diff --git a/src/ui/flutter/lib/common/types.dart b/src/ui/flutter/lib/common/types.dart
index 58aa21a8..192bb0f7 100644
--- a/src/ui/flutter/lib/common/types.dart
+++ b/src/ui/flutter/lib/common/types.dart
@@ -16,3 +16,117 @@
   You should have received a copy of the GNU General Public License
   along with this program.  If not, see <http://www.gnu.org/licenses/>.
 */
+
+import 'package:sanmill/common/misc.dart';
+
+enum MoveType { place, move, remove }
+
+enum Phase { none, ready, placing, moving, gameOver }
+
+enum Action { none, select, place, remove }
+
+enum GameOverReason {
+  noReason,
+  loseReasonlessThanThree,
+  loseReasonNoWay,
+  loseReasonBoardIsFull,
+  loseReasonResign,
+  loseReasonTimeOver,
+  drawReasonThreefoldRepetition,
+  drawReasonRule50,
+  drawReasonBoardIsFull
+}
+
+enum PieceType { none, blackStone, whiteStone, ban, count, stone }
+
+enum Square {
+  SQ_0,
+  SQ_1,
+  SQ_2,
+  SQ_3,
+  SQ_4,
+  SQ_5,
+  SQ_6,
+  SQ_7,
+  SQ_8,
+  SQ_9,
+  SQ_10,
+  SQ_11,
+  SQ_12,
+  SQ_13,
+  SQ_14,
+  SQ_15,
+  SQ_16,
+  SQ_17,
+  SQ_18,
+  SQ_19,
+  SQ_20,
+  SQ_21,
+  SQ_22,
+  SQ_23,
+  SQ_24,
+  SQ_25,
+  SQ_26,
+  SQ_27,
+  SQ_28,
+  SQ_29,
+  SQ_30,
+  SQ_31,
+}
+
+const sqBegin = Square.SQ_8;
+const sqEnd = 32;
+const sqNumber = 40;
+const effectiveSqNumber = 24;
+
+enum MoveDirection { clockwise, anticlockwise, inward, outward }
+
+enum LineDirection { horizontal, vertical, slash }
+
+enum File { A, B, C }
+
+const fileNumber = 3;
+
+enum Rank { rank_1, rank_2, rank_3, rank_4, rank_5, rank_6, rank_7, rank_8 }
+
+const rankNumber = 8;
+
+bool isOk(int sq) {
+  return sq == 0 || (sq >= 8 && sq <= 31); // TODO: SQ_NONE?
+}
+
+int fileOf(int sq) {
+  return (sq >> 3);
+}
+
+int rankOf(int sq) {
+  return (sq & 0x07) + 1;
+}
+
+int fromSq(int move) {
+  move = abs(move);
+  return (move >> 8);
+}
+
+int toSq(int move) {
+  move = abs(move);
+  return (move & 0x00FF);
+}
+
+MoveType typeOf(int move) {
+  if (move < 0) {
+    return MoveType.remove;
+  } else if (move & 0x1f00 > 0) {
+    return MoveType.move;
+  }
+
+  return MoveType.place; // m & 0x00ff
+}
+
+int makeMove(int fromSq, int toSq) {
+  return (fromSq << 8) + toSq;
+}
+
+int reverseMove(int move) {
+  return makeMove(toSq(move), fromSq(move));
+}
diff --git a/src/ui/flutter/lib/mill/mill.dart b/src/ui/flutter/lib/mill/mill.dart
index 4d69f575..858fc754 100644
--- a/src/ui/flutter/lib/mill/mill.dart
+++ b/src/ui/flutter/lib/mill/mill.dart
@@ -17,78 +17,6 @@
   along with this program.  If not, see <http://www.gnu.org/licenses/>.
 */
 
-enum MoveType { place, move, remove }
-
-enum Phase { none, ready, placing, moving, gameOver }
-
-enum Action { none, select, place, remove }
-
-enum GameOverReason {
-  noReason,
-  loseReasonlessThanThree,
-  loseReasonNoWay,
-  loseReasonBoardIsFull,
-  loseReasonResign,
-  loseReasonTimeOver,
-  drawReasonThreefoldRepetition,
-  drawReasonRule50,
-  drawReasonBoardIsFull
-}
-
-enum PieceType { none, blackStone, whiteStone, ban, count, stone }
-
-enum Square {
-  SQ_0,
-  SQ_1,
-  SQ_2,
-  SQ_3,
-  SQ_4,
-  SQ_5,
-  SQ_6,
-  SQ_7,
-  SQ_8,
-  SQ_9,
-  SQ_10,
-  SQ_11,
-  SQ_12,
-  SQ_13,
-  SQ_14,
-  SQ_15,
-  SQ_16,
-  SQ_17,
-  SQ_18,
-  SQ_19,
-  SQ_20,
-  SQ_21,
-  SQ_22,
-  SQ_23,
-  SQ_24,
-  SQ_25,
-  SQ_26,
-  SQ_27,
-  SQ_28,
-  SQ_29,
-  SQ_30,
-  SQ_31,
-}
-
-const sqBegin = Square.SQ_8;
-const sqEnd = 32;
-const sqNumber = 40;
-const effectiveSqNumber = 24;
-
-enum MoveDirection { clockwise, anticlockwise, inward, outward }
-
-enum LineDirection { horizontal, vertical, slash }
-
-enum File { A, B, C }
-
-const fileNumber = 3;
-
-enum Rank { rank_1, rank_2, rank_3, rank_4, rank_5, rank_6, rank_7, rank_8 }
-
-const rankNumber = 8;
-
 Map<int, int> squareToIndex = {
   8: 17,
   9: 18,
diff --git a/src/ui/flutter/lib/mill/position.dart b/src/ui/flutter/lib/mill/position.dart
index ef0d7b53..b4e52d7e 100644
--- a/src/ui/flutter/lib/mill/position.dart
+++ b/src/ui/flutter/lib/mill/position.dart
@@ -17,6 +17,7 @@
   along with this program.  If not, see <http://www.gnu.org/licenses/>.
 */
 
+import '../common/types.dart';
 import '../mill/mill.dart';
 import '../mill/recorder.dart';
 
@@ -139,6 +140,11 @@ class Position {
     nPlayed = other.nPlayed;
   }
 
+  String pieceOnGrid(int index) => _grid[index];
+  String pieceOn(int sq) => _board[sq];
+
+  bool empty(int sq) => pieceOn(sq) == Piece.noPiece;
+
   /// fen() returns a FEN representation of the position.
 
   String fen() {
@@ -338,9 +344,6 @@ class Position {
 
   changeSideToMove() => _sideToMove = Color.opponent(_sideToMove);
 
-  String pieceOnGrid(int index) => _grid[index];
-  String pieceOn(int sq) => _board[sq];
-
   get halfMove => _recorder.halfMove;
 
   get fullMove => _recorder.fullMove;