void neighbour_choose_and_move_to_next( ant_struct *a , long int phase )
/*    
      FUNCTION:      Choose for an ant probabilistically a next city among all 
                     unvisited cities in the current city's candidate list. 
		     If this is not possible, choose the closest next
      INPUT:         pointer to ant the construction step "phase" 
      OUTPUT:        none 
      (SIDE)EFFECT:  ant moves to the chosen city
*/
{ 
    long int i, help; 
    long int current_city;
    double   rnd, partial_sum = 0., sum_prob = 0.0;
    /*  double   *prob_of_selection; */ /* stores the selection probabilities 
	of the nearest neighbor cities */
    double   *prob_ptr;



    if ( (q_0 > 0.0) && (ran01( &seed ) < q_0)  ) {
	/* with a probability q_0 make the best possible choice
	   according to pheromone trails and heuristic information */
	/* we first check whether q_0 > 0.0, to avoid the very common case
	   of q_0 = 0.0 to have to compute a random number, which is
	   expensive computationally */
	neighbour_choose_best_next(a, phase);
	return;
    }

    prob_ptr = prob_of_selection;

    current_city = (*a).tour[phase-1]; /* current_city city of ant k */
    DEBUG( assert ( current_city >= 0 && current_city < n ); )
    for ( i = 0 ; i < nn_ants ; i++ ) {
	if ( (*a).visited[instance.nn_list[current_city][i]] ) 
	    prob_ptr[i] = 0.0;   /* city already visited */
	else {
	    DEBUG( assert ( instance.nn_list[current_city][i] >= 0 && instance.nn_list[current_city][i] < n ); )
	    prob_ptr[i] = total[current_city][instance.nn_list[current_city][i]];
	    sum_prob += prob_ptr[i];
	} 
    }

    if (sum_prob <= 0.0) {
	/* All cities from the candidate set are tabu */
	choose_best_next( a, phase );
    }     
    else {  
	/* at least one neighbor is eligible, chose one according to the
	   selection probabilities */
	rnd = ran01( &seed );
	rnd *= sum_prob;
	i = 0;
	partial_sum = prob_ptr[i];
	while ( partial_sum <= rnd ) {
	    i++;
	    partial_sum += prob_ptr[i]; 
	}
	DEBUG( assert ( 0 <= i && i < nn_ants); );
	DEBUG( assert ( prob_ptr[i] >= 0.0); );
	help = instance.nn_list[current_city][i];
	DEBUG( assert ( help >= 0 && help < n ); )
	DEBUG( assert ( (*a).visited[help] == FALSE ); )
	(*a).tour[phase] = help; /* instance.nn_list[current_city][i]; */
	(*a).visited[help] = TRUE;
    }
}