title: clojure: O(1) time to get size of vector
body: <h2>Context</h2>
  
  <pre><code>(doc count)
  -------------------------
  clojure.core/count
  ([coll])
    Returns the number of items in the collection. (count nil) returns
    0.  Also works on strings, arrays, and Java Collections and Maps
  </code></pre>
  
  <h2>Question</h2>
  
  <p>Is count guaranteed to be constant time on vector?</p>
  
  <p>If not, is there anyway to get the size of a vector in constant time?</p>
  
  <h2>More Context</h2>
  
  <p>I actually want to get the list element of a vector, which I can do in O(1) with nth, assuming I know the vector size.</p>
  
  <h2>EDIT:</h2>
  
  <p>I forgot to mention this. I need this to work on a <em>transient</em> vector.</p>