MDL-32188 question CBM: minimal handling of certainty -1

Certainty -1 has never been used in standard Moodle, but is
used in Tony-Gardiner Medwin's patches to mean 'No idea' which
we intend to implement: MDL-42077. In the mean time, these changes
avoid errors for people who have used TGM's patches.

question/behaviour/behaviourbase.php

@@ -684,6 +684,13 @@ public static function default_certainty() {
      * @return number the adjusted fraction taking the certainty into account.
      */
     public static function adjust_fraction($fraction, $certainty) {
+        if ($certainty == -1) {
+            // Certainty -1 has never been used in standard Moodle, but is
+            // used in Tony-Gardiner Medwin's patches to mean 'No idea' which
+            // we intend to implement: MDL-42077. In the mean time, avoid
+            // errors for people who have used TGM's patches.
+            return 0;
+        }
         if ($fraction <= 0.00000005) {
             return self::$wrongscore[$certainty];
         } else {

question/behaviour/deferredcbm/behaviourtype.php

@@ -85,7 +85,11 @@ public function summarise_usage(question_usage_by_activity $quba, question_displ
             }
 
             $certainty = $qa->get_last_behaviour_var('certainty');
-            if (is_null($certainty)) {
+            if (is_null($certainty) || $certainty == -1) {
+                // Certainty -1 has never been used in standard Moodle, but is
+                // used in Tony-Gardiner Medwin's patches to mean 'No idea' which
+                // we intend to implement: MDL-42077. In the mean time, avoid
+                // errors for people who have used TGM's patches.
                 $certainty = question_cbm::default_certainty();
             }
 

question/behaviour/deferredcbm/lang/en/qbehaviour_deferredcbm.php

@@ -43,9 +43,11 @@
 Best marks are gained by acknowledging uncertainty. For example, if you think there is more than a 1 in 3 chance of being wrong, you should enter C=1 and avoid the risk of a negative mark.
 ';
 $string['certainty_link'] = 'qbehaviour/deferredcbm/certainty';
+$string['certainty-1'] = 'No Idea';
 $string['certainty1'] = 'C=1 (Unsure: <67%)';
 $string['certainty2'] = 'C=2 (Mid: >67%)';
 $string['certainty3'] = 'C=3 (Quite sure: >80%)';
+$string['certaintyshort-1'] = 'No Idea';
 $string['certaintyshort1'] = 'C=1';
 $string['certaintyshort2'] = 'C=2';
 $string['certaintyshort3'] = 'C=3';