title: Algorithm for unique find edges from polygon mesh
body: <p>I'm looking for a good algorithm that can give me the unique edges from a set of polygon data. In this case, the polygons are defined by two arrays. One array is the number of points per polygon, and the other array is a list of vertex indices.</p>
  
  <p>I have a version that is working, but performance gets slow when reaching over 500,000 polys. My version walks over each face and adds each edge's sorted vertices to an stl::set. My data set will be primarily triangle and quad polys, and most edges will be shared.</p>
  
  <p>Is there a smarter algorithm for this?</p>