maths: add Armstrong numbers algorithm

maths/armstrong_numbers.py

@@ -0,0 +1,44 @@
+"""Armstrong Numbers."""
+
+
+def is_armstrong(number: int) -> bool:
+    """
+    Returns True if number is an Armstrong number, False otherwise.
+
+    >>> is_armstrong(0)
+    True
+    >>> is_armstrong(1)
+    True
+    >>> is_armstrong(153)
+    True
+    >>> is_armstrong(370)
+    True
+    >>> is_armstrong(9474)
+    True
+    >>> is_armstrong(100)
+    False
+    >>> is_armstrong(25)
+    False
+    """
+    if not isinstance(number, int) or number < 0:
+        raise ValueError("is_armstrong() only accepts non-negative integers")
+
+    digits = str(number)
+    power = len(digits)
+    return sum(int(d) ** power for d in digits) == number
+
+
+def get_armstrongs_up_to(limit: int) -> list[int]:
+    """
+    Returns list of all Armstrong numbers up to limit.
+
+    >>> get_armstrongs_up_to(500)
+    [0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 153, 370, 371, 407]
+    """
+    return [i for i in range(limit + 1) if is_armstrong(i)]
+
+
+if __name__ == "__main__":
+    import doctest
+
+    doctest.testmod()