Add Armstrong Numbers exercise

config.json

@@ -47,10 +47,20 @@
       "difficulty": 2,
       "topics": [
         "sequences",
-        "strings", 
+        "strings",
         "regular_expressions"
       ]
     },
+    {
+      "slug": "armstrong-numbers",
+      "uuid": "77c5c7e6-265a-4f1a-9bc4-851f8f825420",
+      "core": false,
+      "unlocked_by": "isogram",
+      "difficulty": 3,
+      "topics": [
+        "math"
+      ]
+    },
     {
       "slug": "matrix",
       "uuid": "3de21c18-a533-4150-8a73-49df8fcb8c61",
@@ -209,7 +219,7 @@
         "maps",
         "transforming"
       ]
-    },    
+    },
     {
       "slug": "pangram",
       "uuid": "fcf07149-b2cb-4042-90e6-fb3350e0fdf6",

exercises/armstrong-numbers/.meta/solutions/armstrong_numbers.rb

@@ -0,0 +1,7 @@
+class ArmstrongNumbers
+  def self.is_valid?(number)
+    exponent = number.digits.size
+
+    number.digits.sum { |digit| digit**exponent } == number
+  end
+end

exercises/armstrong-numbers/README.md

@@ -0,0 +1,41 @@
+# Armstrong Numbers
+
+An [Armstrong number](https://en.wikipedia.org/wiki/Narcissistic_number) is a number that is the sum of its own digits each raised to the power of the number of digits.
+
+For example:
+
+- 9 is an Armstrong number, because `9 = 9^1 = 9`
+- 10 is *not* an Armstrong number, because `10 != 1^2 + 0^2 = 1`
+- 153 is an Armstrong number, because: `153 = 1^3 + 5^3 + 3^3 = 1 + 125 + 27 = 153`
+- 154 is *not* an Armstrong number, because: `154 != 1^3 + 5^3 + 4^3 = 1 + 125 + 64 = 190`
+
+Write some code to determine whether a number is an Armstrong number.
+
+* * * *
+
+For installation and learning resources, refer to the
+[Ruby resources page](http://exercism.io/languages/ruby/resources).
+
+For running the tests provided, you will need the Minitest gem. Open a
+terminal window and run the following command to install minitest:
+
+    gem install minitest
+
+If you would like color output, you can `require 'minitest/pride'` in
+the test file, or note the alternative instruction, below, for running
+the test file.
+
+Run the tests from the exercise directory using the following command:
+
+    ruby armstrong_numbers_test.rb
+
+To include color from the command line:
+
+    ruby -r minitest/pride armstrong_numbers_test.rb
+
+## Source
+
+Wikipedia [https://en.wikipedia.org/wiki/Narcissistic_number](https://en.wikipedia.org/wiki/Narcissistic_number)
+
+## Submitting Incomplete Solutions
+It's possible to submit an incomplete solution so you can see how others have completed the exercise.

exercises/armstrong-numbers/armstrong_numbers_test.rb

@@ -0,0 +1,59 @@
+require 'minitest/autorun'
+require_relative 'armstrong_numbers'
+
+class ArmstrongNumbersTest < Minitest::Test
+  def test_one_digit_numbers_are_armstrong_numbers
+    assert ArmstrongNumbers.is_valid?(1)
+    assert ArmstrongNumbers.is_valid?(6)
+    assert ArmstrongNumbers.is_valid?(9)
+  end
+
+  def test_two_digit_numbers_aren_t_armstrong_numbers
+    skip
+    refute ArmstrongNumbers.is_valid?(11)
+    refute ArmstrongNumbers.is_valid?(89)
+    refute ArmstrongNumbers.is_valid?(44)
+  end
+
+  def test_three_digit_numbers_that_are_armstrong_numbers
+    skip
+    assert ArmstrongNumbers.is_valid?(153)
+    assert ArmstrongNumbers.is_valid?(370)
+  end
+
+  def test_three_digit_numbers_that_aren_t_armstrong_numbers
+    skip
+    refute ArmstrongNumbers.is_valid?(555)
+    refute ArmstrongNumbers.is_valid?(662)
+  end
+
+  def test_four_digit_number_that_is_armstrong_number
+    skip
+    assert ArmstrongNumbers.is_valid?(1_634)
+  end
+
+  def test_four_digit_number_that_is_not_armstrong_number
+    skip
+    refute ArmstrongNumbers.is_valid?(9_989)
+  end
+
+  def test_seven_digit_number_that_is_armstrong_number
+    skip
+    assert ArmstrongNumbers.is_valid?(9_926_315)
+  end
+
+  def test_seven_digit_number_that_is_not_armstrong_number
+    skip
+    refute ArmstrongNumbers.is_valid?(2_369_989)
+  end
+
+  def test_ten_digit_number_that_is_armstrong_number
+    skip
+    assert ArmstrongNumbers.is_valid?(4_679_307_774)
+  end
+
+  def test_fourteen_digit_number_that_is_armstrong_number
+    skip
+    assert ArmstrongNumbers.is_valid?(28_116_440_335_967)
+  end
+end