Add Armstrong number check algorithm

maths/armstrong_number.py

@@ -0,0 +1,41 @@
+from typing import Union
+
+
+def is_armstrong_number(n: int) -> bool:
+    """
+    Check whether a non-negative integer is an Armstrong (narcissistic) number.
+
+    An Armstrong number is a number that is the sum of its own digits each raised
+    to the power of the number of digits in the number.
+
+    Reference:
+        Narcissistic number (Armstrong number) — Wikipedia
+        https://en.wikipedia.org/wiki/Narcissistic_number
+
+    >>> is_armstrong_number(0)
+    True
+    >>> is_armstrong_number(1)
+    True
+    >>> is_armstrong_number(153)
+    True
+    >>> is_armstrong_number(370)
+    True
+    >>> is_armstrong_number(9474)
+    True
+    >>> is_armstrong_number(9475)
+    False
+    >>> is_armstrong_number(-1)  # negative numbers are not considered Armstrong
+    False
+    """
+    # Only non-negative integers are considered
+    if n < 0:
+        return False
+
+    # Convert to string to count digits
+    digits = str(n)
+    power = len(digits)
+
+    # Sum of each digit raised to the 'power'
+    total = sum(int(d) ** power for d in digits)
+
+    return total == n