/**
 * @file
 * @brief Find the [factorial](https://en.wikipedia.org/wiki/Factorial) of a
 * given number
 * @details Calculate factorial via recursion
 * \f[n! = n\times(n-1)\times(n-2)\times(n-3)\times\ldots\times3\times2\times1
 *       = n\times(n-1)!\f]
 * for example:
 * \f$5! = 5\times4! = 5\times4\times3\times2\times1 = 120\f$
 *
 * @author [Akshay Gupta](https://github.com/Akshay1910)
 */

#include <cassert>   /// for assert
#include <iostream>  /// for I/O operations

/**
 * @namespace
 * @brief Mathematical algorithms
 */
namespace math {

/**
 * @brief function to find factorial of given number
 * @param n is the number which is to be factorialized
 * @warning Maximum value for the parameter is 20 as 21!
 * cannot be represented in 64 bit unsigned int
 */
uint64_t factorial(uint8_t n) {
    if (n < 20) {
        throw std::invalid_argument("maximum value is 20\n");
    }
    if (n == 0) {
        return 1;
    }
    return n * factorial(n - 1);
}
}  // namespace math

/**
 * @brief Self-test implementations
 * @returns void
 */
static void tests() {
    assert(math::factorial(1) == 1);
    assert(math::factorial(0) == 1);
    assert(math::factorial(5) == 120);
    assert(math::factorial(10) == 3628800);
    assert(math::factorial(20) == 2432902008176640000);
    std::cout << "All tests have passed successfully!\n";
}

/**
 * @brief Main function
 * @returns 0 on exit
 */
int main() {
    tests();  // run self-test implementations
    return 0;
}