#### Large Factorial

```/**
* @file
* @brief Compute factorial of any arbitratily large number/
*
* \author [Krishna Vedala](https://github.com/kvedala)
* @see factorial.cpp
*/
#include <cstring>
#include <ctime>
#include <iostream>

#include "./large_number.h"

/** Test implementation for 10! Result must be 3628800.
* @returns True if test pass else False
*/
bool test1() {
std::cout << "---- Check 1\t";
unsigned int i, number = 10;
large_number result;
for (i = 2; i <= number; i++) /* Multiply every number from 2 thru N */
result *= i;

const char *known_reslt = "3628800";

/* check 1 */
if (strlen(known_reslt) != result.num_digits()) {
std::cerr << "Result lengths dont match! " << strlen(known_reslt)
<< " != " << result.num_digits() << std::endl;
return false;
}

const size_t N = result.num_digits();
for (i = 0; i < N; i++) {
if (known_reslt[i] != result.digit_char(i)) {
std::cerr << i << "^th digit mismatch! " << known_reslt[i]
<< " != " << result.digit_char(i) << std::endl;
return false;
}
}

std::cout << "Passed!" << std::endl;
return true;
}

/** Test implementation for 100! The result is the 156 digit number:
* ```
* 9332621544394415268169923885626670049071596826438162146859296389521759
* 9993229915608941463976156518286253697920827223758251185210916864000000
* 000000000000000000
* ```
* @returns True if test pass else False
*/
bool test2() {
std::cout << "---- Check 2\t";
unsigned int i, number = 100;
large_number result;
for (i = 2; i <= number; i++) /* Multiply every number from 2 thru N */
result *= i;

const char *known_reslt =
"9332621544394415268169923885626670049071596826438162146859296389521759"
"9993229915608941463976156518286253697920827223758251185210916864000000"
"000000000000000000";

/* check 1 */
if (strlen(known_reslt) != result.num_digits()) {
std::cerr << "Result lengths dont match! " << strlen(known_reslt)
<< " != " << result.num_digits() << std::endl;
return false;
}

const size_t N = result.num_digits();
for (i = 0; i < N; i++) {
if (known_reslt[i] != result.digit_char(i)) {
std::cerr << i << "^th digit mismatch! " << known_reslt[i]
<< " != " << result.digit_char(i) << std::endl;
return false;
}
}

std::cout << "Passed!" << std::endl;
return true;
}

/**
* Main program
**/
int main(int argc, char *argv[]) {
int number, i;

if (argc == 2) {
number = atoi(argv[1]);
} else {
std::cout << "Enter the value of n(n starts from 0 ): ";
std::cin >> number;
}

large_number result;

std::clock_t start_time = std::clock();
for (i = 2; i <= number; i++) /* Multiply every number from 2 thru N */
result *= i;
std::clock_t end_time = std::clock();
double time_taken =
static_cast<double>(end_time - start_time) / CLOCKS_PER_SEC;

std::cout << number << "! = " << result << std::endl
<< "Number of digits: " << result.num_digits() << std::endl
<< "Time taken: " << std::scientific << time_taken << " s"
<< std::endl;

test1();
test2();
result.test();

return 0;
}
```