aoc-2024-haskell/day2/Main.hs

module Main where

import System.Environment (getArgs)

import Text.Parsec (parse, many1, char, sepBy, endBy)
import Text.Parsec.Char (endOfLine)
import Text.ParserCombinators.Parsec (GenParser)
import Text.ParserCombinators.Parsec.Number (decimal)

type Report = [Level]
type Level = Int

reports :: GenParser Char st [Report]
reports = endBy report endOfLine

report :: GenParser Char st Report
report = sepBy level (many1 (char ' '))

level :: GenParser Char st Level
level = decimal

pairUp :: [a] -> [(a, a)]
pairUp (a:b:tl) = (a, b) : pairUp (b:tl)
pairUp _ = []

allIncOrDec :: Bool -> Report -> Bool
allIncOrDec dampened (a:b:tl) = if a /= b
    then (if dampened then (>=) 0 else (==) 0)
        $ length
        $ filter (not . uncurry (if a < b then (<) else (>)))
        $ pairUp
        $ b:tl
    else dampened && allIncOrDec False (b:tl)
allIncOrDec _ _ = True

notTooDifferent :: Bool -> Report -> Bool
notTooDifferent dampened (a:b:tl) =
    let dif = abs (a - b)
    in if (1 <= dif) && (dif <= 3)
        then notTooDifferent dampened (b:tl)
        else dampened && notTooDifferent False (b:tl)
notTooDifferent _ _ = True

solve :: Bool -> [Report] -> Int
solve dampened = length
    . filter (allIncOrDec dampened)
    . filter (notTooDifferent dampened)

main :: IO ()
main = do
    args <- getArgs
    (fp:_) <- return args
    content <- readFile fp
    case parse reports fp content of
        Left e -> putStrLn ("Input file is incorrectly formatted : " ++ show e)
        Right r -> do
            putStrLn ("Solution (Part 1) : " ++ show (solve False r))
            putStrLn ("Solution (Part 2) : " ++ show (solve True r))