Add solution to day 2.

2024/haskell/app/day2/Day2.hs

@@ -0,0 +1,47 @@
+module Main (main) where
+
+-- | Part 1
+type Report = [Int]
+
+grabReports :: String -> [Report]
+grabReports = map (map (\x -> read x :: Int) . words) . lines
+
+isReportSafeWith :: (Int -> Int -> Bool) -> Report -> Bool
+isReportSafeWith _ [] = True
+isReportSafeWith _ [_] = True
+isReportSafeWith f (x : y : xs) = f x y && isReportSafeWith f (y : xs)
+
+reportDecreasing :: Int -> Int -> Bool
+reportDecreasing x y = 1 <= x - y && x - y <= 3
+
+reportIncreasing :: Int -> Int -> Bool
+reportIncreasing x y = 1 <= y - x && y - x <= 3
+
+isReportSafe :: Report -> Bool
+isReportSafe report = isReportSafeWith reportDecreasing report || isReportSafeWith reportIncreasing report
+
+part1 :: String -> IO ()
+part1 = print . length . filter isReportSafe . grabReports
+
+-- | Part 2
+removeItem :: Int -> Report -> Report
+removeItem n report = removeItem' n report 0
+  where
+    removeItem' :: Int -> Report -> Int -> Report
+    removeItem' _ [] _ = []
+    removeItem' n' (r : rs) i
+      | n' == i = removeItem' n' rs (i + 1)
+      | otherwise = r : removeItem' n' rs (i + 1)
+
+reportPermutations :: Report -> [Report]
+reportPermutations report = report : [removeItem i report | i <- [0 .. length report - 1]]
+
+isReportSafePerm :: Report -> Bool
+isReportSafePerm report = any isReportSafe (reportPermutations report)
+
+part2 :: String -> IO ()
+part2 = print . length . filter isReportSafePerm . grabReports
+
+-- | Main
+main :: IO ()
+main = readFile "resources/day2.txt" >>= part2

2024/haskell/haskell.cabal

@@ -37,7 +37,7 @@ library
       base >=4.7 && <5
   default-language: Haskell2010
 
-executable aoc2024-day1
+executable day1
   main-is: Day1.hs
   other-modules:
       Paths_haskell
@@ -51,6 +51,20 @@ executable aoc2024-day1
     , haskell
   default-language: Haskell2010
 
+executable day2
+  main-is: Day2.hs
+  other-modules:
+      Paths_haskell
+  autogen-modules:
+      Paths_haskell
+  hs-source-dirs:
+      app/day2
+  ghc-options: -Wall -Wcompat -Widentities -Wincomplete-record-updates -Wincomplete-uni-patterns -Wmissing-export-lists -Wmissing-home-modules -Wpartial-fields -Wredundant-constraints -threaded -rtsopts -with-rtsopts=-N
+  build-depends:
+      base >=4.7 && <5
+    , haskell
+  default-language: Haskell2010
+
 test-suite haskell-test
   type: exitcode-stdio-1.0
   main-is: Spec.hs

2024/haskell/package.yaml

@@ -37,7 +37,7 @@ library:
   source-dirs: src
 
 executables:
-  aoc2024-day1:
+  day1:
     main:                Day1.hs
     source-dirs:         app/day1
     ghc-options:
@@ -47,6 +47,16 @@ executables:
     dependencies:
     - haskell
 
+  day2:
+    main:                Day2.hs
+    source-dirs:         app/day2
+    ghc-options:
+    - -threaded
+    - -rtsopts
+    - -with-rtsopts=-N
+    dependencies:
+    - haskell
+
 tests:
   haskell-test:
     main:                Spec.hs

2024/haskell/resources/day2_example.txt

@@ -0,0 +1,6 @@
+7 6 4 2 1
+1 2 7 8 9
+9 7 6 2 1
+1 3 2 4 5
+8 6 4 4 1
+1 3 6 7 9