From 29d69b5ab1952cb8f59d06cdfa911ba6138a2d3e Mon Sep 17 00:00:00 2001
From: stefiosif <stef8iosif@gmail.com>
Date: Sun, 19 Jan 2025 20:15:23 +0200
Subject: [PATCH] Add repetition detection

---
 src/board/game.rs     | 27 +++++++++++++++++++++++++++
 src/board/history.rs  |  9 +++++++++
 src/search/negamax.rs |  4 ++++
 3 files changed, 40 insertions(+)

diff --git a/src/board/game.rs b/src/board/game.rs
index 19d1253..1b48370 100644
--- a/src/board/game.rs
+++ b/src/board/game.rs
@@ -244,6 +244,14 @@ impl Game {
             }
         }
     }
+
+    pub fn in_repetition(&self) -> bool {
+        if self.board.state.halfmove_clock < 4 {
+            return false;
+        }
+
+        self.history.in_repetition(self.hash)
+    }
 }
 
 impl Default for Game {
@@ -431,4 +439,23 @@ mod tests {
 
         Ok(())
     }
+
+    #[test]
+    fn test_in_repetition() -> Result<(), String> {
+        let mut game = from_fen(FEN)?;
+
+        game.make_move(&Move::new(Square::F3, Square::E3));
+        game.make_move(&Move::new(Square::F7, Square::G7));
+        game.make_move(&Move::new(Square::E3, Square::F3));
+        game.make_move(&Move::new(Square::G7, Square::F7));
+        game.make_move(&Move::new(Square::F3, Square::E3));
+
+        assert!(game.in_repetition());
+
+        game.make_move(&Move::new(Square::H7, Square::H6));
+
+        assert!(!game.in_repetition());
+
+        Ok(())
+    }
 }
diff --git a/src/board/history.rs b/src/board/history.rs
index 895cfc0..33eb58d 100644
--- a/src/board/history.rs
+++ b/src/board/history.rs
@@ -27,6 +27,15 @@ impl History {
     pub fn pop_move_parameters(&mut self) -> Option<MoveParameters> {
         self.move_parameters.pop()
     }
+
+    pub fn in_repetition(&self, hash: ZobristHash) -> bool {
+        self.move_parameters
+            .iter()
+            .rev()
+            .skip(1)
+            .step_by(2)
+            .any(|mp| mp.zobrist_hash.expect("State without hash") == hash)
+    }
 }
 
 #[derive(Debug, Clone, PartialEq, Eq)]
diff --git a/src/search/negamax.rs b/src/search/negamax.rs
index f57c9a8..2b4ca52 100644
--- a/src/search/negamax.rs
+++ b/src/search/negamax.rs
@@ -24,6 +24,10 @@ pub fn negamax(
         bail!("Time is up! In Negamax");
     }
 
+    if plies != 0 && game.in_repetition() {
+        return Ok(0);
+    }
+
     if depth == 0 {
         let q_score = quiescence(game, alpha, beta, time_info).map_err(|e| anyhow!("{e}"))?;
         return Ok(q_score);