tree-building: port

config.json

@@ -590,6 +590,18 @@
         "maybe"
       ]
     },
+    {
+      "slug": "tree-building",
+      "uuid": "508868ec-bd5f-4826-a4c0-ab2a545e7038",
+      "core": false,
+      "unlocked_by": "collatz-conjecture",
+      "difficulty": 5,
+      "topics": [
+        "maybe",
+        "recursion",
+        "trees"
+      ]
+    },
     {
       "slug": "atbash-cipher",
       "uuid": "4b6d04f4-8cfc-4d8a-ac6e-77f3889f27c3",

exercises/tree-building/README.md

@@ -0,0 +1,83 @@
+# Tree Building
+
+Refactor a tree building algorithm.
+
+Some web-forums have a tree layout, so posts are presented as a tree. However
+the posts are typically stored in a database as an unsorted set of records. Thus
+when presenting the posts to the user the tree structure has to be
+reconstructed.
+
+Your job will be to refactor a working but slow and ugly piece of code that
+implements the tree building logic for highly abstracted records. The records
+only contain an ID number and a parent ID number. The ID number is always
+between 0 (inclusive) and the length of the record list (exclusive). All records
+have a parent ID lower than their own ID, except for the root record, which has
+a parent ID that's equal to its own ID.
+
+An example tree:
+
+```text
+root (ID: 0, parent ID: 0)
+|-- child1 (ID: 1, parent ID: 0)
+|    |-- grandchild1 (ID: 2, parent ID: 1)
+|    +-- grandchild2 (ID: 4, parent ID: 1)
++-- child2 (ID: 3, parent ID: 0)
+|    +-- grandchild3 (ID: 6, parent ID: 3)
++-- child3 (ID: 5, parent ID: 0)
+```
+
+
+## Getting Started
+
+For installation and learning resources, refer to the
+[exercism help page](http://exercism.io/languages/haskell).
+
+## Running the tests
+
+To run the test suite, execute the following command:
+
+```bash
+stack test
+```
+
+#### If you get an error message like this...
+
+```
+No .cabal file found in directory
+```
+
+You are probably running an old stack version and need
+to upgrade it.
+
+#### Otherwise, if you get an error message like this...
+
+```
+No compiler found, expected minor version match with...
+Try running "stack setup" to install the correct GHC...
+```
+
+Just do as it says and it will download and install
+the correct compiler version:
+
+```bash
+stack setup
+```
+
+## Running *GHCi*
+
+If you want to play with your solution in GHCi, just run the command:
+
+```bash
+stack ghci
+```
+
+## Feedback, Issues, Pull Requests
+
+The [exercism/haskell](https://github.com/exercism/haskell) repository on
+GitHub is the home for all of the Haskell exercises.
+
+If you have feedback about an exercise, or want to help implementing a new
+one, head over there and create an issue.  We'll do our best to help you!
+
+## Submitting Incomplete Solutions
+It's possible to submit an incomplete solution so you can see how others have completed the exercise.

exercises/tree-building/examples/success-standard/package.yaml

@@ -0,0 +1,21 @@
+name:                tree-building
+version:             1.0.0.0
+
+dependencies:
+  - base
+
+library:
+  exposed-modules: TreeBuilding
+  source-dirs: src
+  ghc-options: -Wall
+  # dependencies:
+  # - foo       # List here the packages you
+  # - bar       # want to use in your solution.
+
+tests:
+  test:
+    main:        Tests.hs
+    source-dirs: test
+    dependencies:
+    - tree-building
+    - hspec

exercises/tree-building/examples/success-standard/src/TreeBuilding.hs

@@ -0,0 +1,77 @@
+module TreeBuilding (newTree, Record(..), Tree(..)) where
+
+import Data.List
+import Data.Maybe (isNothing)
+
+type Id = Int
+type Children = [Tree]
+type ParentGrouping = (Maybe Id, [Id])
+
+data Record = Record Id (Maybe Id) deriving (Eq, Show)
+
+data Tree = Leaf Id | Branch Id Children deriving (Eq, Show)
+
+newTree :: [Record] -> Maybe Tree
+newTree records
+  | cycles records      = Nothing
+  | succIdCheck records = build . groupByParent $ records
+  | otherwise           = Nothing
+
+-- Checks for cycling in newTree
+cycles :: [Record] -> Bool
+cycles = any checkRecord
+  where checkRecord r =
+          case r of
+            (Record i Nothing)  -> if' (i /= 0) True False
+            (Record i (Just p)) -> if' (p >= i) True False
+
+build :: [ParentGrouping] -> Maybe Tree
+build [] = Nothing
+build (x:xs)
+  | not . validRoot $ x = Nothing
+  | null xs             = Just (Leaf 0)
+  | otherwise           = Just (Branch 0 (build' xs))
+
+--  Internal build
+build' :: [ParentGrouping] -> [Tree]
+build' xs
+  | length xs == 1 = map Leaf (snd (head xs))
+  | otherwise      = buildWithChildren xs
+  where
+    buildWithChildren (y:ys) =
+      map (\yid ->
+             let children = filter ((== Just yid) . fst) ys
+             in if not (null children)
+                then Branch yid (build' children)
+                else Leaf yid
+          ) (snd y)
+
+-- Validate the Root node
+validRoot :: ParentGrouping -> Bool
+validRoot (p, ids) = isNothing p && length ids == 1 && sum ids == 0
+
+groupByParent :: [Record] -> [ParentGrouping]
+groupByParent = sortOn fst
+              . map (\xs -> (recordParent (head xs), map recordId xs))
+              . groupBy parentsEq
+              . sortOn recordParent
+              . sortOn recordId
+
+parentsEq :: Record -> Record -> Bool
+parentsEq rx ry = recordParent rx == recordParent ry
+
+recordParent :: Record -> Maybe Int
+recordParent (Record _ p) = p
+
+recordId :: Record -> Id
+recordId (Record i _) = i
+
+-- verification
+succIdCheck :: [Record] -> Bool
+succIdCheck = all (\(x,y) -> succ x == y) . pairwise . sort . map recordId
+  where pairwise = zip <*> tail
+
+-- General Tools
+
+if' :: Bool -> a -> a -> a
+if' p a b = if p then a else b

exercises/tree-building/package.yaml

@@ -0,0 +1,21 @@
+name:                tree-building
+version:             1.0.0.0
+
+dependencies:
+  - base
+
+library:
+  exposed-modules: TreeBuilding
+  source-dirs: src
+  ghc-options: -Wall
+  # dependencies:
+  # - foo       # List here the packages you
+  # - bar       # want to use in your solution.
+
+tests:
+  test:
+    main:        Tests.hs
+    source-dirs: test
+    dependencies:
+    - tree-building
+    - hspec

exercises/tree-building/src/TreeBuilding.hs

@@ -0,0 +1,49 @@
+module TreeBuilding (newTree, Record(..), Tree(..)) where
+
+import Data.List
+import Data.Ord (comparing)
+--import Data.Maybe (fromMaybe)
+
+type Id = Int
+type Children = [Tree]
+
+data Record = Record Id (Maybe Id) deriving (Show)
+
+data Tree = Leaf Id | Branch Id Children deriving (Eq, Show)
+
+newTree :: [Record] -> Maybe Tree
+newTree records
+  | cycleCheck records = Nothing
+  | succIdCheck records == True = build . groupByParent $ records
+  | otherwise = Nothing
+  where
+    recordParent = \(Record _ p) -> p
+    recordId = \(Record i _) -> i
+    succIdCheck = all (\(x,y) -> succ x == y) . pairwise . sort . map recordId
+      where pairwise = zip <*> tail
+    cycleCheck = any id . map (\(r) -> case r of
+                                         (Record i Nothing)   -> if i == 0
+                                                                 then False
+                                                                 else True
+                                         (Record i (Just p)) -> if p >= i
+                                                                 then True
+                                                                 else False)
+    groupByParent = sortOn fst . map (\xs -> (recordParent (head xs), map recordId xs)) . groupBy parentsEq . sortBy (comparing recordParent) . sortBy (comparing recordId)
+    parentsEq rx ry = recordParent rx == recordParent ry
+    build [] = Nothing
+    build (x:xs)
+      | not . rootCheck $ x = Nothing
+      | xs == []    = Just (Leaf 0)
+      | otherwise   = Just (Branch 0 (recursiveBuilding xs))
+      where
+        rootCheck (p,ids)
+          | p /= Nothing || (length ids /= 1 && (head ids) /= 0) = False
+          | otherwise = True
+        recursiveBuilding ys
+          | length ys == 1 = map (\yid -> Leaf yid) (snd (head ys))
+          | otherwise      = (\(z:zs) -> map (\xid ->
+                                                let children = filter ((== (Just xid)) . fst) zs
+                                                in  if length children > 0
+                                                    then Branch xid (recursiveBuilding children)
+                                                    else Leaf xid
+                                             ) (snd z)) ys

exercises/tree-building/stack.yaml

@@ -0,0 +1 @@
+resolver: lts-12.4