aoc-2020-day8

p1.hs

module Main where

import qualified Data.Map.Strict as M
import qualified Data.Set as S

data Inst
  = Nop Int
  | Acc Int
  | Jmp Int

nop = Nop
acc = Acc
jmp = Jmp

program =
  M.fromList $ zip [0..] [
    jmp (336),
    jmp (593),
    jmp (121),
    acc (-8),
    nop (459),
    -- etc... thanks vim
  ]

stopBeforeFirstRepeat prog = go S.empty 0 0
  where
    go seen ip acc | S.member ip seen = (ip, acc)
    go seen ip acc =
      let
        seen' = S.insert ip seen
      in
        case prog M.! ip of
          Nop _ -> go seen' (ip + 1) acc
          Acc v -> go seen' (ip + 1) (acc + v)
          Jmp offset -> go seen (ip + offset) acc

main =
  print $ stopBeforeFirstRepeat program

p2.hs

module Main where

import qualified Data.Map.Strict as M
import qualified Data.Set as S
import Control.Monad (guard)
import Data.Maybe (catMaybes)

data Inst
  = Nop Int
  | Acc Int
  | Jmp Int

isFlippable (Acc _) = False
isFlippable _ = True

flipInst (Nop v) = Jmp v
flipInst (Jmp v) = Nop v
flipInst v = v

nop = Nop
acc = Acc
jmp = Jmp

program =
  M.fromList $ zip [0..] [
    jmp (336),
    jmp (593),
    jmp (121),
    acc (-8),
    nop (459),
    -- etc... thanks vim
  ]

possiblePrograms prog = do
  ip <- [0..M.size prog-1]
  guard $ isFlippable (prog M.! ip)
  pure $ M.adjust flipInst ip prog

finalAcc prog = go S.empty 0 0
  where
    go seen ip acc | S.member ip seen = Nothing
    go seen ip acc =
      let
        seen' = S.insert ip seen
      in
        case M.lookup ip prog  of
          Just (Nop _) -> go seen' (ip + 1) acc
          Just (Acc v) -> go seen' (ip + 1) (acc + v)
          Just (Jmp offset) -> go seen' (ip + offset) acc
          Nothing -> Just acc

main =
  print $ head $ catMaybes $ map finalAcc $ possiblePrograms program