void zz_system::tick_time ()
{
	if (use_fixed_framerate) {
		add_time(80);
	}
#ifdef USE_SMOOTH_FRAMERATE
	else {
		static float accum_error = 0; // accumulated diff error
		const zz_time MAX_ACCUM_ERROR = ZZ_TICK_PER_SEC / 1; // max error = 1 sec delay
		const zz_time MIN_ACCUM_ERROR = ZZ_TICK_PER_SEC / 10; // min error = .1 sec delay

		const int MIN_FRAMERATE = 5; // not to get slower than min_framerate
		const zz_time MAX_DIFF = ZZ_TICK_PER_SEC / MIN_FRAMERATE; // time(1sec) / frame
		const zz_time ERROR_MAX_DIFF = ZZ_TICK_PER_SEC*1; // 1 sec

		zz_time now = timer.get_time();
		zz_time diff = now - last_time_; // new real diff

		if (use_force_diff_time) {
			diff = force_diff_time;
			use_force_diff_time = false;
			force_diff_time = 0;
		}

		if (diff > ERROR_MAX_DIFF) { // zhotest
			diff = ERROR_MAX_DIFF;
		}

		if (0) {
			if (diff > MAX_DIFF) {
				accum_error += diff - MAX_DIFF;
				if (accum_error > MAX_ACCUM_ERROR) { // if error exceeded max error
					diff = zz_time(accum_error) + MAX_DIFF;
					accum_error = 0;
				}
				else {
					diff = MAX_DIFF;
				}
			}
			else {
				if (accum_error < MIN_ACCUM_ERROR) {
					diff += zz_time(accum_error);
					accum_error = 0;
				}
				else {
					diff += MIN_ACCUM_ERROR;
					accum_error -= float(MIN_ACCUM_ERROR);
					assert(accum_error >= 0);
				}
			}
		}
		else if (1) {
			const zz_time ACCUM_ERROR_FULL = 2 * ZZ_TICK_PER_SEC; // not to exceed 2 sec
			const float KP = .3f; // magic constants. damper
			zz_time extra_add = 0;

			accum_error += float(diff); // it's current time

			if (accum_error > ACCUM_ERROR_FULL) {
				extra_add = zz_time(accum_error) - ACCUM_ERROR_FULL;
				accum_error = ACCUM_ERROR_FULL;
			}

			assert(accum_error <= ACCUM_ERROR_FULL);

			assert(KP <= 1.0f);
			diff = zz_time (KP * accum_error * accum_error / ACCUM_ERROR_FULL);


			accum_error -= diff;
			diff += extra_add;

			assert(accum_error >= 0);
		}

		set_current_time(now); // update current_time and last_time
		last_time = now - diff; // overwrite last_time set by set_current_time
		last_time_ = now;

		//ZZ_LOG("system: tick_time(). diff(%10d),last(%10d), now(%10d)\n", diff, last_time, now);
		//ZZ_LOG("%f, \n", accum_error);
		//ZZ_LOG("%d, \n", diff);
	}
#else // NON SMOOTH FRAMERATE
	else { // dynamic frame rate
		set_current_time();
	}
#endif
}