Add solution to the first part of day12

Author: CastilloDel

AdventOfCode2021.cabal

@@ -71,4 +71,10 @@ executable day11
     main-is:          Main.hs
     build-depends:    base ^>=4.14.3.0, matrix, mtl
     hs-source-dirs:   day11/src
+    default-language: Haskell2010
+
+executable day12
+    main-is:          Main.hs
+    build-depends:    base ^>=4.14.3.0, containers, split
+    hs-source-dirs:   day12/src
     default-language: Haskell2010

day12/input

@@ -0,0 +1,22 @@
+zs-WO
+zs-QJ
+WO-zt
+zs-DP
+WO-end
+gv-zt
+iu-SK
+HW-zs
+iu-WO
+gv-WO
+gv-start
+gv-DP
+start-WO
+HW-zt
+iu-HW
+gv-HW
+zs-SK
+HW-end
+zs-end
+DP-by
+DP-iu
+zt-start

day12/src/Main.hs

@@ -0,0 +1,43 @@
+import Data.Char (isLower)
+import Data.List.Split (splitOn)
+import Data.Map as M (Map, empty, insertWith, (!))
+
+{-
+Some considerations about the input:
+* There can't be two adjacent big caves
+* There must be a cave called "start" and another one called "end"
+-}
+
+main :: IO ()
+main = do
+  testInput <- readInput "day12/test_input"
+  input <- readInput "day12/input"
+  print $ "Test input: " ++ show (firstProblem testInput) ++ " == 10"
+  print $ "Problem input: " ++ show (firstProblem input) ++ " == 3738"
+  where
+    readInput file = foldl foldLine M.empty . lines <$> readFile file
+    foldLine acc = insertNew acc . splitOn "-"
+    insertNew map (cave1 : cave2 : _) =
+      insertWith (++) cave1 [cave2] $ insertWith (++) cave2 [cave1] map
+    insertNew _ out = error $ "Invalid split: " ++ show out
+
+type Cave = String
+
+isSmall :: Cave -> Bool
+isSmall = isLower . head
+
+type CaveConections = Map Cave [Cave]
+
+firstProblem :: CaveConections -> Int
+firstProblem = length . getPaths
+
+getPaths :: CaveConections -> [[String]]
+getPaths connections = getPaths' ["start"]
+  where
+    getPaths' path@("end" : _) = [reverse path]
+    getPaths' path =
+      let possibleMovements = filterSmall path $ connections ! head path
+       in if null possibleMovements
+            then []
+            else concatMap (getPaths' . (: path)) possibleMovements
+    filterSmall path = filter $ not . (\cave -> isSmall cave && elem cave path)

day12/test_input

@@ -0,0 +1,7 @@
+start-A
+start-b
+A-c
+A-b
+b-d
+A-end
+b-end