Modified the array of possible jump lengths for moving through questi…

…ons so that all of them are coprime with the number of question bins. This guarantees that users will not repeat a question until they have seen 'bins' questions. Reference Ben Alpert

Reviewers: alpert

CC: eater, jace

Differential Revision: http://phabricator.khanacademy.org/D548

khan-exercise.js

@@ -75,8 +75,9 @@ var Khan = (function() {
         });
     }
 
-    // Prime numbers used for jumping through exercises
-    var primes = [2, 3, 5, 7, 11, 13, 17, 19, 23, 29, 31, 37, 41, 43,
+    // Numbers which are coprime to the number of bins, used for jumping through
+    // exercises
+    var primes = [3, 7, 11, 13, 17, 19, 23, 29, 31, 37, 41, 43,
     47, 53, 59, 61, 67, 71, 73, 79, 83],
 
     /*
@@ -163,7 +164,9 @@ var Khan = (function() {
     exerciseName = deslugify(exerciseId),
 
     // Bin users into a certain number of realms so that
-    // there is some level of reproducability in their questions
+    // there is some level of reproducability in their questions.
+    // If you change this, make sure all entries in the array "primes" 
+    // set above are coprime to the new value.
     bins = 200,
 
     // Number of past problems to consider when avoiding duplicates