Add 2020 day 9

haskellAoC/src/Y2020/Day09.hs

@@ -0,0 +1,39 @@
+module Y2020.Day09 (y20day09) where
+
+import Data.Maybe
+
+hasSumInList :: Int -> [Int] -> Bool
+hasSumInList _ [] = False
+hasSumInList nb (x:xs)
+  | (nb - x) `elem` xs = True
+  | otherwise = hasSumInList nb xs
+
+walkTilInvalid :: [Int] -> [Int] -> Int
+walkTilInvalid (cur:next) last25
+  | hasSumInList cur last25 = walkTilInvalid next (cur:(init last25))
+  | otherwise = cur
+walkTilInvalid _ _ = -1
+
+getSum :: [Int] -> Int -> Maybe [Int]
+getSum [] _ = Nothing
+getSum (y:ys) t
+  | y > t = Nothing
+  | y == t = Just [y]
+  | otherwise = fmap (\zs -> y:zs) (getSum ys (t-y))
+
+findContiguousSum :: [Int] -> Int -> [Int]
+findContiguousSum (x:xs) target
+  | x >= target = findContiguousSum xs target
+  | isJust sumStartingThere = fromJust sumStartingThere
+  | otherwise = findContiguousSum xs target
+  where sumStartingThere = getSum xs target
+
+y20day09 :: [String] -> (String, String)
+y20day09 input = (part1, part2)
+  where part1 = show $ firstInvalid
+        part2 = show $ (minimum contiguousSum + maximum contiguousSum)
+        input' = map read $ input :: [Int]
+        preamble = reverse $ take 25 $ input'
+        rest = drop 25 $ input'
+        firstInvalid = walkTilInvalid rest preamble
+        contiguousSum = findContiguousSum input' firstInvalid

haskellAoC/src/Y2020/Days.hs

@@ -8,6 +8,7 @@ import Y2020.Day05
 import Y2020.Day06
 import Y2020.Day07
 import Y2020.Day08
+import Y2020.Day09
 
 year2020 :: String -> [String] -> (String, String)
 year2020 "01" = y20day01
@@ -18,3 +19,4 @@ year2020 "05" = y20day05
 year2020 "06" = y20day06
 year2020 "07" = y20day07
 year2020 "08" = y20day08
+year2020 "09" = y20day09