Selection of the event

As a preparation for the selection of the event we sum the values in the rate array:

    for (is = 1; is < last; is++)
        queue[is].rate += queue[is - 1].rate;
    sum = queue[last - 1].rate;
    dt = -log(ran(0)) / sum;

queue[n].rate is $\sum_{i=0}^{n} r_i$

dt is the timestep, equation \ref{tau}.

If we scanned all sites and found no possible events, nothing can happen. At first sight this may seem silly, however, it does occur and must be handled correctly:

    if (last == 0)
        return;

We select a random number, level, between 0 and sum, search for the corresponding entry in queue, and decode and execute the action represented by this entry:

        if (tsum + dt * ran(0) <= tstep)
        {
            level = sum * ran(0);

            for (is = 0; is < last; is++)
                if (level <= queue[is].rate)
                    break;

            if (queue[is].action < NBRNUM)
                swapAtm(queue[is].target, queue[is].action);
            else
                desTwo(queue[is].target, queue[is].action - NBRNUM);
        }