stronger threefold repetition prevention

Author: andtsa

README.md

@@ -87,7 +87,7 @@ everything under [`./src/sandy/`](src/sandy/) is part of the frontend (almost)
 - [`./src/sandy/uci/search_controls.rs`](src/sandy/uci/search_controls.rs) does the same for search controls (eg `go depth 4`)
 
 ## Changelog
-- `v0.6.3` TBD, TODO
+- `v0.6.3` one more attempt at fixing 3fold repetition avoidance
 - `v0.6.2` inline move ordering: switch from an allocated `Vec<ChessMove>` to an iterator that only generates moves as needed, performing all move ordering operations on the construction of the iterator.
 - `v0.6.1` variable search depth: when a node has <= 3 children, increase search depth by 1, just for this case. this massively helps lookahead in positions with a lot of checks 
 - lost versions: i did not actually keep a changelog until `v0.6.1`. i do not remember the details here

src/engine/evaluation/mod.rs

@@ -46,15 +46,17 @@ pub fn evaluate(pos: &Position, out_of_moves: bool) -> Value {
     if out_of_moves {
         return if pos.chessboard.checkers().eq(&EMPTY) {
             optlog!(eval;debug;"eval stalemate");
-            // in stalemate, give a slightly negative score to the side that's winning to
+            // in stalemate, give a negative score to the side that's winning to
             // encourage it to keep playing instead
-            value -= material(&pos.chessboard, stm, (0.0, 0.0, 1.0));
-            value += material(&pos.chessboard, stm.not(), (0.0, 0.0, 1.0));
-            value
+            value += material(&pos.chessboard, stm, (0.0, 0.0, 1.0));
+            value -= material(&pos.chessboard, stm.not(), (0.0, 0.0, 1.0));
+            value += piece_position_benefit_for_side(&pos.chessboard, stm, (0.0, 0.0, 1.0));
+            value -= piece_position_benefit_for_side(&pos.chessboard, stm.not(), (0.0, 0.0, 1.0));
+            -2 * value
         } else {
             // Side to move is checkmated
-            optlog!(eval;debug;"eval checkmate");
-            -Value::MATE // Large negative value
+            optlog!(eval;trace;"eval checkmate");
+            -Value::MATE
         };
     }
 

src/engine/lib.rs

@@ -71,7 +71,7 @@ impl Engine {
     /// positions to detect threefold repetition.
     pub fn log_position(&mut self, pos: Position) {
         self.history.push_front(pos);
-        self.history.truncate(6);
+        self.history.truncate(7);
     }
 
     /// set the global [`SEARCHING`]

src/engine/search/main_search.rs

@@ -87,9 +87,12 @@ impl Engine {
                     *engine_history.get(3).unwrap_or(&0),
                     *engine_history.get(4).unwrap_or(&0),
                     *engine_history.get(5).unwrap_or(&0),
+                    *engine_history.get(6).unwrap_or(&0),
                 ],
             };
 
+            optlog!(search;warn;"history:{:?}", initial_options.history);
+
             // SAFETY: if it fails it's due to poison,
             // and that means another thread panicked,
             // so we should panic as well anyway

src/engine/search/mod.rs

@@ -111,7 +111,7 @@ pub struct SearchOptions {
     pub extensions: Depth,
 
     /// previously played position that would cause draw by threefold repetition
-    pub history: [u64; 6],
+    pub history: [u64; 7],
 }
 
 /// wrapper around [`SEARCH_UNTIL`]

src/engine/search/negamax.rs

@@ -179,8 +179,8 @@ pub fn negamax(
         // if theres 3 moves or less, search +1 level deeper
     };
 
-    search_options.history.rotate_left(1);
-    search_options.history[5] = current_hash;
+    search_options.history.rotate_right(1);
+    search_options.history[0] = current_hash;
     search_options = SearchOptions {
         extensions: search_options
             .extensions

src/sandy/player/mod.rs

@@ -74,7 +74,7 @@ pub fn terminal_loop(mut engine: Engine) -> Result<()> {
             engine.best_move(search_depth, search_time)?
         };
         let capture = engine.board.chessboard.piece_on(mv.get_dest()).is_some();
-        engine.board = engine.board.make_move(mv);
+        engine.make_move(mv);
 
         info!("{}", engine.board.print_move(mv, capture));