ds_grid_flood_fill

/// ds_grid_flood_fill(grid,x,y,val)
//
//  Fills a bounded area within a grid with a given value.
//  The area is bounded by values unequal to the initial
//  value found at a given seeded position.
//
//      grid        grid data structure, grid
//      x,y         position to seed fill from, integer
//      val         value filling bounded area, real or string
//
//  Note: Requires short-circuit evaluation enabled.
//
/// GMLscripts.com/license
{
    var grid = argument0;
    var seedx = argument1;
    var seedy = argument2;
    var newval = argument3;

    var w = ds_grid_width(grid);
    var h = ds_grid_height(grid);

    var oldval = grid[# seedx, seedy];

    var stackx = ds_queue_create();
    var stacky = ds_queue_create();

    ds_queue_enqueue(stackx, seedx);
    ds_queue_enqueue(stacky, seedy);

    while (ds_queue_size(stackx))
    {
        var px = ds_queue_dequeue(stackx);
        var py = ds_queue_dequeue(stacky);
        var y1 = py;
        while (y1 >= 0 && grid[# px, y1] == oldval) y1--;
        y1++;
        var spanLeft = false;
        var spanRight = false;
        while (y1 < h && grid[# px, y1] == oldval)
        {
            grid[# px,y1] = newval;
            if (!spanLeft && px > 0 && grid[# px-1, y1] == oldval)
            {
                ds_queue_enqueue(stackx, px-1);
                ds_queue_enqueue(stacky, y1);
                spanLeft = true;
            }
            else if (spanLeft && px > 0 && grid[# px-1, y1] != oldval)
            {
                spanLeft = false;
            }
            if (!spanRight && px < w-1 && grid[# px+1, y1] == oldval)
            {
                ds_queue_enqueue(stackx, px+1);
                ds_queue_enqueue(stacky, y1);
                spanRight = true;
            }
            else if (spanRight && px < w-1 && grid[# px+1, y1] != oldval)
            {
                spanRight = false;
            }
            y1++;
        }
    }

    ds_queue_destroy(stackx);
    ds_queue_destroy(stacky);

    return 0;
}