Iterative double factorial P17913

Write an iterative function that returns the double factorial @n@$!!$ for a natural $n$.

Recall that $n!! = n \times (n - 2) \times (n - 4) \times \dots$. For instance, $9!! = 9 \times 7 \times 5 \times 3 \times 1 = 945$ and $8!! = 8 \times 6 \times 4 \times 2 = 384$. By definition, $0!! = 1!! = 1$.

Interface

int double_factorial(int x);

public static int doubleFactorial(int x);

double_factorial(x)  # returns int

double_factorial(x: int) -> int

Precondition

Assume $0\le$ @n@ $\le 19$.

Observation

You only need to submit the required procedure; your main program will be ignored.