euler14.cpp

#include <stdexcept>
#include <iostream>
#include <chrono>

using namespace std;
using namespace std::chrono;

typedef long long number;
typedef int count_type;

number getNext(number n)
{
	if (n <= 1) {
		throw invalid_argument("Number must be greater than 1.");
	}

	if (n%2 == 0) { // Even
		return n/2;
	} else { // Odd
		return 3*n+1;
	}
}

count_type countChain(number n)
{
	count_type count = 1;

	while ((n = getNext(n)) != 1) {
		++count;
	}

	return count;
}

int main()
{
	auto beginTime = std::chrono::high_resolution_clock::now();

	number maxStart = 0;
	count_type maxCount = 0;
	for (number i = 2; i < 1000000; ++i)
	{
		count_type currCount = countChain(i);
		if (currCount > maxCount) {
			maxCount = currCount;
			maxStart = i;
		}
	}

	auto endTime = std::chrono::high_resolution_clock::now();

	cout << "The number that starts the longest chain is " << maxStart << " with a chain totaling " << maxCount << " elements. "
	<< "It was found in " << std::chrono::duration_cast<milliseconds>(endTime - beginTime).count() << "ms." << endl;
}