* * * * *

There's a reason why a feature that takes five minutes to implement takes ten
                               hours to code …

It seemed like an easy feature to add to the Quick and Dirty B-Movie Plot
Generator [1]. All I wanted was the ability to bookmark a particular plot.

What I didn't realize was that the method I used, saving the random number
seed, was completely and utterly the wrong thing to do, and that the feature
isn't quite as easy to implement as I first thought.

Easy to implement features, aren't.

My first clue? I “bookmarked” a particular plot on my development server:

> Sheffie Les Mastenbrook is a **mute world-famous card sharp from a doomed
> world**. Susette Marie-Jeanne Sanford-Wright is a **virginal green-skinned
> werewolf with nothing left to lose**. And together, they must **defuse a
> bomb in less than a day that's a thousand miles away**.
>

(the bold parts? I'll get to that—and by the way, this is plot #1134)

Later on, I decided to check on the “bookmarked” plot on my deployment
server, and got:

> Sheffie Les Mastenbrook is a **fast talking hunchbacked ad-exec on the
> run**. Susette Marie-Jeanne Sanford-Wright is an **orphaned short-sighted
> boxer with the power to bend men's minds**. And together, they must **fight
> crime**.
>

Oops. (Bold text denotes differences between the two versions)

It seems that the random number generator between the two servers isn't quite
the same; enough to get the names the same (which are the first things to be
generated) but diverge right after that. So, if I wish to keep the URL
(Uniform Resource Locator)s valid, I can't change servers, or even update the
current server!

From this, I realized another problem—I can't change the data files the
program uses! This stems from how I use the random numbers I get, and for
this, I need a slight digression.

In C (which is what the program is written in), I call rand() to get a random
number. rand() takes no parameters, and returns an integer between 0 and some
upper bound RAND_MAX (which is a system and compiler dependant value). The
man page (on my development server, which is an older Linux distribution) has
this to say on using rand() (sad to say, but the man page on current Linux
distributions excludes this excellent bit of advice):

> The versions of rand() and srand() in the Linux C Library use the same
> random number generator as random() and srandom(), so the lower-order bits
> should be as random as the higher-order bits. However, on older rand()
> implementations, the lower-order bits are much less random than the higher-
> order bits.
>
> In _Numerical Recipes in C: The Art of Scientific Computing_ [2] (William
> H. Press, Brian P. Flannery, Saul A. Teukolsky, William T. Vetterling; New
> York: Cambridge University Press, 1990 (1st ed, p. 207)), the following
> comments are made:
>
> > If you want to generate a random integer between 1 and 10, you should
> > always do it by
> >
> > > j=1+(int) (10.0*rand()/(RAND_MAX+1.0));
> > >
> >
> > and never by anything resembling
> >
> > > j=1+((int) (1000000.0*rand()) % 10);
> > >
> >
> > (which uses lower-order bits).
> >
>
> Random-number generation is a complex topic. The _Numerical Recipes in C_
> [3] book (see reference above) provides an excellent discussion of
> practical random-number generation issues in Chapter 7 (Random Numbers).
>
> For a more theoretical discussion which also covers many practical issues
> in depth, please see Chapter 3 (Random Numbers) in Donald E. Knuth's _The
> Art of Computer Programming_ [4], volume 2 (Seminumerical Algorithms), 2nd
> ed.; Reading, Massachusetts: Addison-Wesley Publishing Company, 1981.
>

Since I'm picking a random entry from N items, I basically do

> n = (int)(((double)rand() / (double)RAND_MAX) * (double)N);
>

The upshot?

I add an element to the list, the value of N changes, and for a given output
of rand() you get a diffent answer than with the previous, smaller N.

And it doesn't matter where I add an item. Add a new occupation for our
characters at the end of the list (on the development server) and for plot
#1134 you now get:

> Sheffie Les Mastenbrook is a mute world-famous **inventor** from a doomed
> world. Susette Marie-Jeanne Sanford-Wright is a virginal green-skinned
> **vampire hunter** with nothing left to lose. And together, they must
> defuse a bomb in less than a day that's a thousand miles away.
>

This “easy feature” is turning out not to be so easy. And this on a simple
program. It's stuff like this that makes programming take longer than
expected.

The proper solution is to record the fifteen random numbers generated and
make that part of the URL, and make sure that additions to the various lists
appear at the end, but we finally get “bookmarkable” plots.

Oh, I just realized—only to lose the “bookmarked plots” if the template (and
thus, the number of random numbers selected) changes.

Oh, bother.

[1] https://boston.conman.org/cgi-bin/plotdriver.cgi
[2] http://www.amazon.com/exec/obidos/ASIN/0521431085/conmanlaborat-20
[3] http://www.amazon.com/exec/obidos/ASIN/0521431085/conmanlaborat-20
[4] http://www.amazon.com/exec/obidos/ASIN/0201485419/conmanlaborat-20

Email author at [email protected]

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