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It's the encoder as well, plus converter code for things like JPEG reading as well, and JPEG XL supports animation...

I guess the original comment in TFA is a little bit misleading, then.

> of the reference decoder (currently behind a pref in Firefox Nightly), which weighs in at more than 100,000 lines of multithreaded C++

That number includes all the different extra tools in the repo, and all tests. And as mentioned, the encoder.

There must be a lot of edge cases that were discovered over the years.