Day 9

advent-of-code-2023.cabal

@@ -68,3 +68,11 @@ executable day08
     build-depends:    base ^>=4.15.1.0, hashable, unordered-containers
     hs-source-dirs:   day08
     default-language: Haskell2010
+
+executable day09
+    main-is:          Main.hs
+    other-modules:    Commons Part1 Part2
+
+    build-depends:    base ^>=4.15.1.0, MissingH
+    hs-source-dirs:   day09
+    default-language: Haskell2010

day09/Commons.hs

@@ -0,0 +1,26 @@
+module Commons where
+
+import GHC.IO.Handle (isEOF)
+import Data.List.Utils (split)
+
+
+type History = [Int]
+
+
+parse :: IO [History]
+parse = do done <- isEOF
+           if done
+              then return []
+              else do line <- getLine
+                      let history = map read $ split " " line
+                      otherHistories <- parse
+                      return $ history: otherHistories
+
+
+getDifferences :: History -> History
+getDifferences [h] = []
+getDifferences (h1: h2: t) = h1 - h2: getDifferences (h2: t)
+
+getNextValue :: History -> Int
+getNextValue (0: 0: _) = 0
+getNextValue (h: t) = h + (getNextValue . getDifferences) (h: t)

day09/Main.hs

@@ -0,0 +1,12 @@
+module Main where
+
+import Commons
+import qualified Part1
+import qualified Part2
+
+
+main = do histories <- parse
+          let part1Res = sum $ Part1.getNextValues histories
+          print part1Res
+          let part2Res = sum $ Part2.getNextValues histories
+          print part2Res

day09/Part1.hs

@@ -0,0 +1,7 @@
+module Part1 where
+
+import Commons
+
+
+getNextValues :: [History] -> [Int]
+getNextValues = map $ getNextValue . reverse

day09/Part2.hs

@@ -0,0 +1,7 @@
+module Part2 where
+
+import Commons
+
+
+getNextValues :: [History] -> [Int]
+getNextValues = map getNextValue