Mob coding through hanoi problem, discovered need for state tracking and tests are busted

homework.cabal

@@ -34,7 +34,6 @@ library
   ghc-options: -Wall -Werror -Wcompat -Widentities -Wincomplete-patterns -Wincomplete-record-updates -Wincomplete-uni-patterns -Wmissing-home-modules -Wname-shadowing -Wpartial-fields -Wredundant-constraints -Wunused-packages -Wunused-type-patterns
   build-depends:
       base >=4.14 && <5
-    , unordered-containers
   default-language: Haskell2010
 
 executable homework

lib/Homework/Ch01/Hanoi.hs

@@ -1,26 +1,38 @@
 module Homework.Ch01.Hanoi where
 
-import qualified Data.HashMap.Strict as HashMap
-
 newtype Peg = Peg ()
 
 data Move = Move {moveFrom :: String, moveTo :: String} deriving (Eq, Show)
 
-data Disc = Disc {discSize :: Int}
+data Disc = Disc {discSize :: Int} deriving (Eq, Show, Ord)
+
+type Pegs = [(String, [Disc])]
 
 hanoi :: Int -> String -> String -> String -> Either String [Move]
 hanoi numDisks pegLabelA pegLabelB pegLabelC =
-  let _pegs =
-        HashMap.fromList
-          [ (pegLabelA, fillPegWithDiscs),
+  let pegs =
+        [ (pegLabelA, fillPegWithDiscs numDisks),
           (pegLabelB, []),
           (pegLabelC, [])
         ]
-   in Right
-        [ Move "a" "c",
-          Move "a" "b",
-          Move "c" "b"
-        ]
-  where
-    fillPegWithDiscs :: [Disc]
-    fillPegWithDiscs = Disc <$> [1 .. numDisks] -- start here: make sure this is initialized correctly
+   in Right $ snd $ move pegs
+
+move :: Pegs -> (Pegs, [Move])
+move pegs =
+  let (firstPegLabel, firstPeg) = head pegs
+      (lastPegLabel, lastPeg) = last pegs
+      firstPegDisc = last firstPeg
+      lastPegDisc = last lastPeg
+      canMove = firstPegDisc < lastPegDisc
+   in if canMove
+        then
+          ( [ (firstPegLabel, init firstPeg),
+              head $ tail pegs,
+              (lastPegLabel, lastPeg <> [firstPegDisc])
+            ],
+            [Move firstPegLabel lastPegLabel]
+          )
+        else (pegs, [])
+
+fillPegWithDiscs :: Int -> [Disc]
+fillPegWithDiscs numDisks = Disc <$> reverse [1 .. numDisks]

package.yaml

@@ -16,8 +16,8 @@ dependencies:
 
 library:
   source-dirs: lib
-  dependencies:
-  - unordered-containers
+  # dependencies:
+  # - unordered-containers
 
 executables:
   homework:

test/Homework/Ch01/HanoiSpec.hs

@@ -4,11 +4,23 @@ import Homework.Ch01.Hanoi
 import Test.Hspec
 
 spec :: Spec
-spec = describe "hanoi" $ do
-  it "can solve for stack of 2 and three pegs" $ do
-    hanoi 2 "a" "b" "c"
+spec = describe "Hanoi" $ do
+  describe "hanoi" $ do
+    it "can solve for a stack of 1 and three pegs" $ do
+      hanoi 1 "a" "b" "c"
+        `shouldBe` Right
+          [Move "a" "c"]
+    it "can solve for stack of 3 and three pegs" $ do
+      hanoi 3 "a" "b" "c"
         `shouldBe` Right
           [ Move "a" "c",
             Move "a" "b",
             Move "c" "b"
           ]
+  describe "fillPegWithDiscs" $ do
+    it "creates a list of disks from biggest to smallest" $ do
+      fillPegWithDiscs 3
+        `shouldBe` [ Disc 3,
+                     Disc 2,
+                     Disc 1
+                   ]