We stand on the threshold of the age of the algorithm, a not-too-distant future where machines will make some of our most important decisions - in healthcare, transport, finance, security, what we watch, where we go and even who we send to prison. So how much should we rely on them? What kind of future do we want? Hannah Fry takes us on a tour of the good, the bad and the downright ugly of the algorithms that surround us.