SeasonalityFactor.php

<?php

class SeasonalityFactor
{
	const FORMAT = 'M';
	/**
	 * @var array
	 */
	private $modifiers = array ();

	public function __construct(array $modifiers)
	{
		$date = new \DateTime("1 January");
		do
		{
			$current = strtolower($date->format(self::FORMAT));
			$this->modifiers[$current] = isset($modifiers[$current]) ? (float) $modifiers[$current] : 1;
			$date->modify("next month");
		}
		while ($date->format('M') != "Jan");
	}

	/**
	 * @param \DateTime $dateStart
	 * @param \DateTime $dateEnd
	 *
	 * @return float
	 */
	public function getFactor(\DateTime $dateStart, \DateTime $dateEnd)
	{
		$factor = 0;
		$cnt = 0;
		for ($date = clone $dateStart; $date <= $dateEnd; $date->modify("next day"))
		{
			$month = strtolower($date->format(self::FORMAT));
			$factor += $this->modifiers[$month];
			$cnt++;
		}
		return $cnt ? $factor / $cnt : 1;
	}

}