double_factorial.c

VALUE factorials_num_double_factorial(VALUE num)
{
  long value = NUM2INT(num), result = 1;

  if(value % 2 == 0) {
    return INT2NUM(
      pow(2, value / 2) * NUM2INT(
        factorials_num_factorial(INT2NUM(value / 2))
      )
    );
  } else {
    return INT2NUM(
      NUM2INT(factorials_num_factorial(num)) / (
        pow(2, (value - 1) / 2) * NUM2INT(
          factorials_num_factorial(INT2NUM((value - 1) / 2))
        )
      )
    );
  }
}