student_id and initial_seed are exam-level variables

The variables `student_id` and `initial_seed` are set in the exam scope
and available to all questions. They can be used to coordinate the
randomisation across questions.

The initial seed is either a value from the built-in random number
generator, or loaded from the SCORM element 'numbas.initial_seed'.
The LTI provider can use this to force attempts to use a particular
seed, so they match a printed copy, for example.

fixes #805

runtime/scripts/exam.js

@@ -636,11 +636,15 @@ Exam.prototype = /** @lends Numbas.Exam.prototype */ {
     init: function()
     {
         var exam = this;
+        if(exam.store) {
+            job(exam.store.init,exam.store,exam);        //initialise storage
+            job(exam.set_exam_variables, exam);
+        }
         job(exam.chooseQuestionSubset,exam);            //choose questions to use
         job(exam.makeQuestionList,exam);                //create question objects
         exam.signals.on('question list initialised', function() {
             if(exam.store) {
-                job(exam.store.init,exam.store,exam);        //initialise storage
+                job(exam.store.init_questions,exam.store,exam); //initialise question storage
                 job(exam.store.save,exam.store);            //make sure data get saved to LMS
             }
         });
@@ -671,9 +675,10 @@ Exam.prototype = /** @lends Numbas.Exam.prototype */ {
         }
         this.loading = true;
         var suspendData = this.store.load(this);    //get saved info from storage
+        exam.seed = suspendData.randomSeed || exam.seed;
+        job(exam.set_exam_variables, exam);
         job(function() {
             var e = this;
-            e.seed = suspendData.randomSeed || e.seed;
             var numQuestions = 0;
             if(suspendData.questionGroupOrder) {
                 this.questionGroupOrder = suspendData.questionGroupOrder.slice();
@@ -714,6 +719,15 @@ Exam.prototype = /** @lends Numbas.Exam.prototype */ {
             exam.signals.trigger('ready');
         });
     },
+
+    /** Set exam-level variables.
+     *
+     */
+    set_exam_variables: function() {
+        this.scope.setVariable('initial_seed', Numbas.jme.wrapValue(this.seed));
+        this.scope.setVariable('student_id', Numbas.jme.wrapValue(this.student_id));
+    },
+
     /** Decide which questions to use and in what order.
      *
      * @fires Numbas.Exam#chooseQuestionSubset