Add move ordering heuristic MVV/LVA

src/search/mod.rs

@@ -1,3 +1,4 @@
+pub mod move_ordering;
 pub mod negamax;
 pub mod perft;
 pub mod quiescence;

src/search/move_ordering.rs

@@ -0,0 +1,42 @@
+use crate::{board::board::Board, movegen::r#move::Move};
+
+// Rows: Aggressors (P, N, B, R, Q, K)
+// Columns: Victims (K, Q, R, B, N, P)
+const MVV_LVA: [[usize; 6]; 6] = [
+    [30, 29, 28, 27, 26, 00],
+    [25, 24, 23, 22, 21, 00],
+    [20, 19, 18, 17, 16, 00],
+    [15, 14, 13, 12, 11, 00],
+    [10, 09, 08, 07, 06, 00],
+    [05, 04, 03, 02, 01, 00],
+];
+
+pub fn score_by_mvv_lva(board: &Board, mv: Move) -> usize {
+    match (board.piece_type_at(mv.src), board.piece_type_at(mv.dst)) {
+        (Some(aggressor), Some(victim)) => MVV_LVA[aggressor.idx()][victim.idx()],
+        _ => 0,
+    }
+}
+
+#[cfg(test)]
+mod tests {
+    use crate::{
+        board::{board::PieceType, fen::from_fen, square::Square},
+        movegen::r#move::{Move, MoveType},
+        search::move_ordering::{score_by_mvv_lva, MVV_LVA},
+    };
+
+    const FEN: &'static str = "1r2k2r/2P1pq1p/2npb3/1p3ppP/p3P3/P2B1Q2/1P1PNPP1/R3K2R w KQk g6 0 1";
+
+    #[test]
+    fn test_score_by_mvv_lva() -> Result<(), String> {
+        let game = from_fen(FEN)?;
+        let f3f5 = Move::new_with_type(Square::F3, Square::F5, MoveType::Capture);
+        let actual = score_by_mvv_lva(&game.board, f3f5);
+        let expected = MVV_LVA[PieceType::Queen.idx()][PieceType::Pawn.idx()];
+
+        assert_eq!(expected, actual);
+
+        Ok(())
+    }
+}

src/search/negamax.rs

@@ -4,7 +4,7 @@ use crate::{
     movegen::r#move::Move,
 };
 
-use super::quiescence::quiescence;
+use super::{move_ordering::score_by_mvv_lva, quiescence::quiescence};
 
 pub fn negamax(
     game: &mut Game,
@@ -22,7 +22,10 @@ pub fn negamax(
     let (mut best_move, mut best_score, mate_score) = (None, MIN_SCORE, -MATE_SCORE + plies as i32);
     let mut legal_moves = 0;
 
-    for mv in game.board.pseudo_moves_all() {
+    let mut pseudo_legal_moves = game.board.pseudo_moves_all();
+    pseudo_legal_moves.sort_unstable_by_key(|mv| score_by_mvv_lva(&game.board, *mv));
+
+    for mv in pseudo_legal_moves {
         let move_parameters = MoveParameters::build(&game.board, &mv);
         game.board.make_move(&mv);
 
@@ -41,7 +44,7 @@ pub fn negamax(
         }
 
         if move_score >= beta {
-            return (Some(mv), beta);
+            return (best_move, beta);
         }
 
         alpha = alpha.max(move_score);
@@ -64,12 +67,22 @@ mod tests {
     use crate::movegen::r#move::Move;
     use crate::search::negamax::negamax;
 
-    const FEN_MATE_IN_1: &str = "8/8/8/8/8/4q1k1/8/5K2 b - - 0 1";
+    const FEN_MATE_IN_1: [&str; 2] = [
+        "8/8/8/8/8/4q1k1/8/5K2 b - - 0 1",
+        "8/8/8/8/8/4Q1K1/8/5k2 w - - 0 1",
+    ];
 
     #[test]
     fn test_negamax() -> Result<(), String> {
         init_attacks();
-        let mut game = from_fen(FEN_MATE_IN_1)?;
+        let mut game = from_fen(FEN_MATE_IN_1[0])?;
+
+        let e3f2 = Move::new(Square::E3, Square::F2);
+        let anointed_move = negamax(&mut game, MIN_SCORE, MAX_SCORE, 2, 0).0.unwrap();
+
+        assert_eq!(e3f2, anointed_move);
+
+        let mut game = from_fen(FEN_MATE_IN_1[1])?;
 
         let e3f2 = Move::new(Square::E3, Square::F2);
         let anointed_move = negamax(&mut game, MIN_SCORE, MAX_SCORE, 2, 0).0.unwrap();

src/search/quiescence.rs

@@ -4,6 +4,8 @@ use crate::{
     movegen::r#move::{Move, MoveType},
 };
 
+use super::move_ordering::score_by_mvv_lva;
+
 pub fn quiescence(game: &mut Game, mut alpha: i32, beta: i32) -> (Option<Move>, i32) {
     let color = game.current_player();
     let stand_pat = evaluate_position(&game.board);
@@ -16,11 +18,14 @@ pub fn quiescence(game: &mut Game, mut alpha: i32, beta: i32) -> (Option<Move>,
         alpha = stand_pat;
     }
 
-    let captures = game
-        .board
-        .pseudo_moves_all()
-        .into_iter()
-        .filter(|m| !matches!(m.move_type, MoveType::Quiet));
+    let mut captures: Vec<_> = game.board.pseudo_moves_all().into_iter().filter(|m| {
+        matches!(
+            m.move_type,
+            MoveType::Capture | MoveType::PromotionCapture(_)
+        )
+    }).collect();
+
+    captures.sort_unstable_by_key(|mv| score_by_mvv_lva(&game.board, *mv));
 
     for mv in captures {
         let move_parameters = MoveParameters::build(&game.board, &mv);
@@ -41,5 +46,5 @@ pub fn quiescence(game: &mut Game, mut alpha: i32, beta: i32) -> (Option<Move>,
         alpha = alpha.max(move_score);
     }
 
-    (None, 0)
+    (None, alpha)
 }