/* set value of the surrounding 8 grids until there's no changes */
    while (change != 0){
        change = 0;
        for(j=world->y_siz-1; j>=0; j--){
            for (i=0;i<=world->x_siz;i++){
        //for(i=0;i<world->x_siz;i++){
            //for(j=0;j<world->y_siz;j++){

                /*1*/
                if (((world->grid[i+1][j]) == OBSTACLE)||(i+1>world->x_siz-1)){
                    /*do nothing*/
                }else{
                    if ((world->grid[i+1][j] == UNKNOWN)&&(world->grid[i][j]==0)){
                        world->grid[i+1][j] = world->grid[i][j] + 2;
                        change = 1;
                    }
                    else if ((world->grid[i+1][j] != UNKNOWN) &&(world->grid[i+1][j]>world->grid[i][j]+2)){
                        world->grid[i+1][j] = world->grid[i][j] + 2;
                        change = 1;
                    }
                }