The International Simutrans Forum

/* Calculates (and sets) new akt_speed
 * needed for driving, entering and leaving a depot)
 */
void convoi_t::calc_acceleration(long delta_t)
{
	// Prissi: more pleasant and a little more "physical" model *
	int sum_friction_weight = 0;
	sum_gesamtgewicht = 0;
	// calculate total friction
	for(unsigned i=0; i<anz_vehikel; i++) {
		const vehikel_t* v = fahr[i];
		int total_vehicle_weight = v->get_gesamtgewicht();

		sum_friction_weight += v->get_frictionfactor() * total_vehicle_weight;
		sum_gesamtgewicht += total_vehicle_weight;
	}

	// try to simulate quadratic friction
	if(sum_gesamtgewicht != 0) {
		/*
		 * The parameter consist of two parts (optimized for good looking):
		 *  - every vehicle in a convoi has a the friction of its weight
		 *  - the dynamic friction is calculated that way, that v^2*weight*frictionfactor = 200 kW
		 * => heavier loaded and faster traveling => less power for acceleration is available!
		 * since delta_t can have any value, we have to scale the step size by this value.
		 * however, there is a quadratic friction term => if delta_t is too large the calculation may get weird results
		 * @author prissi
		 */

		/* but for integer, we have to use the order below and calculate actually 64*deccel, like the sum_gear_und_leistung
		 * since akt_speed=10/128 km/h and we want 64*200kW=(100km/h)^2*100t, we must multiply by (128*2)/100
		 * But since the acceleration was too fast, we just deccelerate 4x more => >>6 instead >>8 */
		sint32 deccel = ( ( (akt_speed*sum_friction_weight)>>6 )*(akt_speed>>2) ) / 25 + (sum_gesamtgewicht*64);	// this order is needed to prevent overflows!

		// prissi:
		// integer sucks with planes => using floats ...
		sint32 delta_v =  (sint32)( ( (double)( (akt_speed>akt_speed_soll?0l:sum_gear_und_leistung) - deccel)*(double)delta_t)/(double)sum_gesamtgewicht);

		// we normalize delta_t to 1/64th and check for speed limit */
//		sint32 delta_v = ( ( (akt_speed>akt_speed_soll?0l:sum_gear_und_leistung) - deccel) * delta_t)/sum_gesamtgewicht;

		// we need more accurate arithmetic, so we store the previous value
		delta_v += previous_delta_v;
		previous_delta_v = delta_v & 0x0FFF;
		// and finally calculate new speed
		akt_speed = max(akt_speed_soll>>4, akt_speed+(sint32)(delta_v>>12l) );
	}
	else {
		// very old vehicle ...
		akt_speed += 16;
	}

	// obey speed maximum with additional const brake ...
	if(akt_speed > akt_speed_soll) {
		akt_speed -= 24;
		if(akt_speed > akt_speed_soll+kmh_to_speed(20)) {
			akt_speed = akt_speed_soll+kmh_to_speed(20);
		}
	}

	// new record?
	if(akt_speed > max_record_speed) {
		max_record_speed = akt_speed;
		record_pos = fahr[0]->get_pos().get_2d();
	}
}