Day 9 - Part 2

test/Day9Spec.hs

@@ -29,6 +29,17 @@ expectedPositions = sort [
   (0,0),(1,0),(2,0),(3,0)
   ]
 
+testInput2 :: String
+testInput2 = [str|R 5
+                 |U 8
+                 |L 8
+                 |D 3
+                 |R 17
+                 |D 10
+                 |L 25
+                 |U 20
+                 |]
+
 spec :: Spec
 spec =
   describe "Day9" $ do
@@ -51,7 +62,7 @@ spec =
         step (0, 1) (Step (0, -1)) `shouldBe` (0, 0)
       it "normalizes movement" $ do
         normalizeMovement still `shouldBe` []
-        normalizeMovement (left 1) `shouldBe` [Step (-1, 0)]
+        normalizeMovement (left 2) `shouldBe` [Step (-1, 0),Step (-1, 0)]
         normalizeMovement (right 1) `shouldBe` [Step (1, 0)]
         normalizeMovement (up 1) `shouldBe` [Step (0, 1)]
         normalizeMovement (down 2) `shouldBe` [Step (0, -1), Step (0, -1)]
@@ -74,6 +85,18 @@ spec =
         positions `shouldBe` expectedPositions
       it "solves the riddle" $ do
         input <- readFile "ressources/day09-input"
-        let movements = parseMovements input
-        let positions = concatMap normalizeMovement >>> recordPositions >>> fst >>> length $ movements
+        let headSteps = parseMovements >>> concatMap normalizeMovement $ input
+        let allTails = buildTails headSteps
+        let positions = (!! 1)  >>> fst >>> length $ allTails
         positions `shouldBe` 5878
+      it "solves example of part 2" $ do
+        let headSteps = parseMovements >>> concatMap normalizeMovement $ testInput2
+        let allTails = buildTails headSteps
+        let tail9Positions = fst $ allTails !! 9
+        length tail9Positions `shouldBe` 36
+      it "solves the riddle part 2" $ do
+        input <- readFile "ressources/day09-input"
+        let headSteps = parseMovements >>> concatMap normalizeMovement $ input
+        let allTails = buildTails headSteps
+        let positions = (!! 9)  >>> fst >>> length $ allTails
+        positions `shouldBe` 2405