From d30d966d732440664adb7f6b58fc236bc68306fb Mon Sep 17 00:00:00 2001
From: Matthew Todd <matthewdb.todd@gmail.com>
Date: Thu, 9 Jul 2026 00:00:34 +0100
Subject: [PATCH 1/2] fix: reset halfmove counter on non-pawn captures

---
 src/chess/position.rs | 53 +++++++++++++++++++++++++++++++++++++++++--
 1 file changed, 51 insertions(+), 2 deletions(-)

diff --git a/src/chess/position.rs b/src/chess/position.rs
index 4aabfb2..17e584f 100644
--- a/src/chess/position.rs
+++ b/src/chess/position.rs
@@ -161,6 +161,7 @@ impl Position {
         self.remove_piecetype(self.stm, pt, from);
 
         // capture piece
+        let mut is_capture = false;
         if mv.mtype() != MoveType::Castle {
             let captured = self.piece_at(to);
             if captured != Piece::none() {
@@ -171,7 +172,7 @@ impl Position {
                     self.update_castling_rights_color(!self.stm, from, to);
                 }
 
-                self.halfm = 0;
+                is_capture = true;
             }
         }
 
@@ -233,7 +234,7 @@ impl Position {
         // update state
 
         // Update halfmove clock
-        if pt == PieceType::Pawn {
+        if pt == PieceType::Pawn || is_capture {
             self.halfm = 0;
         } else {
             self.halfm += 1;
@@ -631,4 +632,52 @@ mod tests {
         let pos = Position::new();
         assert_eq!(pos, Position::from_fen(STARTPOS).unwrap());
     }
+
+    #[test]
+    fn test_halfm_reset() {
+        let fen = "r1bqkb1r/pppp1ppp/2n2n2/1B2p3/4P3/5N2/PPPP1PPP/RNBQK2R w KQkq - 4 4";
+
+        let pos = Position::from_fen(fen).unwrap();
+        let mv = Move::new(
+            Square::from_string("b5").unwrap(),
+            Square::from_string("c6").unwrap(),
+            MoveType::Normal,
+            Piece::NONE,
+        );
+        let pos = pos.after_move(mv);
+        assert_eq!(0, pos.rule50_counter());
+
+        let pos = Position::from_fen(fen).unwrap();
+        let mv = Move::new(
+            Square::from_string("d2").unwrap(),
+            Square::from_string("d3").unwrap(),
+            MoveType::Normal,
+            Piece::NONE,
+        );
+        let pos = pos.after_move(mv);
+        assert_eq!(0, pos.rule50_counter());
+
+        let pos = Position::from_fen(fen).unwrap();
+        let mv = Move::new(
+            Square::from_string("b1").unwrap(),
+            Square::from_string("c3").unwrap(),
+            MoveType::Normal,
+            Piece::NONE,
+        );
+        let pos = pos.after_move(mv);
+        assert_eq!(5, pos.rule50_counter());
+
+        let pos = Position::from_fen(
+            "r1bqk2r/pppp1ppp/2n2n2/1Bb1p3/4P3/2N2N2/PPPP1PPP/R1BQK2R w KQkq - 6 5",
+        )
+        .unwrap();
+        let mv = Move::new(
+            Square::from_string("e1").unwrap(),
+            Square::from_string("h1").unwrap(),
+            MoveType::Castle,
+            Piece::NONE,
+        );
+        let pos = pos.after_move(mv);
+        assert_eq!(7, pos.rule50_counter());
+    }
 }

From 22bdde0a411a357e640d1e00ebb906558a587563 Mon Sep 17 00:00:00 2001
From: Matthew Todd <matthewdb.todd@gmail.com>
Date: Thu, 9 Jul 2026 12:51:56 +0100
Subject: [PATCH 2/2] simplify by re-ordering the halfm reset checks

---
 src/chess/position.rs | 17 ++++++++---------
 1 file changed, 8 insertions(+), 9 deletions(-)

diff --git a/src/chess/position.rs b/src/chess/position.rs
index 17e584f..2e12311 100644
--- a/src/chess/position.rs
+++ b/src/chess/position.rs
@@ -160,8 +160,14 @@ impl Position {
         // clear piece from start
         self.remove_piecetype(self.stm, pt, from);
 
+        // Update halfmove clock
+        if pt == PieceType::Pawn {
+            self.halfm = 0;
+        } else {
+            self.halfm += 1;
+        }
+
         // capture piece
-        let mut is_capture = false;
         if mv.mtype() != MoveType::Castle {
             let captured = self.piece_at(to);
             if captured != Piece::none() {
@@ -172,7 +178,7 @@ impl Position {
                     self.update_castling_rights_color(!self.stm, from, to);
                 }
 
-                is_capture = true;
+                self.halfm = 0;
             }
         }
 
@@ -233,13 +239,6 @@ impl Position {
 
         // update state
 
-        // Update halfmove clock
-        if pt == PieceType::Pawn || is_capture {
-            self.halfm = 0;
-        } else {
-            self.halfm += 1;
-        }
-
         // Update fullmove number
         if self.stm == Color::Black {
             self.fullm += 1;