C++, math, gcd_recursive_euclidean.cpp

/**
 * @file
 * @brief Compute the greatest common denominator of two integers using
 * *recursive form* of
 * [Euclidean algorithm](https://en.wikipedia.org/wiki/Euclidean_algorithm)
 *
 * @see gcd_iterative_euclidean.cpp, gcd_of_n_numbers.cpp
 */
#include <iostream>

/**
 * algorithm
 */
int gcd(int num1, int num2) {
    if (num1 <= 0 | num2 <= 0) {
        throw std::domain_error("Euclidean algorithm domain is for ints > 0");
    }

    if (num1 == num2) {
        return num1;
    }

    // Everything divides 0
    if (num1 == 0)
        return num2;
    if (num2 == 0)
        return num1;

    // base case
    if (num1 == num2)
        return num1;

    // a is greater
    if (num1 > num2)
        return gcd(num1 - num2, num2);
    return gcd(num1, num2 - num1);
}

/**
 * Main function
 */
int main() {
    std::cout << "gcd of 120,7 is " << (gcd(120, 7)) << std::endl;
    try {
        std::cout << "gcd of -120,10 is " << gcd(-120, 10) << std::endl;
    } catch (const std::domain_error &e) {
        std::cout << "Error handling was successful" << std::endl;
    }
    std::cout << "gcd of 312,221 is " << (gcd(312, 221)) << std::endl;
    std::cout << "gcd of 289,204 is " << (gcd(289, 204)) << std::endl;
    return 0;
}