/**
* exponentiation_by_square.
*
* @link https://en.wikipedia.org/wiki/Exponentiation_by_squaring.
*
* @param int $x The base
* @param int $n The exponent
*
* @return float The value of $x raised to the power $n
*
* @throws InvalidArgumentException If input is not valid number
*/
function exponentiation_by_square($x, $n)
{
if (!($a = is_numeric($x)) or !is_numeric($n)) {
throw new \InvalidArgumentException(sprintf('%s is not valid number', $a ? $x : $n));
}
if ($n < 0) {
return exponentiation_by_square(1 / $x, -$n);
} elseif ($n == 0) {
return 1;
} elseif ($n == 1) {
return $x;
} elseif ($n % 2 == 0) {
return exponentiation_by_square($x * $x, $n / 2);
} else {
return $x * exponentiation_by_square($x * $x, ($n - 1) / 2);
}
}
/**
* @depends testExponentiation
* @expectedException \InvalidArgumentException
*/
public function testExponentiationException2()
{
exponentiation_by_square(2, 'str');
}