title: Cheap algorithm to find measure of angle between vectors
body: <p>Finding the angle between two vectors is not hard <a href="https://stackoverflow.com/questions/3441782/how-to-calculate-the-angle-of-a-vector-from-the-vertical/3441867#3441867">using the cosine rule</a>. However, because I am programming for a platform with very limited resources, I would like to avoid calculations such as <code>sqrt</code> and <code>arccos</code>. Even simple divisions should be limited as much as possible.</p>
  
  <p>Fortunately, I do not need the angle per se, but only need some value that is proportional to said angle.</p>
  
  <p>So I am looking for some computationally cheap algorithm to calculate a quantity that is related to the angle between two vectors. So far, I haven't found something that fits the bill, nor have I been able to come up with something myself.</p>