[gnome-klotski] Improve a bit tile drop.

[Arnaud B. <arnaudb src gnome org>]

commit aa8e19bb556981c8e54471beed1dcbb4cd06ea71
Author: Arnaud Bonatti <arnaud bonatti gmail com>
Date:   Sun Feb 9 05:20:24 2020 +0100

    Improve a bit tile drop.

 src/puzzle-view.vala |  6 +++++-
 src/puzzle.vala      | 11 +++++++++++
 2 files changed, 16 insertions(+), 1 deletion(-)
---
diff --git a/src/puzzle-view.vala b/src/puzzle-view.vala
index 706f29b..80bbc63 100644
--- a/src/puzzle-view.vala
+++ b/src/puzzle-view.vala
@@ -248,8 +248,12 @@ private class PuzzleView : Gtk.DrawingArea
             if (Puzzle.is_static_tile (new_piece_id) || new_piece_id == piece_id)
                 return false;
 
-            if (already_moving) // TODO skip the new piece selection if it cannot be moved
+            if (already_moving)
+            {
                 validate_move ();
+                if (!puzzle.can_be_moved (new_piece_id))
+                    return false;
+            }
 
             piece_unmoved = true;
             piece_id = new_piece_id;
diff --git a/src/puzzle.vala b/src/puzzle.vala
index 45e4f18..82694b2 100644
--- a/src/puzzle.vala
+++ b/src/puzzle.vala
@@ -287,6 +287,17 @@ private class Puzzle : Object
         return return_value;
     }
 
+    internal bool can_be_moved (int id)
+    {
+        if (is_static_tile (id))
+            return false;
+
+        return check_valid_move (id,  0,  1)
+            || check_valid_move (id,  0, -1)
+            || check_valid_move (id,  1,  0)
+            || check_valid_move (id, -1,  0);
+    }
+
     private bool check_valid_move (int id, int dx, int dy)
     {
         for (var y = 0; y < height; y++)