simple question: how would one go about determining the bpm of a tune?

the end result would be lets say a pendulum that would swing in sync with the current track in a playlist (so it has to process the sound continuously and adjust if necessary).

update: i was thinking of using william brents timbreid and low pass filters to isolate the baseline, learn it, count how many times it occurs in a minute (or less) but it sounds a little complicated for my novice skills, that being the reason i posted here (maybe someone could point me in the right direction)