class.ilCalendarRecurrenceCalculator.php

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<?php
/*
    +-----------------------------------------------------------------------------+
    | ILIAS open source                                                           |
    +-----------------------------------------------------------------------------+
    | Copyright (c) 1998-2006 ILIAS open source, University of Cologne            |
    |                                                                             |
    | This program 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 2              |
    | of the License, or (at your option) any later version.                      |
    |                                                                             |
    | This program 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 this program; if not, write to the Free Software                 |
    | Foundation, Inc., 59 Temple Place - Suite 330, Boston, MA  02111-1307, USA. |
    +-----------------------------------------------------------------------------+
*/
 
include_once './Services/Calendar/classes/class.ilCalendarRecurrence.php';
include_once('./Services/Calendar/classes/class.ilDateList.php');
include_once('./Services/Calendar/classes/class.ilTimeZone.php');
include_once('./Services/Calendar/classes/class.ilCalendarUtil.php');
include_once './Services/Calendar/interfaces/interface.ilCalendarRecurrenceCalculation.php';
 
class ilCalendarRecurrenceCalculator
{
    protected $timezone = null;
    protected $log = null;
    
    protected $limit_reached = false;
    protected $valid_dates = null;
    protected $period_start = null;
    protected $period_end = null;
    protected $start = null;
 
    protected $event = null;
    protected $duration = null;
    protected $recurrence = null;
    
    protected $frequence_context = 0;
 
    public function __construct(ilDatePeriod $entry, ilCalendarRecurrenceCalculation $rec)
    {
        $this->log = $GLOBALS['DIC']->logger()->cal();
        $this->event = $entry;
        $this->recurrence = $rec;
 
        $this->duration = $entry->getEnd()->get(IL_CAL_UNIX) - $entry->getStart()->get(IL_CAL_UNIX);
    }
    
    protected function getDuration()
    {
        return $this->duration;
    }
    
    public function calculateDateListByMonth($a_month, $a_year)
    {
    }
    
    
    public function calculateDateList(ilDateTime $a_start, ilDateTime $a_end, $a_limit = -1)
    {
        #               echo $a_start;
        #               echo $a_end;
        #               echo $this->event->getStart();
        #               echo $this->event->getEnd();
 
 
        $this->valid_dates = $this->initDateList();
 
        // Check invalid settings: e.g no frequence given, invalid start/end dates ...
        if (!$this->validateRecurrence()) {
            $this->valid_dates->add($this->event->getStart());
            return $this->valid_dates;
        }
        
        // Performance fix: Switching the timezone for many dates seems to be
        // quite time consuming.
        // Therfore we adjust the timezone of all input dates (start,end, event start)
        // to the same tz (UTC for fullday events, Recurrence tz for all others).
        $this->adjustTimeZones($a_start, $a_end);
        
        
        
        // Add start of event if it is in the period
        if ((ilDateTime::_after($this->event->getStart(), $this->period_start, IL_CAL_DAY) and
            ilDateTime::_before($this->event->getStart(), $this->period_end, IL_CAL_DAY)) or
            ilDateTime::_equals($this->event->getStart(), $this->period_start, IL_CAL_DAY)) {
            // begin-patch aptar
            $this->valid_dates->add($this->event->getStart());
            #$this->valid_dates->add($this->event->getStart());
            // end patch aptar
        }
 
        // Calculate recurrences based on frequency (e.g. MONTHLY)
        $time = microtime(true);
 
        $start = $this->optimizeStartingTime();
        
        #echo "ZEIT: ADJUST: ".(microtime(true) - $time).'<br>';
        $counter = 0;
        do {
            ++$counter;
            // initialize context for applied rules
            // E.g
            // RRULE:FREQ=YEARLY;BYMONTH=1;BYWEEKNO=1,50        => context for BYWERKNO is monthly because it filters to the weeks in JAN
            // RRULE:FREQ=YEARLY;BYWEEKNO=1,50                          => context for BYWERKNO is yearly because it adds all weeks.
            $this->frequence_context = $this->recurrence->getFrequenceType();
            
            $freq_res = $this->initDateList();
            $freq_res->add($start);
            
            // Fixed sequence of applying rules (RFC 2445 4.3.10)
            $freq_res = $this->applyBYMONTHRules($freq_res);
            #echo "BYMONTH: ".$freq_res;
 
            $freq_res = $this->applyBYWEEKNORules($freq_res);
            #echo "BYWEEKNO: ".$freq_res;
 
            $freq_res = $this->applyBYYEARDAYRules($freq_res);
            #echo "BYYEARDAY: ".$freq_res;
 
 
            $freq_res = $this->applyBYMONTHDAYRules($freq_res);
            #echo "BYMONTHDAY: ".$freq_res;
 
            #$time = microtime(true);
            $freq_res = $this->applyBYDAYRules($freq_res);
            #echo "ZEIT: ".(microtime(true) - $time);
            #echo "BYDAY: ".$freq_res;
 
 
            $freq_res = $this->applyBYSETPOSRules($freq_res);
            #echo "BYSETPOS: ".$freq_res;
            
            $freq_res = $this->applyLimits($freq_res);
            #echo $freq_res;
 
            $start = $this->incrementByFrequency($start);
 
            if (ilDateTime::_after($start, $this->period_end) or $this->limit_reached) {
                break;
            }
        } while (true);
 
        $this->applyExclusionDates();
        
        $this->applyDurationPeriod($this->valid_dates, $this->period_start, $this->period_end);
 
        $this->valid_dates->sort();
            
        // Restore default timezone
        ilTimeZone::_restoreDefaultTimeZone();
        return $this->valid_dates;
    }
    
    protected function applyDurationPeriod(ilDateList $list, ilDateTime $start, ilDateTime $end)
    {
        $list_copy = clone $list;
        foreach ($list_copy as $start_date) {
            $end_date = clone $start_date;
            $end_date->increment(ilDateTime::MINUTE, $this->getDuration() / 60);
            
            if (
                (
                    ilDateTime::_after($start_date, $this->period_end)
                ) ||
                (
                    ilDateTime::_before($end_date, $this->period_start)
                )
            ) {
                $this->log->debug('Removed invalid date ' . (string) $start_date . ' <-> ' . (string) $end_date);
                $list->remove($start_date);
            }
        }
    }
    
    protected function adjustTimeZones(ilDateTime $a_start, ilDateTime $a_end)
    {
        $this->timezone = $this->event->isFullday() ? ilTimeZone::UTC : $this->recurrence->getTimeZone();
        ilTimeZone::_setDefaultTimeZone($this->timezone);
        
        $this->period_start = clone $a_start;
        $this->period_end = clone $a_end;
        $this->start = clone $this->event->getStart();
        
        try {
            if ($this->event->isFullday()) {
                $this->period_start->switchTimeZone(ilTimeZone::UTC);
                $this->period_end->switchTimeZone(ilTimeZone::UTC);
                $this->start->switchTimeZone(ilTimeZone::UTC);
            } else {
                $this->period_start->switchTimeZone($this->recurrence->getTimeZone());
                $this->period_end->switchTimeZone($this->recurrence->getTimeZone());
                $this->start->switchTimeZone($this->recurrence->getTimeZone());
            }
            return true;
        } catch (ilDateTimeException $e) {
            $this->log->write(__METHOD__ . ': ' . $e->getMessage());
            return false;
        }
    }
    
    protected function optimizeStartingTime()
    {
        $time = microtime(true);
        
        // starting time cannot be optimzed if RRULE UNTIL is given.
        // In that case we have to calculate all dates until "UNTIL" is reached.
        if ($this->recurrence->getFrequenceUntilCount() > 0) {
            // Switch the date to the original defined timzone for this recurrence
            return $this->createDate($this->start->get(IL_CAL_UNIX, '', $this->timezone));
        }
        $optimized = $start = $this->createDate($this->start->get(IL_CAL_UNIX, '', $this->timezone));
        while (ilDateTime::_before($start, $this->period_start)) {
            $optimized = clone $start;
            $start = $this->incrementByFrequency($start);
        }
        
        return $optimized;
    }
    
    protected function incrementByFrequency($start)
    {
        global $DIC;
 
        $logger = $DIC->logger()->cal();
 
        switch ($this->recurrence->getFrequenceType()) {
            case ilCalendarRecurrence::FREQ_YEARLY:
                $start->increment(ilDateTime::YEAR, $this->recurrence->getInterval());
                break;
            
            case ilCalendarRecurrence::FREQ_MONTHLY:
                $start->increment(ilDateTime::MONTH, $this->recurrence->getInterval());
                break;
 
            case ilCalendarRecurrence::FREQ_WEEKLY:
                $start->increment(ilDateTime::WEEK, $this->recurrence->getInterval());
                break;
            
            case ilCalendarRecurrence::FREQ_DAILY:
                $start->increment(ilDateTime::DAY, $this->recurrence->getInterval());
                break;
            
            default:
                $logger->warning('No frequence defined.');
                break;
        }
        return $start;
    }
    
    protected function applyBYMONTHRules(ilDateList $list)
    {
        // return unmodified, if no bymonth rules are available
        if (!$this->recurrence->getBYMONTHList()) {
            return $list;
        }
        $month_list = $this->initDateList();
        foreach ($list->get() as $date) {
            #echo "SEED: ".$seed;
            
            foreach ($this->recurrence->getBYMONTHList() as $month) {
                #echo "RULW_MONTH:".$month;
                
                // YEARLY rules extend the seed to every month given in the BYMONTH rule
                // Rules < YEARLY must match the month of the seed
                if ($this->recurrence->getFrequenceType() == ilCalendarRecurrence::FREQ_YEARLY) {
                    $month_date = $this->createDate($date->get(IL_CAL_UNIX, '', $this->timezone));
                    $month_date->increment(ilDateTime::MONTH, -($date->get(IL_CAL_FKT_DATE, 'n', $this->timezone) - $month));
                    
                    #echo "BYMONTH: ".$month_date;
                    $month_list->add($month_date);
                } elseif ($date->get(IL_CAL_FKT_DATE, 'n', $this->timezone) == $month) {
                    $month_list->add($date);
                }
            }
        }
        // decrease the frequence_context for YEARLY rules
        if ($this->recurrence->getFrequenceType() == ilCalendarRecurrence::FREQ_YEARLY) {
            $this->frequence_context = ilCalendarRecurrence::FREQ_MONTHLY;
        }
        return $month_list;
    }
    
    protected function applyBYWEEKNORules(ilDateList $list)
    {
        if ($this->recurrence->getFrequenceType() != ilCalendarRecurrence::FREQ_YEARLY) {
            return $list;
        }
        // return unmodified, if no byweekno rules are available
        if (!$this->recurrence->getBYWEEKNOList()) {
            return $list;
        }
        $weeks_list = $this->initDateList();
        foreach ($list->get() as $seed) {
            $weeks_in_year = date('W', mktime(0, 0, 0, 12, 28, $seed->get(IL_CAL_FKT_DATE, 'Y', $this->timezone)));
            $this->log->write(__METHOD__ . ': Year ' . $seed->get(IL_CAL_FKT_DATE, 'Y', $this->timezone) . ' has ' . $weeks_in_year . ' weeks');
            foreach ($this->recurrence->getBYWEEKNOList() as $week_no) {
                $week_no = $week_no < 0 ? ($weeks_in_year + $week_no + 1) : $week_no;
                
                switch ($this->frequence_context) {
                    case ilCalendarRecurrence::FREQ_MONTHLY:
                        $this->log->write(__METHOD__ . ': Handling BYWEEKNO in MONTHLY context');
                        // Check if week matches
                        if ($seed->get(IL_CAL_FKT_DATE, 'W', $this->timezone) == $week_no) {
                            $weeks_list->add($seed);
                        }
                        break;
                        
                    case ilCalendarRecurrence::FREQ_YEARLY:
                        $this->log->write(__METHOD__ . ': Handling BYWEEKNO in YEARLY context');
                        $week_diff = $week_no - $seed->get(IL_CAL_FKT_DATE, 'W', $this->timezone);
                        
                        // TODO: think about tz here
                        $new_week = $this->createDate($seed->get(IL_CAL_UNIX, '', $this->timezone));
                        $new_week->increment(ilDateTime::WEEK, $week_diff);
                        $weeks_list->add($new_week);
                        break;
                }
            }
        }
        $this->frequence_context = ilCalendarRecurrence::FREQ_WEEKLY;
        
        return $weeks_list;
    }
    
    protected function applyBYYEARDAYRules(ilDateList $list)
    {
        // return unmodified, if no byweekno rules are available
        if (!$this->recurrence->getBYYEARDAYList()) {
            return $list;
        }
        $days_list = $this->initDateList();
        foreach ($list->get() as $seed) {
            $num_days = date('z', mktime(0, 0, 0, 12, 31, $seed->get(IL_CAL_FKT_DATE, 'Y', $this->timezone)));
            $this->log->write(__METHOD__ . ': Year ' . $seed->get(IL_CAL_FKT_DATE, 'Y', $this->timezone) . ' has ' . $num_days . ' days.');
            
            foreach ($this->recurrence->getBYYEARDAYList() as $day_no) {
                $day_no = $day_no < 0 ? ($num_days + $day_no + 1) : $day_no;
                
                $day_diff = $day_no - $seed->get(IL_CAL_FKT_DATE, 'z', $this->timezone);
                $new_day = $this->createDate($seed->get(IL_CAL_UNIX, '', $this->timezone));
                $new_day->increment(ilDateTime::DAY, $day_diff);
 
                switch ($this->frequence_context) {
                    case ilCalendarRecurrence::FREQ_DAILY:
                        // Check if day matches
                        if ($seed->get(IL_CAL_FKT_DATE, 'z', $this->timezone) == $day_no) {
                            $days_list->add($new_day);
                        }
                        break;
                    case ilCalendarRecurrence::FREQ_WEEKLY:
                        // Check if week matches
                        if ($seed->get(IL_CAL_FKT_DATE, 'W', $this->timezone) == $new_day->get(IL_CAL_FKT_DATE, 'W', $this->timezone)) {
                            $days_list->add($new_day);
                        }
                        break;
                    case ilCalendarRecurrence::FREQ_MONTHLY:
                        // Check if month matches
                        if ($seed->get(IL_CAL_FKT_DATE, 'n', $this->timezone) == $new_day->get(IL_CAL_FKT_DATE, 'n', $this->timezone)) {
                            $days_list->add($new_day);
                        }
                        break;
                    case ilCalendarRecurrence::FREQ_YEARLY:
                        // Simply add
                        $days_list->add($new_day);
                        break;
                }
            }
        }
        
        $this->frequence_context = ilCalendarRecurrence::FREQ_DAILY;
        return $days_list;
    }
    
    protected function applyBYMONTHDAYRules(ilDateList $list)
    {
        // return unmodified, if no byweekno rules are available
        if (!$this->recurrence->getBYMONTHDAYList()) {
            return $list;
        }
        $days_list = $this->initDateList();
        foreach ($list->get() as $seed) {
            $num_days = ilCalendarUtil::_getMaxDayOfMonth(
                $seed->get(IL_CAL_FKT_DATE, 'Y', $this->timezone),
                $seed->get(IL_CAL_FKT_DATE, 'n', $this->timezone)
            );
            /*
            $num_days = cal_days_in_month(CAL_GREGORIAN,
                $seed->get(IL_CAL_FKT_DATE,'n',$this->timezone),
                $seed->get(IL_CAL_FKT_DATE,'Y',$this->timezone));
            */
            #$this->log->write(__METHOD__.': Month '.$seed->get(IL_CAL_FKT_DATE,'M',$this->timezone).' has '.$num_days.' days.');
            
            foreach ($this->recurrence->getBYMONTHDAYList() as $bymonth_no) {
                $day_no = $bymonth_no < 0 ? ($num_days + $bymonth_no + 1) : $bymonth_no;
                if ($this->frequence_context != ilCalendarRecurrence::FREQ_YEARLY) {
                    if ($day_no < 1 or $day_no > $num_days) {
                        $this->log->write(__METHOD__ . ': Ignoring BYMONTHDAY rule: ' . $day_no . ' for month ' .
                            $seed->get(IL_CAL_FKT_DATE, 'M', $this->timezone));
                        continue;
                    }
                }
                $day_diff = $day_no - $seed->get(IL_CAL_FKT_DATE, 'j', $this->timezone);
                $new_day = $this->createDate($seed->get(IL_CAL_UNIX, '', $this->timezone));
                $new_day->increment(ilDateTime::DAY, $day_diff);
                
                switch ($this->frequence_context) {
                    case ilCalendarRecurrence::FREQ_DAILY:
                        // Check if day matches
                        #var_dump("<pre>",$seed->get(IL_CAL_FKT_DATE,'z',$this->timezone),$day_no,"</pre>");
                        if ($seed->get(IL_CAL_FKT_DATE, 'j', $this->timezone) == $day_no) {
                            $days_list->add($new_day);
                        }
                        break;
 
                    case ilCalendarRecurrence::FREQ_WEEKLY:
                        // Check if week matches
                        if ($seed->get(IL_CAL_FKT_DATE, 'W', $this->timezone) == $new_day->get(IL_CAL_FKT_DATE, 'W', $this->timezone)) {
                            $days_list->add($new_day);
                        }
                        break;
 
                    case ilCalendarRecurrence::FREQ_MONTHLY:
                        // seed and new day are in the same month.
                        $days_list->add($new_day);
                        break;
 
                    case ilCalendarRecurrence::FREQ_YEARLY:
                        $h = $this->event->isFullday() ? 0 : $seed->get(IL_CAL_FKT_DATE, 'H', $this->timezone);
                        $i = $this->event->isFullday() ? 0 : $seed->get(IL_CAL_FKT_DATE, 'i', $this->timezone);
                        $s = $this->event->isFullday() ? 0 : $seed->get(IL_CAL_FKT_DATE, 's', $this->timezone);
                        $y = $seed->get(IL_CAL_FKT_DATE, 'Y', $this->timezone);
 
                        // TODO: the chosen monthday has to added to all months
                        for ($month = 1;$month <= 12;$month++) {
                            #$num_days = cal_days_in_month(CAL_GREGORIAN,
                            #   $month,
                            #   $y);
                            $num_days = ilCalendarUtil::_getMaxDayOfMonth(
                                $y,
                                $month
                            );
                            $day_no = $bymonth_no < 0 ? ($num_days + $bymonth_no + 1) : $bymonth_no;
                            if ($day_no < 1 or $day_no > $num_days) {
                                $this->log->write(__METHOD__ . ': Ignoring BYMONTHDAY rule: ' . $day_no . ' for month ' . $month);
                            } else {
                                $tz_obj = ilTimeZone::_getInstance($this->timezone);
                                $tz_obj->switchTZ();
                                $unix = mktime($h, $i, $s, $month, $day_no, $y);
                                $tz_obj->restoreTZ();
                                $new_day = $this->createDate($unix);
                                $days_list->add($new_day);
                            }
                        }
                        break;
                }
            }
        }
        $this->frequence_context = ilCalendarRecurrence::FREQ_DAILY;
        return $days_list;
    }
    
    
    protected function applyBYDAYRules(ilDateList $list)
    {
        // return unmodified, if no byday rules are available
        if (!$this->recurrence->getBYDAYList()) {
            return $list;
        }
 
        $days_list = $this->initDateList();
 
        // generate a list of e.g all Sundays for the given year
        // or e.g a list of all week days in a give month (FREQ = MONTHLY,WEEKLY or DAILY)
        foreach ($list->get() as $seed) {
            $seed_info = $seed->get(IL_CAL_FKT_GETDATE);
            
            // TODO: maybe not correct in dst cases
            $date_info = $seed->get(IL_CAL_FKT_GETDATE);
            $date_info['mday'] = 1;
            $date_info['mon'] = 1;
            $start = $this->createDate($date_info, IL_CAL_FKT_GETDATE);
 
            switch ($this->frequence_context) {
                case ilCalendarRecurrence::FREQ_YEARLY:
                    $day_sequence = $this->getYearWeekDays($seed);
                    break;
                    
                case ilCalendarRecurrence::FREQ_MONTHLY:
                    $day_sequence = $this->getMonthWeekDays($seed_info['year'], $seed_info['mon']);
                    break;
 
                case ilCalendarRecurrence::FREQ_WEEKLY:
                    // TODO or RFC bug: FREQ>WEEKLY;BYMONTH=1;BYDAY=FR returns FR 1.2.2008
                    // Ical says: apply BYMONTH rules and after that apply byday rules on that date list.
                    $day_sequence = $this->getWeekWeekDays($seed_info);
                    break;
 
                case ilCalendarRecurrence::FREQ_DAILY:
                    $day_sequence[strtoupper(substr($seed->get(IL_CAL_FKT_DATE, 'D'), 0, 2))] = array($seed_info['yday']);
                    break;
 
            }
            foreach ($this->recurrence->getBYDAYList() as $byday) {
                $year_day = array();
                $day = strtoupper(substr($byday, -2));
                $num_by_day = (int) $byday;
                
                if ($num_by_day) {
                    if ($num_by_day > 0) {
                        if (isset($day_sequence[$day][$num_by_day - 1])) {
                            $year_day = array($day_sequence[$day][$num_by_day - 1]);
                        }
                    } else {
                        if (isset($day_sequence[$day][count($day_sequence[$day]) + $num_by_day])) {
                            $year_day = array($day_sequence[$day][count($day_sequence[$day]) + $num_by_day]);
                        }
                    }
                } else {
                    if (isset($day_sequence[$day])) {
                        $year_day = $day_sequence[$day];
                    }
                }
                foreach ($year_day as $day) {
                    switch ($this->frequence_context) {
                        case ilCalendarRecurrence::FREQ_WEEKLY:
                        case ilCalendarRecurrence::FREQ_DAILY:
                        case ilCalendarRecurrence::FREQ_MONTHLY:
                        case ilCalendarRecurrence::FREQ_YEARLY:
                            $tmp_date = clone $start;
                            $tmp_date->increment(IL_CAL_DAY, $day);
                            $days_list->add($tmp_date);
                            break;
                    }
                }
            }
        }
        #echo $days_list;
 
        return $days_list;
    }
    
    protected function getYearWeekDays(ilDateTime $seed)
    {
        $time = microtime(true);
        
        $year_days = array();
        
        $current_year = $seed->get(IL_CAL_FKT_DATE, 'Y');
        $start = new ilDate($current_year . '-01-01', IL_CAL_DATE);
        $offset = $start->get(IL_CAL_FKT_DATE, 'w');
        $days = array(0 => 'SU',1 => 'MO',2 => 'TU',3 => 'WE',4 => 'TH',5 => 'FR',6 => 'SA');
        for ($i = 0;$i < $offset;$i++) {
            next($days);
        }
        
        $num_days = ilCalendarUtil::_isLeapYear($current_year) ? 366 : 365;
        for ($i = 0;$i < $num_days;$i++) {
            if (($current_day = current($days)) == false) {
                $current_day = reset($days);
            }
            $year_days[$current_day][] = $i;
            next($days);
        }
        return $year_days;
    }
    
    protected function getMonthWeekDays($year, $month)
    {
        static $month_days = array();
 
        if (isset($month_days[$year][$month])) {
            return $month_days[$year][$month];
        }
        
        $month_str = $month < 10 ? ('0' . $month) : $month;
        $begin_month = new ilDate($year . '-' . $month_str . '-01', IL_CAL_DATE);
        $begin_month_info = $begin_month->get(IL_CAL_FKT_GETDATE);
        
        $days = array(0 => 'SU',1 => 'MO',2 => 'TU',3 => 'WE',4 => 'TH',5 => 'FR',6 => 'SA');
        for ($i = 0;$i < $begin_month_info['wday'];$i++) {
            next($days);
        }
        for ($i = $begin_month_info['yday']; $i < $begin_month_info['yday'] + ilCalendarUtil::_getMaxDayOfMonth($year, $month) ; $i++) {
            if (($current_day = current($days)) == false) {
                $current_day = reset($days);
            }
            $month_days[$year][$month][$current_day][] = $i;
            next($days);
        }
        return $month_days[$year][$month];
    }
    
    protected function getWeekWeekDays($seed_info)
    {
        $days = array(0 => 'SU',1 => 'MO',2 => 'TU',3 => 'WE',4 => 'TH',5 => 'FR',6 => 'SA');
        
        $start_day = $seed_info['yday'] - $seed_info['wday'];
        foreach ($days as $num => $day) {
            $week_days[$day][] = $start_day++;
        }
        return $week_days;
    }
    
    
    protected function applyBYSETPOSRules(ilDateList $list)
    {
        // return unmodified, if no bysetpos rules are available
        if (!$this->recurrence->getBYSETPOSList()) {
            return $list;
        }
        $pos_list = $this->initDateList();
        $list->sort();
        $candidates = $list->get();
        $candidates_count = count($candidates);
        foreach ($this->recurrence->getBYSETPOSList() as $position) {
            if ($position > 0 and $date = $list->getAtPosition($position)) {
                $pos_list->add($date);
            }
            if ($position < 0 and $date = $list->getAtPosition($candidates_count + $position + 1)) {
                $pos_list->add($date);
            }
        }
        return $pos_list;
    }
    
    protected function applyLimits(ilDateList $list)
    {
        $list->sort();
 
        #echo "list: ";
        #echo $list;
        #echo '<br />';
 
        // Check valid dates before starting time
        foreach ($list->get() as $check_date) {
            if (ilDateTime::_before($check_date, $this->event->getStart(), IL_CAL_DAY)) {
                $this->log->debug('Removed invalid date: ' . (string) $check_date . ' before starting date:  ' . (string) $this->event->getStart());
                $list->remove($check_date);
            }
        }
        
        #echo 'Until date '.$this->recurrence->getFrequenceUntilDate();
 
        // Check count if given
        if ($this->recurrence->getFrequenceUntilCount()) {
            foreach ($list->get() as $res) {
                // check smaller than since the start time counts as one
                if (count($this->valid_dates->get()) < $this->recurrence->getFrequenceUntilCount()) {
                    $this->valid_dates->add($res);
                } else {
                    $this->limit_reached = true;
                    return false;
                }
            }
            return true;
        } elseif ($this->recurrence->getFrequenceUntilDate()) {
            #echo 'Until date '.$this->recurrence->getFrequenceUntilDate();
            $date = $this->recurrence->getFrequenceUntilDate();
            foreach ($list->get() as $res) {
                #echo 'Check date '.$res;
                if (ilDateTime::_after($res, $date, IL_CAL_DAY)) {
                    #echo 'Limit reached';
                    $this->limit_reached = true;
                    return false;
                }
                $this->valid_dates->add($res);
            }
            return true;
        }
 
        $this->valid_dates->merge($list);
        return true;
    }
    
    protected function applyExclusionDates()
    {
        if (!$this->recurrence->getExclusionDates()) {
            return true;
        }
        foreach ($this->recurrence->getExclusionDates() as $excl) {
            $this->valid_dates->removeByDAY($excl->getDate());
        }
    }
    
    protected function initDateList()
    {
        return new ilDateList($this->event->isFullday() ? ilDateList::TYPE_DATE : ilDateList::TYPE_DATETIME);
    }
    
    protected function createDate($a_date, $a_format_type = IL_CAL_UNIX)
    {
        if ($this->event->isFullday()) {
            return new ilDate($a_date, $a_format_type);
        } else {
            // TODO: the timezone for this recurrence must be stored in the db
            return new ilDateTime($a_date, $a_format_type, $this->timezone);
        }
    }
    
    protected function validateRecurrence()
    {
        return $this->recurrence->validate();
    }
}