gistfile1.php

    /**
     * get random event
     *
     * format: array('event1' => 'chance1', 'event2' => 'chance2', ...)
     *
     * @param array $events
     * @return string
     */
    public function randomEvent(array $events)
    {
        /** sort events according chance */
        asort($events);

        if (!array_filter($events)) {
            throw new \Core\Exception(sprintf('events(%s) are empty', json_encode($events)));
        }

        /**
         * transformation to the range, like
         * 0 .. 0.1 .. 0.3 .. 1
         */
        $range = array();
        $total = 0;
        foreach ($events as $event => $chance) {
            $total += $chance;
            $range[$event] = $total;
        }

        /** normalization */
        if ($total != 1) {
            $ratio = 1 / $total;
            $range = array_map(function($value) use ($ratio) {
                return $value * $ratio;
            }, $range);
        }

        /** select range */
        $random = $this->random();
        $previous = 0;
        foreach ($range as $event => $current) {
            if ($random > $previous && $random <= $current) {
                break;
            } else {
                $previous = $current;
            }
        }

        $this->log(sprintf('random event from %s: %s', json_encode($events), $event));

        return $event;
    }