bugfix: half move clock resetting during castle

Author: dechristopher

position.go

@@ -84,7 +84,7 @@ func (pos *Position) Update(m *Move) *Position {
 ncr := pos.updateCastleRights(m)
 p := pos.board.Piece(m.s1)
 halfMove := pos.halfMoveClock
-if p.Type() == Pawn || m.HasTag(Capture) || cr != ncr {
+if p.Type() == Pawn || isPureCapture(m) || cr != ncr {
 halfMove = 0
 } else {
 halfMove++
@@ -334,6 +334,12 @@ func decodeCastleRights(rights uint8) CastleRights {
 return CastleRights(cr)
 }
 
+// returns true if the move is not a castle move
+func isPureCapture(m *Move) bool {
+return m.HasTag(Capture) && !m.HasTag(FarPawnCastle) &&
+!m.HasTag(ClosePawnCastle) && !m.HasTag(KnightCastle)
+}
+
 func (pos *Position) copy() *Position {
 return &Position{
 board: pos.board.copy(),