Add 2020 day 8

haskellAoC/src/Y2020/Day08.hs

@@ -0,0 +1,49 @@
+module Y2020.Day08 (y20day08) where
+
+import Data.List
+import Data.List.Split
+import qualified Data.Map as M
+
+data Instr = Acc Int | Jmp Int | Nop Int deriving (Show)
+
+-- current offset, current acc, visited offsets
+type State = (Int, Int, [Int])
+
+runInstructions :: M.Map Int Instr -> State -> (Bool, Int)
+runInstructions instr (cur, acc, visited)
+  | cur `elem` visited           = (False, acc)   -- looping
+  | cur `M.notMember` instr      = (True, acc) -- terminated
+  | otherwise                    = runInstructions instr (newCur, newAcc, newVisited)
+  where newVisited       = cur:visited
+        (newCur, newAcc) = case (instr M.! cur) of
+                             Nop _      -> (cur + 1, acc)
+                             Acc qty    -> (cur + 1, acc + qty)
+                             Jmp ofs    -> (cur + ofs, acc)
+
+parseInput :: String -> Instr
+parseInput input
+  | "nop" `isPrefixOf` input = Nop nb
+  | "acc" `isPrefixOf` input = Acc nb
+  | "jmp" `isPrefixOf` input = Jmp nb
+  where nb                    = (sign raw_sign) 0 number :: Int
+        (raw_sign:raw_number) = (splitOn " " input) !! 1
+        sign '+'              = (+)
+        sign '-'              = (-)
+        number                = read raw_number :: Int
+
+testSwappedNopJmp :: M.Map Int Instr -> Int -> Int
+testSwappedNopJmp original offset = case original M.! offset of
+                                      Acc _     -> next
+                                      Jmp qty   -> resWithSwap (Nop qty)
+                                      Nop qty   -> resWithSwap (Jmp qty)
+  where next                     = testSwappedNopJmp original (offset + 1)
+        resWithSwap swapped      = case (runInstructions (swappedInput swapped) (0, 0, [])) of
+                                     (True, acc) -> acc
+                                     (False, _) -> next
+        swappedInput new         = M.insert offset new original
+
+y20day08 :: [String] -> (String, String)
+y20day08 input = (part1, part2)
+  where part1    = show $ snd $ runInstructions ops (0, 0, [])
+        part2    = show $ testSwappedNopJmp ops 0
+        ops      = M.fromList $ zip (iterate (+ 1) 0) $ map parseInput input

haskellAoC/src/Y2020/Days.hs

@@ -7,6 +7,7 @@ import Y2020.Day04
 import Y2020.Day05
 import Y2020.Day06
 import Y2020.Day07
+import Y2020.Day08
 
 year2020 :: String -> [String] -> (String, String)
 year2020 "01" = y20day01
@@ -16,3 +17,4 @@ year2020 "04" = y20day04
 year2020 "05" = y20day05
 year2020 "06" = y20day06
 year2020 "07" = y20day07
+year2020 "08" = y20day08