Malevolent Cartography

Write C

Drink your coffee black

Sleep on the floor

Burn your rolling luggage

Contact

github.com/qpfiffer

qpfiffer+qpweb@gmail.com

Links

ReverseGPX, A Weekend Project

2022-04-09 by Quinlan Pfifferprogramminggentoocsoftware

For a while now, I’ve noticed that theres no good tool online to flip GPX files around. gpx.studio exists but it’s big and clunky and graphical, and does a lot more than what I want. There are also a handful of weird java tools and random code in GitHub repos, but nothing simple and concise. This need actually comes up a lot for me: Say I need to reverse a bikepacking route, or create a route from a track that was done the wrong way, or who knows. Theres not a simple, easy, fast way to do it.

So now that I’m unemployed and wandering the earth like a Wraith, I finally got around to building ReverseGPX.com. The concept is simple: You upload a file, wait for a second while it does it’s thing, and you get back the same file but the timestamps and trackpoint order is reversed. It’s simple and bespoke and fast. Theres no fucking around with a javascript map or sliders or anything, it just does what it says. I figured I’d talk a little bit about how it’s built and dpeloyed, since it’s done with my own particular brand of brain-damage.

The Code

I hacked it together in a rainy weekend at a hostel here in Pucón, Chile. The frontend is written in Python3 using the Sanic framework because I’m familiar with it. It’s mostly uninteresting. It serves up some HTML and ingests files, making sure they’re mostly benign. Theres some ugly vanilla Javascript and inline CSS because I don’t need any frontend dependencies.

The File Daemon

The interesting part is the bespoke C file-processing daemon, that reads more like a script. You can check it out for yourself here. It’s almost one file, but I needed to add in my vector library to handle arbitrary lists. This one does use dependencies, because I didn’t want to mess around with parsing XML. LibXML2 does that job, and was fun to try and figure out. It’s not a bad library, and more than enough for what I’m doing.

The daemon itself reverses two things: The node order, and the timestamps in those nodes. So far I haven’t found a GPX file that messes it up, but I’m sure they exist. I haven’t even bothered to really check it for memory leaks, since it’s short-lived and just called by inotify. It uses the filesystem for processing.

Deployment

For now it’s using the classic bombproof running-in-tmux deployment method, which I’ve found to be one of the best ways to just get something going. Eventually I’ll get around to sticking it into my private Overlay, but for now it’s just quietly chugging along in the backround and it’s very easy to keep an eye on it, so I’m not too worried.

If you do find a bug in it, please email me here. I’m very interested in finding things that break it, since it’s a very simple and basic utility.