chore: import upstream snapshot with attribution

others/happy_number.cpp

@@ -0,0 +1,39 @@
+/**
+ * @file
+ * @brief A happy number is a number whose sum of digits is calculated until the
+ * sum is a single digit, and this sum turns out to be 1
+ */
+
+#include <iostream>
+
+/**
+ * Checks if a decimal number is a happy number
+ * \returns true if happy else false
+ */
+template <typename T>
+bool is_happy(T n) {
+    T s = 0;              // stores sum of digits
+    while (n > 9) {       // while number is > 9, there are more than 1 digit
+        while (n != 0) {  // get digit
+            T d = n % 10;
+            s += d;
+            n /= 10;
+        }
+        n = s;
+        s = 0;
+    }
+    return (n == 1) ? true : false;  // true if k == 1
+}
+
+/** Main function */
+int main() {
+    int n;
+    std::cout << "Enter a number:";
+    std::cin >> n;
+
+    if (is_happy(n))
+        std::cout << n << " is a happy number" << std::endl;
+    else
+        std::cout << n << " is not a happy number" << std::endl;
+    return 0;
+}