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There are various commercial offerings that do this, including Chordify and Capo.
If you want source code, consider the Chordino plugin (http://www.isophonics.net/nnls-chroma, C++ code at https://github.com/c4dm/nnls-chroma) which you can use in Sonic Visualiser (https://www.sonicvisualiser.org/).
There is a reference for the Chordino method in the page linked above, but roughly it's not that far from the description of the parent poster - a non-negative least-squares method produces a frame-by-frame semitone-scaled decomposition which is then matched against templates and turned into a chord sequence using a hidden Markov model.