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fix: not escaping checkmates

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dogeystamp 2024-10-26 21:05:51 -04:00
parent 7d0d81905e
commit 39d5ebc2b3
1 changed files with 16 additions and 4 deletions
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20
src/search.rs
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@ -14,10 +14,13 @@ Copyright © 2024 dogeystamp <dogeystamp@disroot.org>
//! Game-tree search. //! Game-tree search.
use crate::eval::{Eval, EvalInt}; use crate::eval::{Eval, EvalInt};
use crate::movegen::{Move, MoveGen, MoveGenType}; use crate::movegen::{Move, MoveGen, MoveGenType, ToUCIAlgebraic};
use crate::Board; use crate::Board;
use std::cmp::max; use std::cmp::max;
// min can't be represented as positive
const EVAL_WORST: EvalInt = -(EvalInt::MAX);
/// Search the game tree to find the absolute (positive good) eval for the current player. /// Search the game tree to find the absolute (positive good) eval for the current player.
fn minmax(board: &mut Board, depth: usize) -> EvalInt { fn minmax(board: &mut Board, depth: usize) -> EvalInt {
if depth == 0 { if depth == 0 {
@ -30,7 +33,16 @@ fn minmax(board: &mut Board, depth: usize) -> EvalInt {
let mvs: Vec<_> = board.gen_moves(MoveGenType::Legal).into_iter().collect(); let mvs: Vec<_> = board.gen_moves(MoveGenType::Legal).into_iter().collect();
let mut abs_best = EvalInt::MIN; let mut abs_best = EVAL_WORST;
if mvs.is_empty() {
if board.is_check(board.turn) {
return EVAL_WORST;
} else {
// stalemate
return 0;
}
}
for mv in mvs { for mv in mvs {
let anti_mv = mv.make(board); let anti_mv = mv.make(board);
@ -47,13 +59,13 @@ fn search(board: &mut Board) -> Option<Move> {
let mvs: Vec<_> = board.gen_moves(MoveGenType::Legal).into_iter().collect(); let mvs: Vec<_> = board.gen_moves(MoveGenType::Legal).into_iter().collect();
// absolute eval value // absolute eval value
let mut best_eval = EvalInt::MIN; let mut best_eval = EVAL_WORST;
let mut best_mv: Option<Move> = None; let mut best_mv: Option<Move> = None;
for mv in mvs { for mv in mvs {
let anti_mv = mv.make(board); let anti_mv = mv.make(board);
let abs_eval = -minmax(board, DEPTH); let abs_eval = -minmax(board, DEPTH);
if abs_eval > best_eval { if abs_eval >= best_eval {
best_eval = abs_eval; best_eval = abs_eval;
best_mv = Some(mv); best_mv = Some(mv);
} }