LJ Archive

Work the Shell

Spreading Out Numbers

Dave Taylor

Issue #173, September 2008

The hardest part of any game is coming up with likely, but incorrect, answers. In this month's column, Dave looks at ways to calculate probable wrong answers for the evolving movie trivia game.

The past few months, we've been writing a movie trivia game with the intent of having it be a Twitter client and sporadically spit out questions on its Twitter feed of the form “The film Sunset Blvd. was released in 1943, 1946, or 1950?”

What initially seemed like the most difficult task, finding the list of films and then extracting release dates, turned out to be a manageable one through the expedient of utilizing the terrific Internet Movie Database site (imdb.com) and pushing the data through some filters and transformations.

The end result is that with a simple invocation of a script, we can generate a data file called top-250-films-with-release-dates.db that looks like this: “Sunset Blvd. | 1950” (and now you know the answer to the question in paragraph one).

Generating Interesting Adjacent Numbers

Last column left off with the puzzle of generating good “adjacent” release years. That is, if we're talking about a movie like Prince Caspian, released in 2008, we want the adjacent values to be quite close—maybe 2005 and 2007. If we're talking about Rear Window, released back in 1954, we want the adjacent values to be spread out more, because offering up 1951, 1954 and 1955 is going to be more annoying and nit-picking than 1940, 1950 and 1954 or similar. See what I mean?

What we could do is simply subtract the release year from the current year, then apply some sort of multiple to tweak the delta. Then, Prince Caspian would have an “adjacency” of zero, and Rear Window would have one of 54. Let's consider dividing the value by five and using the ceiling value to see what the calculation for a half-dozen movies produces (Table 1).

Table 1. Calculating Adjacency for the Movie Trivia Game

TitleRelease DateAdjacencyFactor
Der Untergang200441
Sin City200531
Some Like It Hot19594910

That's not bad. Sin City could have incorrect year values within one year of the actual release, while Metropolis could be off by as much as 17 without most people realizing. I mean, if I asked you right now, “Did Fritz Lang's masterwork Metropolis come out in 1927, 1931 or 1947?”, would you know the answer?

This leads to an important realization: we can't have the values perfectly spaced out, so the Factor above is the upper range of a 1..Factor choice. So, the amusing Some Like It Hot can have incorrect guesses that are anywhere from one year to nine years off.

Okay, enough discussion. How do we implement this in code?

Well, we have the release date of the movie in releasedate, and we have the current year in thisyear, so here's a simple test script:

thisyear="$(date +%Y)"
adjacency="$(( $thisyear - $releasedate ))"
if [ $adjacency -lt 5 ] ; then
  factor="$(( $adjacency / 5 + 1 ))"
echo "For release $releasedate we have factor = $factor"

This demonstrates an important facet of shell scripting: sometimes thinking through the solution is more time consuming than actually coding your resultant algorithm. I could share an anecdote about my boss telling me to “stop thinking and start coding” in one of my earlier jobs, but I'll skip it. Just keep in mind that thinking through solution paths is a critical step in any job.

Now that we have a way to calculate our adjacency factor for a given movie release year, let's take the next step and actually calculate possible values:

delta="$(( $RANDOM % $factor + 1))"
add="$(( $RANDOM % 2 ))"
if [ $add -eq 1 ] ; then
  closeyear="$(( $releasedate + $delta ))"
  closeyear="$(( $releasedate - $delta ))"

That isn't too bad as a first step.

There are two problems I see with this algorithm as is, however. First, we can end up with release years in the future (that is, Iron Man could end up with a release year of 2009, which is wrong). Second, for movies released in the last five years, we also could end up with the actual release year always sandwiched in the middle once we de-dupe the results. (I hope you can see why that's the case.)

To fix the first problem, we need to add a test to ensure that the closeyear is never greater than thisyear, which is straightforward. For the second problem, I think that having a minimum delta of two, rather than one, gives us a bit more wiggle space, though any movie released in the current year is basically a gimme anyway for people who are paying even minimal attention.

Here's how I implemented these tweaks:

if [ $adjacency -lt 5 ] ; then
  factor="$(( $adjacency / 5 + 1 ))"

And, a bit later in the code:

if [ $closeyear -gt $thisyear ] ; then
  closeyear="$(( $releasedate - $delta ))"

That seems to work pretty well. Now when we give the script a few different release years, here's what we see:

Release Year    First Five Generated Results
1962            1970, 1967, 1958, 1960, 1971
1994            1996, 1996, 1995, 1993, 1993
2002            2004, 2001, 2000, 2001, 2003
1927            1915, 1925, 1937, 1936, 1911
2008            2006, 2007, 2007, 2006, 2007

I think we can live with this—not bad at all, actually.

Now we have all the building blocks, and next month, we'll put them all together and create the movie trivia game. With luck, we'll have space to start pushing it out on Twitter too. In the meantime, if you want to sign up on Twitter for the game and watch as I develop it, follow FilmBuzz.

Dave Taylor is a 26-year veteran of UNIX, creator of The Elm Mail System, and most recently author of both the best-selling Wicked Cool Shell Scripts and Teach Yourself Unix in 24 Hours, among his 16 technical books. His main Web site is at www.intuitive.com, and he also offers up tech support at AskDaveTaylor.com. Follow him on Twitter if you'd like: twitter.com/DaveTaylor.

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