title: Dynamic perfect hashing and universal hash functions - explanation please?
body: <p>So I'm reading up about hash tables, hash functions etc.  I was intrigued to read on wikipedia about how "dynamic perfect hashing" involves using a second hash table as the data structure to store multiple values within a particular bucket.</p>
  
  <p>Where I get lost however, is when it comes to how a universal hash function is selected to perform the hashing for that second hash table.  Can anyone explain how this universal hash function is determined from the values being stored in the bucket?  I vaguely following the reasoning and logic in wikipedia's "universal hash function" page, but am struggling to have any intuition on it.  In particular, how do these functions guarantee no clashes?  Or atleast, if they're disposed of and a new one generated if a clash is detected, how do we know this can be done in a realistic amount of time if at all?</p>
  
  <p>Ladybird book explanation please?</p>