This is an implementation in C by Eugene K. Ressler of the turning function metric for comparing polygonal shapes developed by Arkin, Chew, Huttenlocher, Kedem, and Mitchell. It expands a little on the cited reference to achieve O(n) space and O(mn log n) run time. This source may be freely distributed and used for non-commercial purposes, so long as this comment is attached to any code copied or derived from it.