aoc-2024-haskell/day2/Main.hs

module Main where

import System.Environment (getArgs)

import Control.Exception (assert)

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 _ = []

isSafe :: Report -> Bool
isSafe (a:b:tl) = a /= b && (==) 0 (
    length $ filter (not . (\(l, r) -> let
                dif = abs (l - r)
                cmp = if a < b then (<) else assert (b < a) (>)
            in (1 <= dif) && (dif <= 3) && l `cmp` r))
        $ pairUp
        $ a:b:tl)
isSafe _ = True

with1Removed :: Report -> [Report]
with1Removed (h:tl) = tl : map (h :) (with1Removed tl)
with1Removed [] = []

isKindaSafe :: Report -> Bool
isKindaSafe = any isSafe . with1Removed

part1 :: [Report] -> Int
part1 = length . filter isSafe

part2 :: [Report] -> Int
part2 = length . filter isKindaSafe

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 (part1 r))
            putStrLn ("Solution (Part 2) : " ++ show (part2 r))