Solve day 5, parts 1 and 2

AdventOfCode2020.cabal

@@ -17,7 +17,7 @@ maintainer:          gael@depreeuw.dev
 extra-source-files:  CHANGELOG.md, README.md
 
 library
-  exposed-modules:     Day1, Day2, Day3, Day4
+  exposed-modules:     Day1, Day2, Day3, Day4, Day5
   -- other-modules:
   -- other-extensions:
   build-depends:       base ^>=4.13.0.0

Main.hs

@@ -4,6 +4,7 @@ import Day1 (day1)
 import Day2 (day2)
 import Day3 (day3)
 import Day4 (day4)
+import Day5 (day5)
 
 main :: IO ()
 main = do
@@ -11,3 +12,4 @@ main = do
   day2
   day3
   day4
+  day5

input/day5

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src/Day5.hs

@@ -0,0 +1,30 @@
+module Day5 (day5) where
+
+import Data.List (sort, (\\))
+
+getRow :: String -> Integer
+getRow = snd . foldr go (1, 0)
+  where
+    go 'F' (m, s) = (m * 2, s)
+    go 'B' (m, s) = (m * 2, s + m)
+    go _ _ = error "not possible"
+
+getCol :: String -> Integer
+getCol = snd . foldr go (1, 0)
+  where
+    go 'L' (m, s) = (m * 2, s)
+    go 'R' (m, s) = (m * 2, s + m)
+    go _ _ = error "not possible"
+
+getSeat :: String -> (Integer, Integer)
+getSeat s = let (row, col) = take 3 <$> splitAt 7 s in (getRow row, getCol col)
+
+day5 :: IO ()
+day5 = do
+  r <- readFile "./input/day5"
+  let m = lines r
+  putStr "[Day 5-1] max id: "
+  let ids = sort $ map ((\(a, b) -> 8 * a + b) . getSeat) m
+  print . maximum $ ids
+  putStr "[Day 5-2] my id: "
+  print . head $ (enumFrom (head ids) \\ ids)