diff --git a/src/ui/flutter/lib/board/pieces_painter.dart b/src/ui/flutter/lib/board/pieces_painter.dart
index 3b2a62d7..8da9e185 100644
--- a/src/ui/flutter/lib/board/pieces_painter.dart
+++ b/src/ui/flutter/lib/board/pieces_painter.dart
@@ -96,7 +96,7 @@ class PiecesPainter extends PainterBase {
       //
       for (var col = 0; col < 7; col++) {
         //
-        final piece = position.pieceAt(row * 7 + col); // 初始状态无棋子
+        final piece = position.pieceOn(row * 7 + col); // 初始状态无棋子
 
         if (piece == Piece.noPiece) continue;
 
diff --git a/src/ui/flutter/lib/mill/mill.dart b/src/ui/flutter/lib/mill/mill.dart
index cd34159d..863b485a 100644
--- a/src/ui/flutter/lib/mill/mill.dart
+++ b/src/ui/flutter/lib/mill/mill.dart
@@ -89,15 +89,48 @@ 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,
+  10: 25,
+  11: 32,
+  12: 31,
+  13: 30,
+  14: 23,
+  15: 16,
+  16: 10,
+  17: 12,
+  18: 26,
+  19: 40,
+  20: 38,
+  21: 36,
+  22: 22,
+  23: 8,
+  24: 3,
+  25: 6,
+  26: 27,
+  27: 48,
+  28: 45,
+  29: 42,
+  30: 21,
+  31: 0
+};
+
+Map<int, int> indexToSquare = squareToIndex.map((k, v) => MapEntry(v, k));
+
+int makeSquare(int file, int rank) {
+  return (file << 3) + rank - 1;
+}
+
 /// 对战结果：未决、赢、输、和
 enum GameResult { pending, win, lose, draw }
 
 class Color {
   //
   static const unknown = '-';
-  static const black = 'b';
-  static const white = 'w';
-  static const ban = 'x';
+  static const black = '@';
+  static const white = 'O';
+  static const ban = 'X';
 
   static String of(String piece) {
     if (black.contains(piece)) return black;
@@ -121,15 +154,15 @@ class Color {
 
 class Piece {
   //
-  static const noPiece = ' ';
+  static const noPiece = '*';
   //
-  static const blackStone = 'b';
-  static const whiteStone = 'w';
-  static const ban = 'x';
+  static const blackStone = '@';
+  static const whiteStone = 'O';
+  static const ban = 'X';
 
-  static bool isBlack(String c) => 'b'.contains(c);
+  static bool isBlack(String c) => '@'.contains(c);
 
-  static bool isWhite(String c) => 'w'.contains(c);
+  static bool isWhite(String c) => 'O'.contains(c);
 }
 
 class Move {
diff --git a/src/ui/flutter/lib/mill/position.dart b/src/ui/flutter/lib/mill/position.dart
index e19a8843..c8322ab4 100644
--- a/src/ui/flutter/lib/mill/position.dart
+++ b/src/ui/flutter/lib/mill/position.dart
@@ -20,35 +20,6 @@
 import '../mill/recorder.dart';
 import 'mill.dart';
 
-Map<int, int> sqToLoc = {
-  8: 17,
-  9: 18,
-  10: 25,
-  11: 32,
-  12: 31,
-  13: 30,
-  14: 23,
-  15: 16,
-  16: 10,
-  17: 12,
-  18: 26,
-  19: 40,
-  20: 38,
-  21: 36,
-  22: 22,
-  23: 8,
-  24: 3,
-  25: 6,
-  26: 27,
-  27: 48,
-  28: 45,
-  29: 42,
-  30: 21,
-  31: 0
-};
-
-Map<int, int> locToSq = sqToLoc.map((k, v) => MapEntry(v, k));
-
 class Position {
   GameResult result = GameResult.pending;
   String _sideToMove = Color.black;
@@ -73,44 +44,32 @@ class Position {
     _recorder = other._recorder;
   }
 
+  /// fen() returns a FEN representation of the position.
+
   String fen() {
     // TODO
-    var fen = '';
+    var ss = '';
 
     for (var file = 1; file <= 3; file++) {
-      //
-      var emptyCounter = 0;
-
       for (var rank = 1; rank <= 8; rank++) {
         //
-        final piece = pieceAt((file - 1) * 8 + rank + 8);
+        final piece = pieceOn((file - 1) * 8 + rank + 8);
 
         if (piece == Piece.noPiece) {
-          //
-          emptyCounter++;
-          //
         } else {
-          //
-          if (emptyCounter > 0) {
-            fen += emptyCounter.toString();
-            emptyCounter = 0;
-          }
-
-          fen += piece;
+          ss += piece;
         }
       }
 
-      if (emptyCounter > 0) fen += emptyCounter.toString();
-
-      if (file < 9) fen += '/';
+      if (file < 9) ss += '/';
     }
 
-    fen += ' $side';
+    ss += ' $side';
 
     // step counter
-    fen += '${_recorder?.halfMove ?? 0} ${_recorder?.fullMove ?? 0}';
+    ss += '${_recorder?.halfMove ?? 0} ${_recorder?.fullMove ?? 0}';
 
-    return fen;
+    return ss;
   }
 
   void putPiece(var pt, int index) {
@@ -215,7 +174,7 @@ class Position {
 
   changeSideToMove() => _sideToMove = Color.opponent(_sideToMove);
 
-  String pieceAt(int index) => _board[index];
+  String pieceOn(int index) => _board[index];
 
   get halfMove => _recorder.halfMove;
 
diff --git a/src/ui/flutter/lib/routes/battle_page.dart b/src/ui/flutter/lib/routes/battle_page.dart
index 7287c4ed..16c4e75d 100644
--- a/src/ui/flutter/lib/routes/battle_page.dart
+++ b/src/ui/flutter/lib/routes/battle_page.dart
@@ -73,23 +73,23 @@ class _BattlePageState extends State<BattlePage> {
 
     //position
     flag++;
-    position.putPiece(flag % 2 == 0 ? 'b' : 'w', index);
+    position.putPiece(flag % 2 == 0 ? '@' : 'O', index);
 
     // 仅 Position 中的 side 指示一方能动棋
     if (position.side != Color.black) return;
 
-    final tapedPiece = position.pieceAt(index);
+    final tapedPiece = position.pieceOn(index);
     print("Tap piece $tapedPiece at <$index>");
 
     // 之前已经有棋子被选中了
     if (Battle.shared.focusIndex != Move.invalidIndex &&
-        Color.of(position.pieceAt(Battle.shared.focusIndex)) == Color.black) {
+        Color.of(position.pieceOn(Battle.shared.focusIndex)) == Color.black) {
       //
       // 当前点击的棋子和之前已经选择的是同一个位置
       if (Battle.shared.focusIndex == index) return;
 
       // 之前已经选择的棋子和现在点击的棋子是同一边的，说明是选择另外一个棋子
-      final focusPiece = position.pieceAt(Battle.shared.focusIndex);
+      final focusPiece = position.pieceOn(Battle.shared.focusIndex);
 
       if (Color.isSameColor(focusPiece, tapedPiece)) {
         //