From a980739fd47126ebf0502f3aef935c216fcd938d Mon Sep 17 00:00:00 2001
From: Tim Hunt <T.J.Hunt@open.ac.uk>
Date: Wed, 17 Aug 2011 12:33:00 +0100
Subject: [PATCH] MDL-28942 qtype multichoice, multi-response. Avoid computing
 the wrong random guess score.

It is effectively impossible to compute the right random guess score, so we should not compute anything, rather than computing a number that we know is wrong.

This patch also fixes a multianswer unit test error that was shown up by this fix.
---
 question/type/multianswer/simpletest/testquestiontype.php | 4 ++--
 question/type/multichoice/questiontype.php                | 6 ++++++
 question/type/multichoice/simpletest/testquestiontype.php | 6 ++++++
 3 files changed, 14 insertions(+), 2 deletions(-)

diff --git a/question/type/multianswer/simpletest/testquestiontype.php b/question/type/multianswer/simpletest/testquestiontype.php
index add7a165507..b8f85819772 100644
--- a/question/type/multianswer/simpletest/testquestiontype.php
+++ b/question/type/multianswer/simpletest/testquestiontype.php
@@ -77,7 +77,7 @@ class qtype_multianswer_test extends UnitTestCase {
         $sadata->id = 1;
         $sadata->qtype = 'shortanswer';
         $sadata->defaultmark = 1;
-        $sadata->options->single = true;
+        $sadata->options->usecase = true;
         $sadata->options->answers[1] = (object) array('answer' => 'Bow-wow', 'fraction' => 0);
         $sadata->options->answers[2] = (object) array('answer' => 'Wiggly worm', 'fraction' => 0);
         $sadata->options->answers[3] = (object) array('answer' => 'Pussy-cat', 'fraction' => 1);
@@ -86,7 +86,7 @@ class qtype_multianswer_test extends UnitTestCase {
         $mcdata->id = 1;
         $mcdata->qtype = 'multichoice';
         $mcdata->defaultmark = 1;
-        $mcdata->options->usecase = true;
+        $mcdata->options->single = true;
         $mcdata->options->answers[1] = (object) array('answer' => 'Dog', 'fraction' => 0);
         $mcdata->options->answers[2] = (object) array('answer' => 'Owl', 'fraction' => 1);
         $mcdata->options->answers[3] = (object) array('answer' => '*', 'fraction' => 0);
diff --git a/question/type/multichoice/questiontype.php b/question/type/multichoice/questiontype.php
index 240bc4c0384..e155366f157 100644
--- a/question/type/multichoice/questiontype.php
+++ b/question/type/multichoice/questiontype.php
@@ -182,6 +182,12 @@ class qtype_multichoice extends question_type {
     }
 
     public function get_random_guess_score($questiondata) {
+        if (!$questiondata->options->single) {
+            // Pretty much impossible to compute for _multi questions. Don't try.
+            return null;
+        }
+
+        // Single choice questions - average choice fraction.
         $totalfraction = 0;
         foreach ($questiondata->options->answers as $answer) {
             $totalfraction += $answer->fraction;
diff --git a/question/type/multichoice/simpletest/testquestiontype.php b/question/type/multichoice/simpletest/testquestiontype.php
index 1afa1249fa0..8123972fbb6 100644
--- a/question/type/multichoice/simpletest/testquestiontype.php
+++ b/question/type/multichoice/simpletest/testquestiontype.php
@@ -71,6 +71,12 @@ class qtype_multichoice_test extends UnitTestCase {
         $this->assertEqual(0.5, $this->qtype->get_random_guess_score($q));
     }
 
+    public function test_get_random_guess_score_multi() {
+        $q = $this->get_test_question_data();
+        $q->options->single = false;
+        $this->assertNull($this->qtype->get_random_guess_score($q));
+    }
+
     public function test_get_possible_responses_single() {
         $q = $this->get_test_question_data();
         $responses = $this->qtype->get_possible_responses($q);