Solve 'Can you get the loop ?' kata (#10)

Author: borisskert

src/CanYouGetTheLoop.hs

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+module CanYouGetTheLoop (loopSize) where
+
+import CanYouGetTheLoop.Types
+
+-- https://www.codewars.com/kata/52a89c2ea8ddc5547a000863/train/haskell
+
+loopSize :: (Eq a) => Node a -> Int
+loopSize = length . findLoop
+
+findLoop :: (Eq a) => Node a -> [Node a]
+findLoop node = go [] . detectLoop node . next $ node
+  where
+    detectLoop :: (Eq a) => Node a -> Node a -> Node a
+    detectLoop slow fast
+      | slow == fast = slow
+      | otherwise = detectLoop (next slow) (next (next fast))
+
+    go :: (Eq a) => [Node a] -> Node a -> [Node a]
+    go [] node = go [node] (next node)
+    go found node
+      | node `elem` found = found
+      | otherwise = go (found ++ [node]) (next node)

src/CanYouGetTheLoop/Types.hs

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+module CanYouGetTheLoop.Types (Node (..), next) where
+
+data Node a = Node a (Node a)
+
+instance (Eq a) => Eq (Node a) where
+  (Node x _) == (Node y _) = x == y
+
+next :: Node a -> Node a
+next (Node x node) = node

test/CanYouGetTheLoopSpec.hs

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+module CanYouGetTheLoopSpec (spec, main) where
+import CanYouGetTheLoop.Types
+import CanYouGetTheLoop
+import Test.Hspec
+
+main = hspec spec
+spec = do
+  describe "loopChain" $ do
+    it "finds loop in list of 4 nodes" $ do
+      let node1 = Node 1 node2
+          node2 = Node 2 node3
+          node3 = Node 3 node4
+          node4 = Node 4 node2
+      loopSize node1 `shouldBe` 3