This is a simple standalone lossless JPEG XL encoder, based on libjxl effort 1 (cjxl -d 0 -e 1) but with all the handwritten SIMD removed.
It is a single C++ file without any dependencies (besides the standard library). The command line tool takes a PPM file (or PAM, to allow for alpha) as input, and produces a JPEG XL file as output.