`numbermind`: a web app written with z3.rkt

You memorize a number. We guess it. You tell us how many digits we got right. We get to the final answer.

How does it work?

We present your inputs as constraints to z3.rkt. That's about it. The solver's just 30 lines of straightforward Racket code and the web server's another 50. (Continuation-based web programming is an absolute joy.)

What practical problems does it solve?

None that we know of. It was written over a couple of days as a fun exercise.

Where can I see it in action?

Here, hosted on Heroku. If you know the right incantations, you can host it yourself too.

Wait, I thought Heroku was only for hipsters using Ruby?

Heroku's Cedar stack supports absolutely anything that'll run on Linux x64. All you need to do is:

On a Linux x64 install, raco exe the server.rkt file and raco distribute it to a subdirectory. I use deploy/racket.
git init in the deploy subdirectory.
Create a Heroku app with the Cedar stack and the null buildpack:

$ heroku create -s cedar --buildpack http://github.com/ryandotsmith/null-buildpack.git

Add a file called Procfile to the directory with the following contents:

web: racket/bin/server

Check everything into your newly created Git repository.
git push heroku master.

If the web process isn't running, type in heroku ps:scale web=1. Note that since this is a stateful server (Z3's internal state can't really be serialized), anything more than web=1 probably won't work.

What license is it under?

The core code is under the Simplified BSD license, and we use a couple of frontend JavaScript libraries under MIT.