title: Making a customizable LCG that travels backward and forward
body: <p>How would i go about making an LCG (type of pseudo random number generator) travel in both directions?
  I know that travelling forward is <code>(a*x+c)%m</code> but how would i be able to reverse it? 
  I am using this so i can store the seed at the position of the player in a map and be able to generate things around it by propogating backward and forward in the LCG (like some sort of randomized number line).</p>