Sorry, I don't know of one. You could start with the Wikipedia description at https://en.wikipedia.org/wiki/Lanczos_resampling but that is nowhere near enough detail to do an actual implementation. You might try searching for a Lanczos-2 or Lanczos-3 implementation.

I wrote my first image resizer in 1986, so I've been immersed in this for a long time. I'm considering writing a blog post to go into the finer points, but my blog is kind of empty right now and has been for a long time - don't get your hopes up.