Black hat Black hat A black hat hacker (or black-hat hacker) is a hacker who "violates computer security for little reason beyond maliciousness or for personal gain". The term's origin is often attributed to hacker culture theorist Richard Stallman (though he denies coining it) to contrast the exploitative hacker with the white hat hacker who hacks protectively by drawing attention to vulnerabilities in computer systems that require repair. The black hat/white hat terminology originates in the Western genre of popular American culture, in which black and white hats denote villainous and heroic cowboys respectively. Black hat hackers are the stereotypical illegal hacking
Item-item collaborative filtering Earlier collaborative filtering systems based on rating similarity between users (known as user-user collaborative filtering) had several problems: Item-item models resolve these problems in systems that have more users than items. Item-item models use rating distributions "per item", not "per user". With more users than items, each item tends to have more ratings than each user, so an item's average rating usually doesn't change quickly. This leads to more stable rating distributions in the model, so the model doesn't have to be rebuilt as often. When users consume and then rate an item, that item's similar items are picked from