engine: implement basic minimax

src/board.rs

@@ -59,6 +59,15 @@ pub const fn is_piece(square: u8, piece: u8) -> bool { has_flag(square, piece) }
 #[inline]
 pub const fn opposite(color: u8) -> u8 { color ^ SQ_COLOR_MASK }
 
+/// Pretty-print a color.
+pub fn color_to_string(color: u8) -> String {
+    match color {
+        SQ_WH => "white".to_string(),
+        SQ_BL => "black".to_string(),
+        _ => panic!("Unknown color {}", color),
+    }
+}
+
 /// Minimum allowed value for stored Pos components.
 pub const POS_MIN: i8 = 0;
 /// Maximum allowed value for stored Pos components.

src/engine.rs

@@ -9,6 +9,9 @@ use crate::notation;
 use crate::rules;
 use crate::uci;
 
+const MIN_F32: f32 = std::f32::NEG_INFINITY;
+const MAX_F32: f32 = std::f32::INFINITY;
+
 /// Analysis engine.
 pub struct Engine {
     /// Debug mode, log some data.
@@ -229,13 +232,13 @@ impl Engine {
         }
 
         self.working.store(true, atomic::Ordering::Relaxed);
-        let node = self.node.clone();
+        let mut node = self.node.clone();
         let args = args.clone();
         let working = self.working.clone();
         let tx = match &self.mode { Mode::Uci(_, _, tx) => tx.clone(), _ => return };
         let debug = self.debug;
         thread::spawn(move || {
-            analyze(&node, &args, working, tx, debug);
+            analyze(&mut node, &args, working, tx, debug);
         });
     }
 
@@ -299,7 +302,7 @@ impl Engine {
 }
 
 fn analyze(
-    node: &Node,
+    node: &mut Node,
     _args: &WorkArgs,
     working: Arc<atomic::AtomicBool>,
     tx: mpsc::Sender<Cmd>,
@@ -309,38 +312,28 @@ fn analyze(
         return;
     }
     if debug {
-        let state_str = format!("Analysing node: {:?}", node);
-        tx.send(Cmd::Log(state_str)).unwrap();
-        let mut s = vec!();
-        board::draw(&node.board, &mut s);
-        let draw_str = String::from_utf8_lossy(&s).to_string();
-        tx.send(Cmd::Log(draw_str)).unwrap();
-    }
-
-    let moves = rules::get_player_moves(&node.board, &node.game_state, true);
-    if debug {
-        let moves_str = format!("Legal moves: {}", notation::move_list_to_string(&moves));
+        tx.send(Cmd::Log(format!("\tAnalyzing node:\n{}", node))).unwrap();
+        let moves = rules::get_player_moves(&node.board, &node.game_state, true);
+        let moves_str = format!("\tLegal moves: {}", notation::move_list_to_string(&moves));
         tx.send(Cmd::Log(moves_str)).unwrap();
     }
 
-    let mut best_e = if board::is_white(node.game_state.color) { -999.0 } else { 999.0 };
-    let mut best_move = None;
-    for m in moves {
-        let mut board = node.board.clone();
-        let mut game_state = node.game_state.clone();
-        rules::apply_move_to(&mut board, &mut game_state, &m);
-        let stats = board::compute_stats(&board);
-        let e = evaluate(&stats);
-        if
-            (board::is_white(node.game_state.color) && e > best_e) ||
-            (board::is_black(node.game_state.color) && e < best_e)
-        {
-            best_e = e;
-            best_move = Some(m.clone());
-        }
+    let (max_score, best_move) = minimax(node, 0, 3, board::is_white(node.game_state.color));
+
+    if best_move.is_some() {
+        let log_str = format!(
+            "\tBest move {} evaluated {}",
+            notation::move_to_string(&best_move.unwrap()), max_score
+        );
+        tx.send(Cmd::Log(log_str)).unwrap();
+        tx.send(Cmd::TmpBestMove(best_move)).unwrap();
+    } else {
+        // If no best move could be found, checkmate is unavoidable; send the first legal move.
+        tx.send(Cmd::Log("Checkmate is unavoidable.".to_string())).unwrap();
+        let moves = rules::get_player_moves(&node.board, &node.game_state, true);
+        let m = if moves.len() > 0 { Some(moves[0]) } else { None };
+        tx.send(Cmd::TmpBestMove(m)).unwrap();
     }
-    thread::sleep(time::Duration::from_millis(500u64));
-    tx.send(Cmd::TmpBestMove(best_move)).unwrap();
 
     // thread::sleep(time::Duration::from_secs(1));
     // for _ in 0..4 {
@@ -353,6 +346,39 @@ fn analyze(
 
 }
 
+fn minimax(
+    node: &mut Node,
+    depth: u32,
+    max_depth: u32,
+    maximizing: bool
+) -> (f32, Option<rules::Move>) {
+    if depth == max_depth {
+        board::compute_stats_into(&node.board, &mut node.stats);
+        return (evaluate(&node.stats), None);
+    }
+    let mut minmax = if maximizing { MIN_F32 } else { MAX_F32 };
+    let mut minmax_move = None;
+    let moves = rules::get_player_moves(&node.board, &node.game_state, true);
+    for m in moves {
+        let mut sub_node = node.clone();
+        rules::apply_move_to(&mut sub_node.board, &mut sub_node.game_state, &m);
+        if maximizing {
+            let (score, _) = minimax(&mut sub_node, depth + 1, max_depth, false);
+            if score > minmax {
+                minmax = score;
+                minmax_move = Some(m);
+            }
+        } else {
+            let (score, _) = minimax(&mut sub_node, depth + 1, max_depth, true);
+            if score < minmax {
+                minmax = score;
+                minmax_move = Some(m);
+            }
+        }
+    }
+    (minmax, minmax_move)
+}
+
 fn evaluate(stats: &(board::BoardStats, board::BoardStats)) -> f32 {
     let (ws, bs) = stats;
 
@@ -373,6 +399,30 @@ fn evaluate(stats: &(board::BoardStats, board::BoardStats)) -> f32 {
 #[cfg(test)]
 mod tests {
     use super::*;
+    use board::pos;
+
+    #[test]
+    fn test_minimax() {
+        let mut node = Node::new();
+        node.game_state.castling = 0;
+
+        // White mates in 1 move, queen to d7.
+        board::set_square(&mut node.board, &pos("a1"), board::SQ_WH_K);
+        board::set_square(&mut node.board, &pos("c6"), board::SQ_WH_P);
+        board::set_square(&mut node.board, &pos("h7"), board::SQ_WH_Q);
+        board::set_square(&mut node.board, &pos("d8"), board::SQ_BL_K);
+        let (_, m) = minimax(&mut node, 0, 2, true);
+        assert_eq!(m.unwrap(), notation::parse_move("h7d7"));
+
+        // Check that it works for black as well.
+        board::set_square(&mut node.board, &pos("a1"), board::SQ_BL_K);
+        board::set_square(&mut node.board, &pos("c6"), board::SQ_BL_P);
+        board::set_square(&mut node.board, &pos("h7"), board::SQ_BL_Q);
+        board::set_square(&mut node.board, &pos("d8"), board::SQ_WH_K);
+        node.game_state.color = board::SQ_BL;
+        let (_, m) = minimax(&mut node, 0, 2, true);
+        assert_eq!(m.unwrap(), notation::parse_move("h7d7"));
+    }
 
     #[test]
     fn test_evaluate() {

src/rules.rs

@@ -40,7 +40,8 @@ impl std::fmt::Display for GameState {
              - en_passant: {}\n\
              - halfmove: {}\n\
              - fullmove: {}",
-            self.color, self.castling, notation::en_passant_to_string(self.en_passant),
+            color_to_string(self.color), self.castling,
+            notation::en_passant_to_string(self.en_passant),
             self.halfmove, self.fullmove
         )
     }