<?php
// This file is part of Moodle - http://moodle.org/
//
// Moodle is free software: you can redistribute it and/or modify
// it under the terms of the GNU General Public License as published by
// the Free Software Foundation, either version 3 of the License, or
// (at your option) any later version.
//
// Moodle is distributed in the hope that it will be useful,
// but WITHOUT ANY WARRANTY; without even the implied warranty of
// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
// GNU General Public License for more details.
//
// You should have received a copy of the GNU General Public License
// along with Moodle.  If not, see <http://www.gnu.org/licenses/>.

/**
 * Defines the question behaviour base class
 *
 * @package    moodlecore
 * @subpackage questionbehaviours
 * @copyright  2009 The Open University
 * @license    http://www.gnu.org/copyleft/gpl.html GNU GPL v3 or later
 */


defined('MOODLE_INTERNAL') || die();


/**
 * The base class for question behaviours.
 *
 * A question behaviour is used by the question engine, specifically by
 * a {@link question_attempt} to manage the flow of actions a student can take
 * as they work through a question, and later, as a teacher manually grades it.
 * In turn, the behaviour will delegate certain processing to the
 * relevant {@link question_definition}.
 *
 * @copyright  2009 The Open University
 * @license    http://www.gnu.org/copyleft/gpl.html GNU GPL v3 or later
 */
abstract class question_behaviour {

    /** @var question_attempt the question attempt we are managing. */
    protected $qa;

    /** @var question_definition shortcut to $qa->get_question(). */
    protected $question;

    /**
     * Normally you should not call this constuctor directly. The appropriate
     * behaviour object is created automatically as part of
     * {@link question_attempt::start()}.
     * @param question_attempt $qa the question attempt we will be managing.
     * @param string $preferredbehaviour the type of behaviour that was actually
     *      requested. This information is not needed in most cases, the type of
     *      subclass is enough, but occasionally it is needed.
     */
    public function __construct(question_attempt $qa, $preferredbehaviour) {
        $this->qa = $qa;
        $this->question = $qa->get_question();
        if (!$this->is_compatible_question($this->question)) {
            throw new coding_exception('This behaviour (' . $this->get_name() .
                    ') cannot work with this question (' . get_class($this->question) . ')');
        }
    }

    /**
     * Some behaviours can only work with certing types of question. This method
     * allows the behaviour to verify that a question is compatible.
     *
     * This implementation is only provided for backwards-compatibility. You should
     * override this method if you are implementing a behaviour.
     *
     * @param question_definition $question the question.
     */
    public abstract function is_compatible_question(question_definition $question);

    /**
     * @return string the name of this behaviour. For example the name of
     * qbehaviour_mymodle is 'mymodel'.
     */
    public function get_name() {
        return substr(get_class($this), 11);
    }

    /**
     * Whether the current attempt at this question could be completed just by the
     * student interacting with the question, before $qa->finish() is called.
     *
     * @return boolean whether the attempt can finish naturally.
     */
    public function can_finish_during_attempt() {
        return false;
    }

    /**
     * Cause the question to be renderered. This gets the appropriate behaviour
     * renderer using {@link get_renderer()}, and adjusts the display
     * options using {@link adjust_display_options()} and then calls
     * {@link core_question_renderer::question()} to do the work.
     * @param question_display_options $options controls what should and should not be displayed.
     * @param string|null $number the question number to display.
     * @param core_question_renderer $qoutput the question renderer that will coordinate everything.
     * @param qtype_renderer $qtoutput the question type renderer that will be helping.
     * @return string HTML fragment.
     */
    public function render(question_display_options $options, $number,
            core_question_renderer $qoutput, qtype_renderer $qtoutput) {
        $behaviouroutput = $this->get_renderer($qoutput->get_page());
        $options = clone($options);
        $this->adjust_display_options($options);
        return $qoutput->question($this->qa, $behaviouroutput, $qtoutput, $options, $number);
    }

    /**
     * Checks whether the users is allow to be served a particular file.
     * @param question_display_options $options the options that control display of the question.
     * @param string $component the name of the component we are serving files for.
     * @param string $filearea the name of the file area.
     * @param array $args the remaining bits of the file path.
     * @param bool $forcedownload whether the user must be forced to download the file.
     * @return bool true if the user can access this file.
     */
    public function check_file_access($options, $component, $filearea, $args, $forcedownload) {
        $this->adjust_display_options($options);

        if ($component == 'question' && $filearea == 'response_bf_comment') {
            foreach ($this->qa->get_step_iterator() as $attemptstep) {
                if ($attemptstep->get_id() == $args[0]) {
                    return true;
                }
            }

            return false;
        }

        return $this->question->check_file_access($this->qa, $options, $component,
                $filearea, $args, $forcedownload);
    }

    /**
     * @param moodle_page $page the page to render for.
     * @return qbehaviour_renderer get the appropriate renderer to use for this model.
     */
    public function get_renderer(moodle_page $page) {
        return $page->get_renderer(get_class($this));
    }

    /**
     * Make any changes to the display options before a question is rendered, so
     * that it can be displayed in a way that is appropriate for the statue it is
     * currently in. For example, by default, if the question is finished, we
     * ensure that it is only ever displayed read-only.
     * @param question_display_options $options the options to adjust. Just change
     * the properties of this object - objects are passed by referece.
     */
    public function adjust_display_options(question_display_options $options) {
        if (!$this->qa->has_marks()) {
            $options->correctness = false;
            $options->numpartscorrect = false;
        }
        if ($this->qa->get_state()->is_finished()) {
            $options->readonly = true;
            $options->numpartscorrect = $options->numpartscorrect &&
                    $this->qa->get_state()->is_partially_correct() &&
                    !empty($this->question->shownumcorrect);
        } else {
            $options->hide_all_feedback();
        }
    }

    /**
     * Get the most applicable hint for the question in its current state.
     * @return question_hint the most applicable hint, or null, if none.
     */
    public function get_applicable_hint() {
        return null;
    }

    /**
     * What is the minimum fraction that can be scored for this question.
     * Normally this will be based on $this->question->get_min_fraction(),
     * but may be modified in some way by the behaviour.
     *
     * @return number the minimum fraction when this question is attempted under
     * this behaviour.
     */
    public function get_min_fraction() {
        return 0;
    }

    /**
     * Return the maximum possible fraction that can be scored for this question.
     * Normally this will be based on $this->question->get_max_fraction(),
     * but may be modified in some way by the behaviour.
     *
     * @return number the maximum fraction when this question is attempted under
     * this behaviour.
     */
    public function get_max_fraction() {
        return $this->question->get_max_fraction();
    }

    /**
     * Return an array of the behaviour variables that could be submitted
     * as part of a question of this type, with their types, so they can be
     * properly cleaned.
     * @return array variable name => PARAM_... constant.
     */
    public function get_expected_data() {
        if (!$this->qa->get_state()->is_finished()) {
            return array();
        }

        $vars = array('comment' => question_attempt::PARAM_RAW_FILES, 'commentformat' => PARAM_INT);
        if ($this->qa->get_max_mark()) {
            $vars['mark'] = PARAM_RAW_TRIMMED;
            $vars['maxmark'] = PARAM_FLOAT;
        }
        return $vars;
    }

    /**
     * Return an array of question type variables for the question in its current
     * state. Normally, if {@link adjust_display_options()} would set
     * {@link question_display_options::$readonly} to true, then this method
     * should return an empty array, otherwise it should return
     * $this->question->get_expected_data(). Thus, there should be little need to
     * override this method.
     * @return array|string variable name => PARAM_... constant, or, as a special case
     *      that should only be used in unavoidable, the constant question_attempt::USE_RAW_DATA
     *      meaning take all the raw submitted data belonging to this question.
     */
    public function get_expected_qt_data() {
        $fakeoptions = new question_display_options();
        $fakeoptions->readonly = false;
        $this->adjust_display_options($fakeoptions);
        if ($fakeoptions->readonly) {
            return array();
        } else {
            return $this->question->get_expected_data();
        }
    }

    /**
     * Return an array of any im variables, and the value required to get full
     * marks.
     * @return array variable name => value.
     */
    public function get_correct_response() {
        return array();
    }

    /**
     * Generate a brief, plain-text, summary of this question. This is used by
     * various reports. This should show the particular variant of the question
     * as presented to students. For example, the calculated quetsion type would
     * fill in the particular numbers that were presented to the student.
     * This method will return null if such a summary is not possible, or
     * inappropriate.
     *
     * Normally, this method delegates to {question_definition::get_question_summary()}.
     *
     * @return string|null a plain text summary of this question.
     */
    public function get_question_summary() {
        return $this->question->get_question_summary();
    }

    /**
     * Generate a brief, plain-text, summary of the correct answer to this question.
     * This is used by various reports, and can also be useful when testing.
     * This method will return null if such a summary is not possible, or
     * inappropriate.
     *
     * @return string|null a plain text summary of the right answer to this question.
     */
    public function get_right_answer_summary() {
        return null;
    }

    /**
     * Used by {@link start_based_on()} to get the data needed to start a new
     * attempt from the point this attempt has go to.
     * @return array name => value pairs.
     */
    public function get_resume_data() {
        $olddata = $this->qa->get_step(0)->get_all_data();
        $olddata = $this->qa->get_last_qt_data() + $olddata;
        $olddata = $this->get_our_resume_data() + $olddata;
        return $olddata;
    }

    /**
     * Used by {@link start_based_on()} to get the data needed to start a new
     * attempt from the point this attempt has go to.
     * @return unknown_type
     */
    protected function get_our_resume_data() {
        return array();
    }

    /**
     * Classify responses for this question into a number of sub parts and response classes as defined by
     * {@link \question_type::get_possible_responses} for this question type.
     *
     * @param string $whichtries         which tries to analyse for response analysis. Will be one of
     *                                   question_attempt::FIRST_TRY, LAST_TRY or ALL_TRIES.
     *                                   Defaults to question_attempt::LAST_TRY.
     * @return (question_classified_response|array)[] If $whichtries is question_attempt::FIRST_TRY or LAST_TRY index is subpartid
     *                                   and values are question_classified_response instances.
     *                                   If $whichtries is question_attempt::ALL_TRIES then first key is submitted response no
     *                                   and the second key is subpartid.
     */
    public function classify_response($whichtries = question_attempt::LAST_TRY) {
        if ($whichtries == question_attempt::LAST_TRY) {
            return $this->question->classify_response($this->qa->get_last_qt_data());
        } else {
            $stepswithsubmit = $this->qa->get_steps_with_submitted_response_iterator();
            if ($whichtries == question_attempt::FIRST_TRY) {
                $firsttry = $stepswithsubmit[1];
                if ($firsttry) {
                    return $this->question->classify_response($firsttry->get_qt_data());
                } else {
                    return $this->question->classify_response(array());
                }
            } else {
                $classifiedresponses = array();
                foreach ($stepswithsubmit as $submittedresponseno => $step) {
                    $classifiedresponses[$submittedresponseno] = $this->question->classify_response($step->get_qt_data());
                }
                return $classifiedresponses;
            }
        }
    }

    /**
     * Generate a brief textual description of the current state of the question,
     * normally displayed under the question number.
     *
     * @param bool $showcorrectness Whether right/partial/wrong states should
     * be distinguised.
     * @return string a brief summary of the current state of the qestion attempt.
     */
    public function get_state_string($showcorrectness) {
        return $this->qa->get_state()->default_string($showcorrectness);
    }

    public abstract function summarise_action(question_attempt_step $step);

    /**
     * Initialise the first step in a question attempt when a new
     * {@link question_attempt} is being started.
     *
     * This method must call $this->question->start_attempt($step, $variant), and may
     * perform additional processing if the behaviour requries it.
     *
     * @param question_attempt_step $step the first step of the
     *      question_attempt being started.
     * @param int $variant which variant of the question to use.
     */
    public function init_first_step(question_attempt_step $step, $variant) {
        $this->question->start_attempt($step, $variant);
        $step->set_state(question_state::$todo);
    }

    /**
     * When an attempt is started based on a previous attempt (see
     * {@link question_attempt::start_based_on}) this method is called to setup
     * the new attempt.
     *
     * This method must call $this->question->apply_attempt_state($step), and may
     * perform additional processing if the behaviour requries it.
     *
     * @param question_attempt_step The first step of the {@link question_attempt}
     *      being loaded.
     */
    public function apply_attempt_state(question_attempt_step $step) {
        $this->question->apply_attempt_state($step);
        $step->set_state(question_state::$todo);
    }

    /**
     * Checks whether two manual grading actions are the same. That is, whether
     * the comment, and the mark (if given) is the same.
     *
     * @param question_attempt_step $pendingstep contains the new responses.
     * @return bool whether the new response is the same as we already have.
     */
    protected function is_same_comment($pendingstep) {
        $previouscomment = $this->qa->get_last_behaviour_var('comment');
        $newcomment = $pendingstep->get_behaviour_var('comment');

        // When the teacher leaves the comment empty, $previouscomment is an empty string but $newcomment is null,
        // therefore they are not equal to each other. That's why checking if $previouscomment != $newcomment is not enough.
        if (($previouscomment != $newcomment) && !(is_null($previouscomment) && html_is_blank($newcomment))) {
            // The comment has changed.
            return false;
        }

        if (!html_is_blank($newcomment)) {
            // Check comment format.
            $previouscommentformat = $this->qa->get_last_behaviour_var('commentformat');
            $newcommentformat = $pendingstep->get_behaviour_var('commentformat');
            if ($previouscommentformat != $newcommentformat) {
                return false;
            }
        }

        // So, now we know the comment is the same, so check the mark, if present.
        $previousfraction = $this->qa->get_fraction();
        $newmark = question_utils::clean_param_mark($pendingstep->get_behaviour_var('mark'));

        if (is_null($previousfraction)) {
            return is_null($newmark) || $newmark === '';
        } else if (is_null($newmark) || $newmark === '') {
            return false;
        }

        $newfraction = $newmark / $pendingstep->get_behaviour_var('maxmark');

        return abs($newfraction - $previousfraction) < 0.0000001;
    }

    /**
     * The main entry point for processing an action.
     *
     * All the various operations that can be performed on a
     * {@link question_attempt} get channeled through this function, except for
     * {@link question_attempt::start()} which goes to {@link init_first_step()}.
     * {@link question_attempt::finish()} becomes an action with im vars
     * finish => 1, and manual comment/grade becomes an action with im vars
     * comment => comment text, and mark => ..., max_mark => ... if the question
     * is graded.
     *
     * This method should first determine whether the action is significant. For
     * example, if no actual action is being performed, but instead the current
     * responses are being saved, and there has been no change since the last
     * set of responses that were saved, this the action is not significatn. In
     * this case, this method should return {@link question_attempt::DISCARD}.
     * Otherwise it should return {@link question_attempt::KEEP}.
     *
     * If the action is significant, this method should also perform any
     * necessary updates to $pendingstep. For example, it should call
     * {@link question_attempt_step::set_state()} to set the state that results
     * from this action, and if this is a grading action, it should call
     * {@link question_attempt_step::set_fraction()}.
     *
     * This method can also call {@link question_attempt_step::set_behaviour_var()} to
     * store additional infomation. There are two main uses for this. This can
     * be used to store the result of any randomisation done. It is important to
     * store the result of randomisation once, and then in future use the same
     * outcome if the actions are ever replayed. This is how regrading works.
     * The other use is to cache the result of expensive computations performed
     * on the raw response data, so that subsequent display and review of the
     * question does not have to repeat the same expensive computations.
     *
     * Often this method is implemented as a dispatching method that examines
     * the pending step to determine the kind of action being performed, and
     * then calls a more specific method like {@link process_save()} or
     * {@link process_comment()}. Look at some of the standard behaviours
     * for examples.
     *
     * @param question_attempt_pending_step $pendingstep a partially initialised step
     *      containing all the information about the action that is being peformed. This
     *      information can be accessed using {@link question_attempt_step::get_behaviour_var()}.
     * @return bool either {@link question_attempt::KEEP} or {@link question_attempt::DISCARD}
     */
    public abstract function process_action(question_attempt_pending_step $pendingstep);

    /**
     * Auto-saved data. By default this does nothing. interesting processing is
     * done in {@link question_behaviour_with_save}.
     *
     * @param question_attempt_pending_step $pendingstep a partially initialised step
     *      containing all the information about the action that is being peformed. This
     *      information can be accessed using {@link question_attempt_step::get_behaviour_var()}.
     * @return bool either {@link question_attempt::KEEP} or {@link question_attempt::DISCARD}
     */
    public function process_autosave(question_attempt_pending_step $pendingstep) {
        return question_attempt::DISCARD;
    }

    /**
     * Implementation of processing a manual comment/grade action that should
     * be suitable for most subclasses.
     * @param question_attempt_pending_step $pendingstep a partially initialised step
     *      containing all the information about the action that is being peformed.
     * @return bool either {@link question_attempt::KEEP}
     */
    public function process_comment(question_attempt_pending_step $pendingstep) {
        if (!$this->qa->get_state()->is_finished()) {
            throw new coding_exception('Cannot manually grade a question before it is finshed.');
        }

        if ($this->is_same_comment($pendingstep)) {
            return question_attempt::DISCARD;
        }

        if ($pendingstep->has_behaviour_var('mark')) {
            $mark = question_utils::clean_param_mark($pendingstep->get_behaviour_var('mark'));
            if ($mark === null) {
                throw new coding_exception('Inalid number format ' . $pendingstep->get_behaviour_var('mark') .
                        ' when processing a manual grading action.', 'Question ' . $this->question->id .
                        ', slot ' . $this->qa->get_slot());

            } else if ($mark === '') {
                $fraction = null;

            } else {
                $fraction = $mark / $pendingstep->get_behaviour_var('maxmark');
                if ($fraction > $this->qa->get_max_fraction() || $fraction < $this->qa->get_min_fraction()) {
                    throw new coding_exception('Score out of range when processing ' .
                            'a manual grading action.', 'Question ' . $this->question->id .
                            ', slot ' . $this->qa->get_slot() . ', fraction ' . $fraction);
                }
            }

            $pendingstep->set_fraction($fraction);
        }

        $pendingstep->set_state($this->qa->get_state()->corresponding_commented_state(
                $pendingstep->get_fraction()));
        return question_attempt::KEEP;
    }

    /**
     * @param $comment the comment text to format. If omitted,
     *      $this->qa->get_manual_comment() is used.
     * @param $commentformat the format of the comment, one of the FORMAT_... constants.
     * @param $context the quiz context.
     * @return string the comment, ready to be output.
     */
    public function format_comment($comment = null, $commentformat = null, $context = null) {
        $formatoptions = new stdClass();
        $formatoptions->noclean = true;
        $formatoptions->para = false;

        if (is_null($comment)) {
            list($comment, $commentformat, $commentstep) = $this->qa->get_manual_comment();
        }

        if ($context !== null) {
            $comment = $this->qa->rewrite_response_pluginfile_urls($comment, $context->id, 'bf_comment', $commentstep);
        }

        return format_text($comment, $commentformat, $formatoptions);
    }

    /**
     * @return string a summary of a manual comment action.
     * @param question_attempt_step $step
     */
    protected function summarise_manual_comment($step) {
        $a = new stdClass();
        if ($step->has_behaviour_var('comment')) {
            $comment = question_utils::to_plain_text($step->get_behaviour_var('comment'),
                    $step->get_behaviour_var('commentformat'));
            $a->comment = shorten_text($comment, 200);
        } else {
            $a->comment = '';
        }

        $mark = question_utils::clean_param_mark($step->get_behaviour_var('mark'));
        if (is_null($mark) || $mark === '') {
            return get_string('commented', 'question', $a->comment);
        } else {
            $a->mark = $mark / $step->get_behaviour_var('maxmark') * $this->qa->get_max_mark();
            return get_string('manuallygraded', 'question', $a);
        }
    }

    public function summarise_start($step) {
        return get_string('started', 'question');
    }

    public function summarise_finish($step) {
        return get_string('attemptfinished', 'question');
    }

    /**
     * Does this step include a response submitted by a student?
     *
     * This method should return true for any attempt explicitly submitted by a student. The question engine itself will also
     * automatically recognise any last saved response before the attempt is finished, you don't need to return true here for these
     * steps with responses which are not explicitly submitted by the student.
     *
     * @param question_attempt_step $step
     * @return bool is this a step within a question attempt that includes a submitted response by a student.
     */
    public function step_has_a_submitted_response($step) {
        return false;
    }
}


/**
 * A subclass of {@link question_behaviour} that implements a save
 * action that is suitable for most questions that implement the
 * {@link question_manually_gradable} interface.
 *
 * @copyright  2009 The Open University
 * @license    http://www.gnu.org/copyleft/gpl.html GNU GPL v3 or later
 */
abstract class question_behaviour_with_save extends question_behaviour {
    public function required_question_definition_type() {
        return 'question_manually_gradable';
    }

    public function apply_attempt_state(question_attempt_step $step) {
        parent::apply_attempt_state($step);
        if ($this->question->is_complete_response($step->get_qt_data())) {
            $step->set_state(question_state::$complete);
        }
    }

    /**
     * Work out whether the response in $pendingstep are significantly different
     * from the last set of responses we have stored.
     * @param question_attempt_step $pendingstep contains the new responses.
     * @return bool whether the new response is the same as we already have.
     */
    protected function is_same_response(question_attempt_step $pendingstep) {
        return $this->question->is_same_response(
                $this->qa->get_last_step()->get_qt_data(), $pendingstep->get_qt_data());
    }

    /**
     * Work out whether the response in $pendingstep represent a complete answer
     * to the question. Normally this will call
     * {@link question_manually_gradable::is_complete_response}, but some
     * behaviours, for example the CBM ones, have their own parts to the
     * response.
     * @param question_attempt_step $pendingstep contains the new responses.
     * @return bool whether the new response is complete.
     */
    protected function is_complete_response(question_attempt_step $pendingstep) {
        return $this->question->is_complete_response($pendingstep->get_qt_data());
    }

    public function process_autosave(question_attempt_pending_step $pendingstep) {
        // If already finished. Nothing to do.
        if ($this->qa->get_state()->is_finished()) {
            return question_attempt::DISCARD;
        }

        // If the new data is the same as we already have, then we don't need it.
        if ($this->is_same_response($pendingstep)) {
            return question_attempt::DISCARD;
        }

        // Repeat that test discarding any existing autosaved data.
        if ($this->qa->has_autosaved_step()) {
            $this->qa->discard_autosaved_step();
            if ($this->is_same_response($pendingstep)) {
                return question_attempt::DISCARD;
            }
        }

        // OK, we need to save.
        return $this->process_save($pendingstep);
    }

    /**
     * Implementation of processing a save action that should be suitable for
     * most subclasses.
     * @param question_attempt_pending_step $pendingstep a partially initialised step
     *      containing all the information about the action that is being peformed.
     * @return bool either {@link question_attempt::KEEP} or {@link question_attempt::DISCARD}
     */
    public function process_save(question_attempt_pending_step $pendingstep) {
        if ($this->qa->get_state()->is_finished()) {
            return question_attempt::DISCARD;
        } else if (!$this->qa->get_state()->is_active()) {
            throw new coding_exception('Question is not active, cannot process_actions.');
        }

        if ($this->is_same_response($pendingstep)) {
            return question_attempt::DISCARD;
        }

        if ($this->is_complete_response($pendingstep)) {
            $pendingstep->set_state(question_state::$complete);
        } else {
            $pendingstep->set_state(question_state::$todo);
        }
        return question_attempt::KEEP;
    }

    public function summarise_submit(question_attempt_step $step) {
        return get_string('submitted', 'question',
                $this->question->summarise_response($step->get_qt_data()));
    }

    public function summarise_save(question_attempt_step $step) {
        $data = $step->get_submitted_data();
        if (empty($data)) {
            return $this->summarise_start($step);
        }
        return get_string('saved', 'question',
                $this->question->summarise_response($step->get_qt_data()));
    }


    public function summarise_finish($step) {
        $data = $step->get_qt_data();
        if ($data) {
            return get_string('attemptfinishedsubmitting', 'question',
                    $this->question->summarise_response($data));
        }
        return get_string('attemptfinished', 'question');
    }
}

abstract class question_behaviour_with_multiple_tries extends question_behaviour_with_save {
    public function step_has_a_submitted_response($step) {
        return $step->has_behaviour_var('submit') && $step->get_state() != question_state::$invalid;
    }
}

/**
 * This helper class contains the constants and methods required for
 * manipulating scores for certainty based marking.
 *
 * @copyright  2009 The Open University
 * @license    http://www.gnu.org/copyleft/gpl.html GNU GPL v3 or later
 */
abstract class question_cbm {
    /**#@+ @var integer named constants for the certainty levels. */
    const LOW = 1;
    const MED = 2;
    const HIGH = 3;
    /**#@-*/

    /** @var array list of all the certainty levels. */
    public static $certainties = array(self::LOW, self::MED, self::HIGH);

    /**#@+ @var array coefficients used to adjust the fraction based on certainty. */
    protected static $rightscore = array(
        self::LOW  => 1,
        self::MED  => 2,
        self::HIGH => 3,
    );
    protected static $wrongscore = array(
        self::LOW  =>  0,
        self::MED  => -2,
        self::HIGH => -6,
    );
    /**#@-*/

    /**#@+ @var array upper and lower limits of the optimal window. */
    protected static $lowlimit = array(
        self::LOW  => 0,
        self::MED  => 0.666666666666667,
        self::HIGH => 0.8,
    );
    protected static $highlimit = array(
        self::LOW  => 0.666666666666667,
        self::MED  => 0.8,
        self::HIGH => 1,
    );
    /**#@-*/

    /**
     * @return int the default certaintly level that should be assuemd if
     * the student does not choose one.
     */
    public static function default_certainty() {
        return self::LOW;
    }

    /**
     * Given a fraction, and a certainty, compute the adjusted fraction.
     * @param number $fraction the raw fraction for this question.
     * @param int $certainty one of the certainty level constants.
     * @return number the adjusted fraction taking the certainty into account.
     */
    public static function adjust_fraction($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[$certainty];
        } else {
            return self::$rightscore[$certainty] * $fraction;
        }
    }

    /**
     * @param int $certainty one of the LOW/MED/HIGH constants.
     * @return string a textual description of this certainty.
     */
    public static function get_string($certainty) {
        return get_string('certainty' . $certainty, 'qbehaviour_deferredcbm');
    }

    /**
     * @param int $certainty one of the LOW/MED/HIGH constants.
     * @return string a short textual description of this certainty.
     */
    public static function get_short_string($certainty) {
        return get_string('certaintyshort' . $certainty, 'qbehaviour_deferredcbm');
    }

    /**
     * Add information about certainty to a response summary.
     * @param string $summary the response summary.
     * @param int $certainty the level of certainty to add.
     * @return string the summary with information about the certainty added.
     */
    public static function summary_with_certainty($summary, $certainty) {
        if (is_null($certainty)) {
            return $summary;
        }
        return $summary . ' [' . self::get_short_string($certainty) . ']';
    }

    /**
     * @param int $certainty one of the LOW/MED/HIGH constants.
     * @return float the lower limit of the optimal probability range for this certainty.
     */
    public static function optimal_probablility_low($certainty) {
        return self::$lowlimit[$certainty];
    }

    /**
     * @param int $certainty one of the LOW/MED/HIGH constants.
     * @return float the upper limit of the optimal probability range for this certainty.
     */
    public static function optimal_probablility_high($certainty) {
        return self::$highlimit[$certainty];
    }
}