大数阶乘

factorial.cpp

#include<Windows.h>
#include<iostream>
#include<string>
unsigned short c[22167];
void testmain(std::string&param)
{
	bool enable_cout = true;
	if (param[0] == '@')
	{
		enable_cout = false;
		param.erase(param.begin());
	}
	for (char&ch : param)
	{
		if (!isdigit(ch))
		{
			puts("Not a num. (NaN)");
			return;
		}
	}
	if (atoi(param.c_str()) > 6553)
	{
		puts("Unacceptable num.");
		return;
	}
	memset(&c, 0, sizeof c);
	c[0] = 1;
	int maxi = 1;
	for (int top = atoi(param.c_str()); top > 1; top--)
	{
		for (int i = 0; i < maxi; i++)
			c[i] *= top;
		for (int i = 0; i < maxi; i++)
		{
			c[i + 1] += c[i] / 10;
			c[i] %= 10;
			if (c[i + 1] && i + 2 > maxi)
				maxi = i + 2;
		}
	}
	for (int i = maxi - 1; i != -1; i--)
	{
		if (c[i] > 9)
		{
			puts("Found num. greater than 9.");
			return;
		}
		if (enable_cout)
			std::cout << c[i];
	}
	std::cout << std::endl << maxi << " digits.\n";
}
int main()
{
	DWORD t_start, t_end;
	std::string param;
	std::getline(std::cin, param);
	t_start = GetTickCount();
	testmain(param);
	t_end = GetTickCount();
	printf("%dms\n", t_end - t_start);
	return 0;
}