Alter treatment of moves with empty squares

In Edit Position mode one can move empty squares, and use them to
'capture' pieces as a means to clear their square. But this often leads
to unintended disappearence of pieces one wants to grab for dragging, if
a previous click had inadvertantly left an empty square selected: the
click on the piece is then seen as the to-click of a click-click move.
  This patch treats to-clicks of moves with an empty-square differently
from other to-clicks: Rather than performing the move (in this case
clearing of the to-square) on the down-click, it defers that to the
up-click, and only does it if that up-click is in the same square. This
allows the user to grab the piece on the down-click, and drag it to
another square, if that is what he wanted to do, without being disturbed
by sudden implosion of the piece he tried to grab.
  If the to-square is in the holdings, grabbing the piece is the only
action, as moves of empty squares into the holdings are forbidden anyway.

diff --git a/backend.c b/backend.c
index b1df197..1165932 100644 (file)
--- a/backend.c
+++ b/backend.c
@@ -6445,7 +6445,7 @@ void LeftClick(ClickType clickType, int xPix, int yPix)
 {
     int x, y;
     Boolean saveAnimate;
-    static int second = 0, promotionChoice = 0;
+    static int second = 0, promotionChoice = 0, clearFlag = 0;
     char promoChoice = NULLCHAR;
     ChessSquare piece;
 
@@ -6602,6 +6602,14 @@ void LeftClick(ClickType clickType, int xPix, int yPix)
 
     if (clickType == Release && x == fromX && y == fromY) {
        DragPieceEnd(xPix, yPix); dragging = 0;
+       if(clearFlag) {
+           // a deferred attempt to click-click move an empty square on top of a piece
+           boards[currentMove][y][x] = EmptySquare;
+           ClearHighlights();
+           DrawPosition(FALSE, boards[currentMove]);
+           fromX = fromY = -1; clearFlag = 0;
+           return;
+       }
        if (appData.animateDragging) {
            /* Undo animation damage if any */
            DrawPosition(FALSE, NULL);
@@ -6621,12 +6629,20 @@ void LeftClick(ClickType clickType, int xPix, int yPix)
        return;
     }
 
+    clearFlag = 0;
+
     /* we now have a different from- and (possibly off-board) to-square */
     /* Completed move */
     toX = x;
     toY = y;
     saveAnimate = appData.animate;
     if (clickType == Press) {
+       if(gameMode == EditPosition && boards[currentMove][fromY][fromX] == EmptySquare) {
+           // must be Edit Position mode with empty-square selected
+           fromX = x; fromY = y; DragPieceBegin(xPix, yPix); dragging = 1; // consider this a new attempt to drag
+           if(x >= BOARD_LEFT && x < BOARD_RGHT) clearFlag = 1; // and defer click-click move of empty-square to up-click
+           return;
+       }
        /* Finish clickclick move */
        if (appData.animate || appData.highlightLastMove) {
            SetHighlights(fromX, fromY, toX, toY);