title: Similarity Between Users Based On Votes
body: <p>lets say i have a set of users, a set of songs, and a set of votes on each song:</p>
  
  <pre><code>=========== =========== =======
  User        Song        Vote
  =========== =========== =======
  user1       song1       [score]
  user1       song2       [score]
  user1       song3       [score]
  user2       song1       [score]
  user2       song2       [score]
  user2       song3       [score]
  user3       song1       [score]
  user3       song2       [score]
  user3       song3       [score]
  user-n      song-n      [score]
  =========== =========== =======
  </code></pre>
  
  <p>whats the most efficient way to calculate user similarity based on song-votes? is there a better way than iterating over every user and every vote for every song?</p>