CASText now checks before creating new names for variables, and when …

…creating questions only some of the variables are really stored in the session.

doc/en/Students/Answer_input.md

@@ -78,6 +78,12 @@ If you type `a+b/c+d`, then STACK will think that you mean
 
 \[a+\frac{b}{c}+d.\]
 
+Think carefully about the expression `a/b/c`.  What do you think this means?  There are two options
+
+\[\frac{a}{b}\cdot\frac{1}{c} = \frac{a}{bc}\quad\mbox{or}\quad\frac{a}{\frac{b}{c}}=\frac{ac}{b}.\]
+
+Maxima interprets this as $\frac{a}{bc}$.  If in doubt use brackets.
+
 Note that in this context you should always use ordinary round bracket (like (a+b)), not square or curly ones (like [a+b] or {a+b}).
 
 * `{a+b}` means a set,

question.php

@@ -195,42 +195,46 @@ public function start_attempt(question_attempt_step $step, $variant) {
             $response[$name] = $cs;
         }
         $session->add_vars($response);
-
-        // Now instantiate the session:
-        $session->instantiate();
-        $this->session = clone $session;
-        if ($session->get_errors()) {
-            throw new Exception('qtype_stack_question : CAS error when instantiating the session: ' .
-            $session->get_errors());
-        }
+        $session_length = count($session->get_session());
 
         // 3. CAS bits inside the question text.
-        $questiontext = new stack_cas_text($this->questiontext, clone $session, $this->seed, 't', false, true);
+        $questiontext = new stack_cas_text($this->questiontext, $session, $this->seed, 't', false, true);
         if ($questiontext->get_errors()) {
             throw new Exception('qtype_stack_question : Error in the the question text: ' .
                     $questiontext->get_errors());
         }
 
         // 4. CAS bits inside the specific feedback.
-        $feedbacktext = new stack_cas_text($this->specificfeedback, clone $session, $this->seed, 't', false, true);
+        $feedbacktext = new stack_cas_text($this->specificfeedback, $session, $this->seed, 't', false, true);
         if ($questiontext->get_errors()) {
             throw new Exception('qtype_stack_question : Error in the feedback text: ' .
                     $feedbacktext->get_errors());
         }
 
         // 5. CAS bits inside the question note.
-        $notetext = new stack_cas_text($this->questionnote, clone $session, $this->seed, 't', false, true);
+        $notetext = new stack_cas_text($this->questionnote,  $session, $this->seed, 't', false, true);
         if ($questiontext->get_errors()) {
             throw new Exception('qtype_stack_question : Error in the question note: ' .
                     $notetext->get_errors());
         }
 
+        // Now instantiate the session:
+        $session->instantiate();
+        if ($session->get_errors()) {
+            throw new Exception('qtype_stack_question : CAS error when instantiating the session: ' .
+            $session->get_errors());
+        }
+
         // Now store the values that depend on the instantiated session.
         $step->set_qt_var('_questionvars', $session->get_keyval_representation());
         $step->set_qt_var('_questiontext', $questiontext->get_display_castext());
         $step->set_qt_var('_feedback', $feedbacktext->get_display_castext());
         $this->questionnoteinstantiated = $notetext->get_display_castext();
         $step->set_qt_var('_questionnote', $this->questionnoteinstantiated);
+
+        // Finally, store only those values really needed for later.
+        $session->prune_session($session_length);
+        $this->session = $session;
     }
 
     public function apply_attempt_state(question_attempt_step $step) {

stack/cas/cassession.class.php

@@ -375,6 +375,14 @@ public function get_session() {
         return $this->session;
     }
 
+    public function prune_session($len) {
+        if (!is_int($len)) {
+            throw new Exception('stack_cas_session: prune_session $len must be an integer.');
+        }
+        $new_session = array_slice($this->session, 0, $len);
+        $this->session = $new_session;
+    }
+
     public function get_all_keys() {
         if (null===$this->valid) {
             $this->validate();

stack/cas/castext.class.php

@@ -209,15 +209,22 @@ private function extract_cas_commands() {
             $cmdarray = array();
             $labels   = array();
 
+            $session_keys = array();
+            if (is_a($this->session, 'stack_cas_session')) {
+                $session_keys = $this->session->get_all_keys();
+            }
             foreach ($temp as $cmd) {
                 // Trim of surrounding white space and CAS commands.
                 $cmd = stack_utils::trim_commands($cmd);
 
                 $cs = new stack_cas_casstring($cmd);
                 $cs->validate($this->security, $this->insertstars, $this->syntax);
 
-                $key = 'caschat'.$i;
-                $i++;
+                do { // ... make sure names are not already in use.
+                    $key = 'caschat'.$i;
+                    $i++;
+                } while (in_array($key, $session_keys));
+                $sesion_keys[] = $key;
                 $labels[] = $key;
                 $cs->set_key($key, true);
                 $cmdarray[] = $cs;

stack/cas/simpletest/test.cassession.class.php

@@ -79,6 +79,9 @@ public function test_keyval_representation_1() {
         $at1 = new stack_cas_session($s1, null, 0);
         $this->assertEqual('a=x^2; b=1/(1+x^2); c=e^(i*pi);', $at1->get_keyval_representation());
         $this->assertEqual(array('a', 'b', 'c'), $at1->get_all_keys());
+
+        $at1->prune_session(1);
+        $this->assertEqual(array('a'), $at1->get_all_keys());
     }
 
     public function test_keyval_representation_2() {

stack/cas/simpletest/test.castext.class.php

@@ -117,6 +117,23 @@ public function check_external_forbidden_words($ct, $val, $words) {
 
     }
 
+    public function test_auto_generated_key_names() {
+
+        $a2=array('a:x^2', 'caschat0:x^3');
+        $s2=array();
+        foreach ($a2 as $s) {
+            $cs = new stack_cas_casstring($s);
+            $cs->validate('t');
+            $s2[] = $cs;
+        }
+        $cs2 = new stack_cas_session($s2, null, 0);
+
+        $at1 = new stack_cas_text("This is some text @x^2@, @x^3@", $cs2, 0);
+        $at1->get_display_castext();
+        $session = $at1->get_session();
+        $this->assertEqual(array('a', 'caschat0', 'caschat1', 'caschat2'), $session->get_all_keys());
+    }
+
     public function testcheck_external_forbidden_words() {
         $cases =  array(
             array('', false, array()),