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David Nickerson reports that it's possible to lay a line over a 'no
line here' cross mark by dragging, and furthermore, that doing so puts
that grid edge into a stuck state that no UI action short of undo can
get it back out of. Fix drags to stop at crosses, and fix execute_move
to fault any move string that nonetheless somehow managed to try to
set a line over a cross without explicitly tagging it 'R'.

[originally from svn r9400]

--- a/pearl.c

+++ b/pearl.c

@@ -1836,7 +1836,15 @@

     if (ox == gx || oy == gy) {

         int dx = (gx < ox ? -1 : gx > ox ? +1 : 0);

         int dy = (gy < oy ? -1 : gy > oy ? +1 : 0);

+        int dir = (dy>0 ? D : dy<0 ? U : dx>0 ? R : L);

         while (ox != gx || oy != gy) {

+            /*

+             * If the drag attempts to cross a 'no line here' mark,

+             * stop there. We physically don't allow the user to drag

+             * over those marks.

+             */

+            if (state->marks[oy*w+ox] & dir)

+                break;

             ox += dx;

             oy += dy;

             ui->dragcoords[ui->ndragcoords++] = oy * w + ox;

@@ -2037,9 +2045,6 @@

             if (!INGRID(state, x, y)) goto badmove;

             if (l < 0 || l > 15) goto badmove;

-            /* TODO trying to set a line over a no-line mark should be

-             * a failed move? */

-

             if (c == 'L')

                 ret->lines[y*w + x] |= (char)l;

             else if (c == 'N')

@@ -2051,6 +2056,16 @@

                 ret->lines[y*w + x] ^= (char)l;

             else if (c == 'M')

                 ret->marks[y*w + x] ^= (char)l;

+

+            /*

+             * If we ended up trying to lay a line _over_ a mark,

+             * that's a failed move: interpret_move() should have

+             * ensured we never received a move string like that in

+             * the first place.

+             */

+            if ((ret->lines[y*w + x] & (char)l) &&

+                (ret->marks[y*w + x] & (char)l))

+                goto badmove;

             move += n;

         } else if (strcmp(move, "H") == 0) {