Credit card problem from Chapter 01

hie.yaml

@@ -0,0 +1,10 @@
+cradle:
+  stack:
+    - path: "lib"
+      component: "homework:lib"
+
+    - path: "app"
+      component: "homework:exe"
+
+    - path: "test"
+      component: "homework:test"

homework.cabal

@@ -23,6 +23,7 @@ source-repository head
 library
   exposed-modules:
       Homework
+      Homework.Ch01
   other-modules:
       Paths_homework
   hs-source-dirs:
@@ -48,6 +49,7 @@ test-suite test
   type: exitcode-stdio-1.0
   main-is: Spec.hs
   other-modules:
+      Homework.Ch01Spec
       Paths_homework
   hs-source-dirs:
       test

lib/Homework/Ch01.hs

@@ -0,0 +1,39 @@
+module Homework.Ch01 where
+
+-- This calls reverse on toDigitsRev because I somehow did this backwards by default.
+toDigits :: Integer -> [Integer]
+toDigits = rev [] . toDigitsRev
+  where
+    rev acc (x : xs) = rev (x : acc) xs
+    rev acc [] = acc
+
+-- I don't know how I managed it, but turning the number into an array already reversed it?
+toDigitsRev :: Integer -> [Integer]
+toDigitsRev n
+  | n < 1 = []
+  | otherwise =
+    let leftDigit = n `mod` 10
+        shiftedDigits = n `div` 10
+     in leftDigit : toDigitsRev shiftedDigits
+
+doubleEveryOther :: [Integer] -> [Integer]
+doubleEveryOther (odd' : even' : rest) = odd' : (even' * 2) : doubleEveryOther rest
+doubleEveryOther rest = rest
+
+sumDigits :: [Integer] -> Integer
+sumDigits = sumDigits' 0 . flattenDigits [] . eachToDigits []
+  where
+    eachToDigits acc (x : xs) = eachToDigits (toDigits x : acc) xs
+    eachToDigits acc [] = acc
+
+    flattenDigits acc ((x : xs) : rest) = flattenDigits (x : acc) (xs : rest)
+    flattenDigits acc ([] : rest) = flattenDigits acc rest
+    flattenDigits acc [] = acc
+
+    sumDigits' acc (x : xs) = sumDigits' (acc + x) xs
+    sumDigits' acc [] = acc
+
+validate :: Integer -> Bool
+validate n = f n `mod` 10 == 0
+  where
+    f = sumDigits . doubleEveryOther . toDigitsRev

test/Homework/Ch01Spec.hs

@@ -0,0 +1,53 @@
+module Homework.Ch01Spec where
+
+import Homework.Ch01
+import Test.Hspec
+
+spec :: Spec
+spec = describe "Credit Card Validation" $ do
+  describe "toDigits" $ do
+    context "n is less than 1" $ do
+      it "returns an empty list" $ do
+        toDigits 0 `shouldBe` []
+
+    context "n contains a single digit" $ do
+      it "creates a list with one digit" $ do
+        toDigits 9 `shouldBe` [9]
+
+    context "n contains more than one digit" $ do
+      it "creates a list of digits" $ do
+        toDigits 12345 `shouldBe` [1, 2, 3, 4, 5]
+
+  describe "toDigitsRev" $ do
+    context "n is less than 1" $ do
+      it "returns an empty list" $ do
+        toDigitsRev 0 `shouldBe` []
+
+    context "n contains a single digit" $ do
+      it "creates a list with one digit" $ do
+        toDigitsRev 9 `shouldBe` [9]
+
+    context "n contains more than one digit" $ do
+      it "creates a list of the digits in reverse order" $ do
+        toDigitsRev 12345 `shouldBe` [5, 4, 3, 2, 1]
+
+  describe "doubleEveryOther" $ do
+    context "list length is odd" $ do
+      it "does not double the last digit" $ do
+        doubleEveryOther [1, 2, 3, 4, 5] `shouldBe` [1, 4, 3, 8, 5]
+
+    context "list length is even" $ do
+      it "doubles every other digit" $ do
+        doubleEveryOther [1, 2, 3, 4, 5, 6] `shouldBe` [1, 4, 3, 8, 5, 12]
+
+  describe "sumDigits" $ do
+    it "splits doubled digits into single digits and sums all single digits together" $ do
+      sumDigits [16, 7, 12, 5] `shouldBe` 22
+
+  describe "validate" $ do
+    it "returns True for a valid credit card number" $ do
+      validate 5105105105105100 `shouldBe` True
+      validate 2223577120017656 `shouldBe` True
+    it "returns False for invalid credit card number" $ do
+      validate 5105105105105101 `shouldBe` False
+      validate 2223573420017656 `shouldBe` False