testsAndMisc/C/lichess_random_engine/search.c

#include "search.h"
#include "movegen.h"
#include <limits.h>
#include <stddef.h>

static int piece_value(Piece p)
{
    switch (p)
    {
    case WK:
    case BK:
        return 0; /* king is invaluable; PST handled later if needed */
    case WP:
    case BP:
        return 100;
    case WN:
    case BN:
        return 320;
    case WB:
    case BB:
        return 330;
    case WR:
    case BR:
        return 500;
    case WQ:
    case BQ:
        return 900;
    default:
        return 0;
    }
}

int evaluate(const Position *pos)
{
    int score = 0;
    for (int sq = 0; sq < BOARD_SIZE; ++sq)
    {
        if ((sq & 0x88))
        {
            sq = (sq | 7);
            continue;
        }
        Piece p = pos->board[sq];
        int   v = piece_value(p);
        if (p >= WP && p <= WK)
        {
            score += v;
        }
        else if (p >= BP && p <= BK)
        {
            score -= v;
        }
    }
    // Score from side-to-move perspective
    return (pos->side == WHITE) ? score : -score;
}

int alphabeta(Position pos, int depth, int alpha, int beta, PrincipalVariation *pv)
{
    if (depth <= 0)
    {
        return evaluate(&pos);
    }
    Move moves[256];
    int  n = gen_moves(&pos, moves, 256, 0);
    if (n == 0)
    {
        // Checkmate or stalemate
        if (in_check(&pos, pos.side))
        {
            return -30000 + (10 - depth); // checkmated
        }
        return 0; // stalemate
    }

    int best_score = INT_MIN / 2;
    int best_from  = -1;
    int best_to    = -1;
    for (int i = 0; i < n; i++)
    {
        Position child = pos;
        Piece    cap   = EMPTY;
        make_move(&child, &moves[i], &cap);
        int score = -alphabeta(child, depth - 1, -beta, -alpha, NULL);
        if (score > best_score)
        {
            best_score = score;
            best_from  = moves[i].from;
            best_to    = moves[i].to;
        }
        if (best_score > alpha)
        {
            alpha = best_score;
        }
        if (alpha >= beta)
        {
            break; // beta cutoff
        }
    }
    if (pv != NULL)
    {
        pv->from = best_from;
        pv->to   = best_to;
    }
    return best_score;
}