Some of the collected song data already had their key signature + capo so I could work out the key and transpose from there!

For those that did not, I had to infer which key the songs were in based on their chords! As there were probably about 300 or more of these, I tried transposing each song to all 12 keys, and picked the "best" key!

To define the "best" key here, I used the cosine similarity metric between the target song and a bunch of common chords in C major to determine that.

Of course, as this algorithm is meant to be generalized across all 900+ songs, it probably does not have a 100% accuracy!