material) + optimism * (7777 + material)) / 77777;

// Damp down the evaluation linearly when shuffling

v -= v * pos.rule50_count() / 212;

// Guarantee evaluation does not hit the tablebase range

v = std::clamp(v, VALUE_TB_LOSS_IN_MAX_PLY + 1, VALUE_TB_WIN_IN_MAX_PLY - 1);

return v;

}

// Like evaluate(), but instead of returning a value, it returns

// a string (suitable for outputting to stdout) that contains the detailed

// descriptions and values of each evaluation term. Useful for debugging.

// Trace scores are from white's point of view

std::string Eval::trace(Position& pos, const Eval::NNUE::Networks& networks) {

if (pos.b.inCheck())

return "Final evaluation: none (in check)";

Eval::NNUE::AccumulatorStack accumulators;

auto caches = std::make_unique<Eval::NNUE::AccumulatorCaches>(networks);

std::stringstream ss;

ss << std::showpoint << std::noshowpos << std::fixed << std::setprecision(2);

ss << '\n' << NNUE::trace(pos, networks, *caches) << '\n';

ss << std::showpoint << std::showpos << std::fixed << std::setprecision(2) << std::setw(15);

auto [psqt, positional] = networks.big.evaluate(pos, accumulators, &caches->big);

Value v = psqt + positional;

v = pos.side_to_move() == WHITE ? v : -v;

ss << "NNUE evaluation " << 0.01 * to_cp(v, pos) << " (white side)\n";

v = evaluate(networks, pos, accumulators, *caches, VALUE_ZERO);

v = pos.side_to_move() == WHITE ? v : -v;

ss << "Final evaluation " << 0.01 * to_cp(v, pos) << " (white side)";

ss << " [with scaled NNUE, ...]";

ss << "\n";

return ss.str();

}

} // namespace Stockfish