There are many, but mostly focused on output quality, and slow, and written in matlab using fancy tools.
If somebody submits a simple patch (written in pure C without dependences) to the jpeg reference distribution, and the code is very fast and good enough, it will most surely become mainstream. The thing is, that the people who know enough math to create a good jpeg decompressor do not tend to be well-versed nor interested in C programming.
They should at least grasp a bit of the math involved, which is possible but not elementary. A full re-implementation would not be hard, the hard thing would be to remove the non-essential steps that increase the quality by a negligible amount but have a huge impact in the running time. Knowing what parts of the algorithm are essential and what parts are superfluous is not easy if you don't understand the algorithm.
Yeah that's what figured but hoped against. I still might have a curious look later...
any pointers for where a good resource might be?
Look at the two-part article from 2015 here:
I've been planning to write an adversarial neural network based JPEG decoder that would find the most realistic possible image that would get compressed to the given JPEG... I would love to have such a decoder integrated into browsers!