Day 2

.gitignore

@@ -1,2 +1,3 @@
 result*
 *.cabal
+dist-newstyle

Main.hs

@@ -3,10 +3,12 @@
 module Main where
 
 import Day1
+import Day2
 
 main :: IO ()
 main = do
   putStrLn "AoC 2024"
   putStrLn "Day 1"
   Day1.main
-
+  putStrLn "Day 2"
+  Day2.main

inputs/day2-example.input

@@ -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

package.yaml

@@ -25,7 +25,7 @@ library:
   source-dirs: src
   exposed-modules:
     - Day1
-    # - Day2
+    - Day2
     # - Day3
     # - Day4
     # - Day5

src/Day2.hs

@@ -0,0 +1,55 @@
+{-# LANGUAGE OverloadedStrings #-}
+
+module Day2 where
+
+-- import Debug.Trace
+
+import Data.Scientific (coefficient)
+import Data.Attoparsec.Text (Parser)
+import Data.Either (fromRight)
+import Data.List (subsequences)
+import qualified Data.Attoparsec.Text as P
+import qualified Data.Text.IO as T
+
+type Report = [Integer]
+
+parseInput :: Parser [Report]
+parseInput = (coefficient <$> P.scientific) `P.sepBy1` " " `P.sepBy1` P.endOfLine
+
+-- Count how many reports are safe
+solveA :: [Report] -> Integer
+solveA = toInteger . length . filter id . fmap isSafe
+
+-- The levels are either all increasing or all decreasing.
+-- Any two adjacent levels differ by at least one and at most three.
+isSafe :: Report -> Bool
+isSafe = check . reportDiff
+  where
+    check dx = (all (>= 0) dx || all (<= 0) dx)
+            && all (\d -> 1 <= abs d && abs d <= 3) dx
+
+reportDiff :: Report -> [Integer]
+reportDiff r = tail $ zipWith (-) r (0:r)
+
+-- Count how many reports are safe, if one can remove up to one
+-- A report can be safe directly OR it can be safe if any of its levels can be
+-- removed to make it safe.
+solveB :: [Report] -> Integer
+solveB = toInteger . length . filter id . fmap isSafeOrAboutSafe
+
+isSafeOrAboutSafe :: Report -> Bool
+isSafeOrAboutSafe r = isSafe r || solveA dropOneLevel > 0
+  where
+    dropOneLevel :: [Report]
+    dropOneLevel = filter ((== (l-1)) . length) $ subsequences r
+    l = length r
+
+main :: IO ()
+main = do
+  _input <- T.readFile "inputs/day2-example.input"
+  input <- T.readFile "inputs/day2.input"
+  putStrLn "Part 1"
+  let xs = fromRight [] $ P.parseOnly parseInput input
+  print $ solveA xs
+  putStrLn "Part 2"
+  print $ solveB xs