Support ability to parse a negative BYDAY $day_number value such as -2MO, etc.

[u01jmg3]

• -1 is already supported as by using relative date formats we can use 'last day of ...'; however anything less than -1 won't work because penultimate, etc. is not offered with relative date formats

---

• For example:
  • FREQ=MONTHLY;BYDAY=-2MO;COUNT=7
    • Every month on the 2nd last Monday for 7 times
  • FREQ=YEARLY;BYDAY=-3SU;BYMONTH=10
    • Every October on the 3rd last Sunday

---

• The code below will convert a negative BYDAY value such as -2MO to its appropriate positive day ordinal such as the 'third'.
  • Will leave the implementation to someone else but this snippet should provide a start.

<?php
    /**
     * Get the number of days between a
     * start and end date
     *
     * @param  $days
     * @param  $start
     * @param  $end
     * @return integer
     */
    function numberOfDays($days, $start, $end){
        $w       = array(date('w', $start), date('w', $end));
        $oneWeek = 604800; // 7 * 24 * 60 * 60
        $x       = floor(($end - $start) / $oneWeek);
        $sum     = 0;

        for ($day = 0; $day < 7; ++$day) {
            if ($days & pow(2, $day)) {
                $sum += $x + ($w[0] > $w[1] ? $w[0] <= $day || $day <= $w[1] : $w[0] <= $day && $day <= $w[1]);
            }
        }

        return $sum;
    }

    /**
     * Convert a negative day ordinal to
     * its equivalent positive form
     *
     * @param  $dayNumber
     * @param  $weekday
     * @param  $timestamp
     * @return string
     */
    function convertDayOrdinalToPositive($dayNumber, $weekday, $timestamp){
        $dayNumber = empty($dayNumber) ? 1 : $dayNumber; // Returns 0 when no number defined in BYDAY

        $dayOrdinals = array(1 => 'first', 2 => 'second', 3 => 'third', 4 => 'fourth', 5 => 'fifth');

        // We only care about negative BYDAY values
        if ($dayNumber >= 1) {
            return $dayOrdinals[$dayNumber];
        }

        $timestamp = (is_object($timestamp)) ? $timestamp : \DateTime::createFromFormat('U', $timestamp);
        $start = strtotime('first day of ' . $timestamp->format('F Y H:i:s'));
        $end   = strtotime('last day of ' . $timestamp->format('F Y H:i:s'));

        // Used with pow(2, X) so pow(2, 4) is THURSDAY
        $weekdays = array('SU' => 0, 'MO' => 1, 'TU' => 2, 'WE' => 3, 'TH' => 4, 'FR' => 5, 'SA' => 6);

        $numberOfDays = numberOfDays(pow(2, $weekdays[$weekday]), $start, $end);

        // Create subset
        $dayOrdinals = array_slice($dayOrdinals, 0, $numberOfDays, true);

        //Reverse only the values
        $dayOrdinals = array_combine(array_keys($dayOrdinals), array_reverse(array_values($dayOrdinals)));

        return $dayOrdinals[$dayNumber * -1];
    }

    echo convertDayOrdinalToPositive(-2, 'MO', date_create('now'));