title: unique integer/long hash key generation over strings for faster compairson
body: <p>I'm curious how others have solved this problem, and what problems might lurk behind the naive solution:</p>
  
  <p>I have a system which processes stock market data.  There are tens of thousands of symbols, with associated prices/sizes, flowing into the system at the rate of several thousand every millisecond.</p>
  
  <p>One of the basic operations that needs to happen on every tick is string comparison to see if the incoming matches the symbol we are interested in.  At such high frequency, optimization of these string comparisons can make a measurable difference in the performance of the whole system.</p>
  
  <p>I am thinking of generating a hash of the symbol string, and storing it with the record.  For subsequent comparison, the system should use this hash (being an int or a long, the comparison should be a single operation, rather than iterating through each character of the string until a mismatch is found).</p>
  
  <p>Let's ignore the cost of generating the hash itself (which, in reality, may actually be prohibitive).  The only problem I can see is that with a large number of unique symbols, a hash collision (two separate symbols generate the same hash) would be devastating.  Is there a hashing algorithm which guarantees that strings which match certain constraints (such as limit on the number of characters) are unique?</p>
  
  <p>EDIT: I'll write this code in Java.  Not sure of the (collision) quality of hashCode or the speed with which it is calculated.</p>