[FIX] Rewrite Armstrong Number algorithm to follow contribution guidelines

Author: shiwans-k-yadav

math/armstrong_number_shiwans.cpp

@@ -0,0 +1,64 @@
+/*
+ * Armstrong Number Checker in C++
+ *
+ * An Armstrong number (narcissistic number) is a number equal to the sum of
+ * its digits raised to the power of the number of digits.
+ *
+ * Example: 153 -> 1^3 + 5^3 + 3^3 = 153
+ *
+ * This implementation is beginner-friendly, with interactive input and tests.
+ */
+
+#include <cassert>
+#include <cmath>
+#include <iostream>
+using namespace std;
+
+// Function to count digits
+int count_digits(int num) {
+    return (num == 0) ? 1 : static_cast<int>(log10(num)) + 1;
+}
+
+// Function to check Armstrong number
+bool isArmstrong(int num) {
+    if (num < 0)
+        return false;
+
+    int sum = 0;
+    int n = count_digits(num);
+    int temp = num;
+
+    while (temp > 0) {
+        int digit = temp % 10;
+        sum += pow(digit, n);
+        temp /= 10;
+    }
+    return sum == num;
+}
+
+// Self-test function
+void test() {
+    assert(isArmstrong(0) == true);
+    assert(isArmstrong(153) == true);
+    assert(isArmstrong(370) == true);
+    assert(isArmstrong(225) == false);
+    assert(isArmstrong(-23) == false);
+    assert(isArmstrong(12) == false);
+    cout << "All self-tests passed!\n";
+}
+
+// Main function for interactive usage
+int main() {
+    test();  // Run automated tests
+
+    int number;
+    cout << "Enter a number to check if it is an Armstrong number: ";
+    cin >> number;
+
+    if (isArmstrong(number))
+        cout << number << " is an Armstrong number!\n";
+    else
+        cout << number << " is NOT an Armstrong number.\n";
+
+    return 0;
+}