Session similarity

session_sim.rs

// 'most_recent_neighbors' is a set of ~500 similar sessions
for neighbor_session in most_recent_neighbors.into_iter() {
  let mut similarity = 0_f64;
  // This returns a HashSet of the items contained in the session
  let other_session_items = index.items_for_session(&neighbor_session.id);
  // The 'evolving_session' is an array of items of length 'num_items_in_evolving_session'   
  for (pos, item_id) in evolving_session.iter().enumerate() {
    if other_session_items.contains(&item_id) {
      let weight = (pos + 1) as f64 / num_items_in_evolving_session as f64;
      similarity += decay_factor;
    }
  }
}