This is quite a good Open Source Project:
https://patterns.enm.bris.ac.uk/hpa-software-package
It detects chords based on a chromagram – a good solution, breaks down a window of the whole spectrum onto an array of pitch classes (size: 12) with float values. Then, chords can be detected by a Hidden Markov Model.
.. should provide you with everything you need. 🙂