add board_canFlip and model_canFlip -- which seem to work though

there's no test that when a single tile is on the board it's not in
the middle.  Sue me.

xwords4/common/board.c

@@ -523,6 +523,13 @@ board_getYOffset( const BoardCtxt* board )
     return vsd->offset;
 } /* board_getYOffset */
 
+XP_Bool
+board_canFlip( const BoardCtxt* board )
+{
+    return model_canFlip( board->model, board->selPlayer, 
+                          TRAY_REVEALED == board->trayVisState );
+}
+
 static XP_U16
 adjustOffset( XP_U16 curOffset, XP_S16 zoomBy )
 {

xwords4/common/board.h

@@ -78,6 +78,8 @@ void board_reset( BoardCtxt* board );
 XP_Bool board_setYOffset( BoardCtxt* board, XP_U16 newOffset );
 XP_U16 board_getYOffset( const BoardCtxt* board );
 
+XP_Bool board_canFlip( const BoardCtxt* board );
+
 /* zoomBy: >0: zoom in; < 0: zoom out; 0: query only */
 XP_Bool board_zoom( BoardCtxt* board, XP_S16 zoomBy, XP_Bool* canInOut );
 

xwords4/common/model.c

@@ -480,13 +480,14 @@ undoFromMoveInfo( ModelCtxt* model, XP_U16 turn, Tile blankTile, MoveInfo* mi )
 
         setModelTileRaw( model, col, row, EMPTY_TILE );
         notifyBoardListeners( model, turn, col, row, XP_FALSE );
+        --model->vol.nTilesOnBoard;
 
         if ( IS_BLANK(tile) ) {
             tile = blankTile;
         }
         model_addPlayerTile( model, turn, -1, tile );
     }
-
+    XP_LOGF( "%s: %d tiles on board", __func__, model->vol.nTilesOnBoard );
     adjustScoreForUndone( model, mi, turn );
 } /* undoFromMoveInfo */
 
@@ -842,9 +843,9 @@ model_trayContains( ModelCtxt* model, XP_S16 turn, Tile tile )
 } /* model_trayContains */
 
 XP_U16
-model_getCurrentMoveCount( ModelCtxt* model, XP_S16 turn )
+model_getCurrentMoveCount( const ModelCtxt* model, XP_S16 turn )
 {
-    PlayerCtxt* player;
+    const PlayerCtxt* player;
     XP_ASSERT( turn >= 0 );
     player = &model->players[turn];
     return player->nPending;
@@ -1104,6 +1105,14 @@ model_getNMoves( const ModelCtxt* model )
     return result;
 }
 
+XP_Bool
+model_canFlip( const ModelCtxt* model, XP_U16 turn, XP_Bool trayVisible )
+{
+    XP_Bool canFlip = 0 < model->vol.nTilesOnBoard
+        || (trayVisible && (0 < model_getCurrentMoveCount( model, turn )));
+    return canFlip;
+}
+
 static void
 incrPendingTileCountAt( ModelCtxt* model, XP_U16 col, XP_U16 row )
 {
@@ -1229,6 +1238,8 @@ commitTurn( ModelCtxt* model, XP_S16 turn, TrayTileSet* newTiles,
         setModelTileRaw( model, col, row, tile );
 
         notifyBoardListeners( model, turn, col, row, XP_FALSE );
+
+        ++model->vol.nTilesOnBoard;
     }
 
     (void)getCurrentMoveScoreIfLegal( model, turn, stream, &score );
@@ -1247,6 +1258,7 @@ commitTurn( ModelCtxt* model, XP_S16 turn, TrayTileSet* newTiles,
         model_addPlayerTile( model, turn, -1, *newTilesP++ );
     }
 
+    XP_LOGF( "%s: %d tiles on board", __func__, model->vol.nTilesOnBoard );
     return score;
 } /* commitTurn */
 

xwords4/common/model.h

@@ -160,7 +160,7 @@ XP_U16 model_numCols( const ModelCtxt* model );
 void model_addToCurrentMove( ModelCtxt* model, XP_S16 turn, 
                              XP_U16 col, XP_U16 row, 
                              Tile tile, XP_Bool isBlank );
-XP_U16 model_getCurrentMoveCount( ModelCtxt* model, XP_S16 turn );
+XP_U16 model_getCurrentMoveCount( const ModelCtxt* model, XP_S16 turn );
 
 void model_getCurrentMoveTile( ModelCtxt* model, XP_S16 turn, XP_S16* index,
                                Tile* tile, XP_U16* col, XP_U16* row, 
@@ -190,6 +190,10 @@ void model_makeTurnFromMoveInfo( ModelCtxt* model, XP_U16 playerNum,
 void model_resetCurrentTurn( ModelCtxt* model, XP_S16 turn );
 XP_S16 model_getNMoves( const ModelCtxt* model );
 
+/* Are there two or more tiles visible */
+XP_Bool model_canFlip( const ModelCtxt* model, XP_U16 turn, 
+                       XP_Bool trayVisible );
+
 /********************* notification ********************/
 typedef void (*BoardListener)(void* data, XP_U16 turn, XP_U16 col, 
                               XP_U16 row, XP_Bool added );

xwords4/common/modelp.h

@@ -53,6 +53,7 @@ typedef struct ModelVolatiles {
     void* trayListenerData;
     DictListener dictListenerFunc;
     void* dictListenerData;
+    XP_U16 nTilesOnBoard;
     MPSLOT
 } ModelVolatiles;