Tobscure\JsonApi\Parameters::getOffset PHP Method

    public function getOffset($perPage = null)
    {
        if ($perPage && ($offset = $this->getOffsetFromNumber($perPage))) {
            return $offset;
        }
        $offset = (int) $this->getPage('offset');
        if ($offset < 0) {
            throw new InvalidParameterException('page[offset] must be >=0', 2, null, 'page[offset]');
        }
        return $offset;
    }

 public function testGetOffsetParsesThePageNumber()
 {
     $parameters = new Parameters(['page' => ['number' => 2]]);
     $this->assertEquals(20, $parameters->getOffset(20));
 }