Chain::move

protected function move($letterId, $offset)
{
	$letter = $this->getLetter($letterId);
	if (!$letter)
	{
		return $this;
	}

	$index = array_search($letter, $this->letters, true);
	$previousIndex = $index + $offset;
	if (!isset($this->letters[$previousIndex]))
	{
		return $this;
	}
	$previousLetter = $this->letters[$previousIndex];

	$this->letters[$previousIndex] = $letter;
	$this->letters[$index] = $previousLetter;

	$this->sort();

	return $this;
}