day 9

aoc.sh

@@ -51,3 +51,6 @@ stack exec day07 < input/day07.input
 
 echo -e "${IRed}--- Day 8: Haunted Wasteland ---${Color_Off}"
 stack exec day08 < input/day08.input
+
+echo -e "${IYellow}--- Day 9: Mirage Maintenance ---${Color_Off}"
+stack exec day09 < input/day09.input

aoc2023.cabal

@@ -361,6 +361,7 @@ test-suite spec
       TestDay06
       TestDay07
       TestDay08
+      TestDay09
       Paths_aoc2023
   autogen-modules:
       Paths_aoc2023

src/Day09.hs

@@ -1,10 +1,34 @@
 module Day09 (main, part1, part2) where
 
+parse :: String -> [[Int]]
+parse = map (map read . words) . lines
+
+diff :: [Int] -> [Int]
+diff = zipWith (-) <$> tail <*> id
+
+prediction :: [Int] -> Int
+prediction xs = fst $ until stop step (0, xs)
+  where
+    step :: (Int, [Int]) -> (Int, [Int])
+    step (p, ys) = (p + last ys, diff ys)
+    stop :: (Int, [Int]) -> Bool
+    stop (_, ys) = all (== 0) ys
+
 part1 :: String -> String
-part1 = id
+part1 = show . sum . map prediction . parse
+
+prediction' :: [Int] -> Int
+prediction' xs = foldl merge 0 $ fst $ until stop step ([], xs)
+  where
+    step :: ([Int], [Int]) -> ([Int], [Int])
+    step (ps, ys) = (head ys : ps, diff ys)
+    stop :: ([Int], [Int]) -> Bool
+    stop (_, ys) = all (== 0) ys
+    merge :: Int -> Int -> Int
+    merge acc x = x - acc
 
 part2 :: String -> String
-part2 = const ""
+part2 = show . sum . map prediction' . parse
 
 solve :: String -> String
 solve input = "Part 1: " ++ part1 input ++ "\nPart 2: " ++ part2 input ++ "\n"

test/Main.hs

@@ -8,6 +8,7 @@ import TestDay05
 import TestDay06
 import TestDay07
 import TestDay08
+import TestDay09
 
 main :: IO ()
 main = do
@@ -19,3 +20,4 @@ main = do
     TestDay06.test
     TestDay07.test
     TestDay08.test
+    TestDay09.test

test/TestDay09.hs

@@ -0,0 +1,22 @@
+module TestDay09 (test) where
+
+import Day09
+import Test.Hspec
+
+input :: String
+input = "0 3 6 9 12 15\n 1 3 6 10 15 21\n10 13 16 21 30 45"
+
+test1 :: Expectation
+test1 = part1 input `shouldBe` "114"
+
+test2 :: Expectation
+test2 = part2 input `shouldBe` "2"
+
+test :: IO ()
+test = hspec $ do
+    describe "day09" $ do
+        describe "part1" $ do
+            it "should work for the examples" test1
+
+        describe "part2" $ do
+            it "should work for the examples" test2