I'm not sure that this is ok but it definitively prevents impossible to solve...

I'm not sure that this is ok but it definitively prevents impossible to solve puzzles from being generated here.

modules/video_filter/puzzle.c

@@ -118,6 +118,18 @@ static vlc_bool_t finished( vout_sys_t *p_sys )
     }
     return VLC_TRUE;
 }
+static vlc_bool_t is_valid( vout_sys_t *p_sys )
+{
+    int i, s;
+    if( p_sys->b_blackslot == VLC_FALSE ) return VLC_TRUE;
+    for( i = 0; i < p_sys->i_cols * p_sys->i_rows; i++ )
+    {
+        if( (p_sys->i_cols*p_sys->i_rows+i-p_sys->pi_order[i])%2 ) s++;
+        else s--;
+    }
+    if( s!=0 ) return VLC_FALSE;
+    else return VLC_TRUE;
+}
 static void shuffle( vout_sys_t *p_sys )
 {
     int i, c;
@@ -140,7 +152,8 @@ static void shuffle( vout_sys_t *p_sys )
             }
         }
         p_sys->b_finished = finished( p_sys );
-    } while( p_sys->b_finished == VLC_TRUE );
+    } while(    p_sys->b_finished == VLC_TRUE
+             && is_valid( p_sys ) == VLC_FALSE );
 
     if( p_sys->b_blackslot == VLC_TRUE )
     {