Engine.cpp

#include <stdexcept>
#include <vector>
#include "Engine.h"

Engine::Engine() 
    : rng(std::random_device{}()){

}

Move Engine::chooseRandomMove(const Board& board) {
    std::vector<Move> legalMoves = board.generateLegalMoves();

    if (legalMoves.empty())
        throw std::runtime_error("Cannot choose a move: no legal moves available.");

    std::uniform_int_distribution<std::size_t> distribution(
        0,
        legalMoves.size() - 1
    );

    return legalMoves[distribution(rng)];
}

int Engine::evaluateMaterial(const Board& board) const {
    static_assert(
        static_cast<int>(Piece::PIECE_COUNT) == 12,
        "Expected 12 piece bitboards"
    );

    const auto& pieces = board.getPieces();

    constexpr int FIRST_BLACK_PIECE = static_cast<int>(Piece::BP);

    int score = 0;

    for (int piece = 0; piece < static_cast<int>(Piece::PIECE_COUNT); ++piece) {
        int count = __builtin_popcountll(pieces[piece]);
        int material = PIECE_VALUES[piece] * count;

        score += piece < FIRST_BLACK_PIECE ? material : -material;
    }

    return score;
}

int Engine::evaluate(const Board& board, int plyFromRoot) const {
    if (board.isCheckmate()) {
        if (board.isWhiteToMove()) {
            return -MATE_SCORE + plyFromRoot;
        }

        return MATE_SCORE - plyFromRoot;
    }

    if (board.isStalemate()) {
        return 0;
    }

    return evaluateMaterial(board);
}