adventofcode-2020/day5/main.hs

#! /usr/bin/env -S"ANSWER=42" nix-shell
#! nix-shell -p ghcid
#! nix-shell -p "haskellPackages.ghcWithPackages (p: with p; [])"
#! nix-shell -i "ghcid -c 'ghci -Wall' -T main"

{-# LANGUAGE OverloadedStrings #-}

-- import Debug.Trace (trace)
import Data.List (sort)

binaryPartitionWalker :: String -> Int -> Int -> (String, Int, Int)
binaryPartitionWalker ('F':s) begin end =
  binaryPartitionWalker s begin (((end - begin) `div` 2) + begin)
binaryPartitionWalker ('B':s) begin end =
  binaryPartitionWalker s (((end - begin) `div` 2) + begin + 1) end
binaryPartitionWalker ('L':s) begin end =
  binaryPartitionWalker ('F':s) begin end
binaryPartitionWalker ('R':s) begin end =
  binaryPartitionWalker ('B':s) begin end
binaryPartitionWalker "" b e = ("",b,e)
binaryPartitionWalker _ _ _ = undefined

seatID :: String -> Int
seatID str = row * 8 + seat
  where
    (rowStr, seatStr) = splitAt 7 str
    (_,row,_) = binaryPartitionWalker rowStr 0 127
    (_,seat,_) = binaryPartitionWalker seatStr 0 7

solvePart1 :: [String] -> Int
solvePart1 = maximum . (map seatID)

solvePart2 :: [String] -> Int
solvePart2 = fst . head . (filter (\(x,y) -> x /= y)) . (zip [13..]) . sort . (map seatID)

main :: IO ()
main = do
  putStrLn "Day 5 - Part 1"
  print $ seatID "FBFBBFFRLR"
  print $ seatID "BFFFBBFRRR"
  print $ seatID "FFFBBBFRRR"
  print $ seatID "BBFFBBFRLL"
  input <- lines <$> readFile "day5/input"
  putStrLn $ "Solution of part 1"
  print $ solvePart1 input
  putStrLn $ "Solution of part 2"
  print $ solvePart2 input