adventofcode-2020/day3/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 Data.List (intersperse)

testData :: [String]
testData = [ "..##......."
           , "#...#...#.."
           , ".#....#..#."
           , "..#.#...#.#"
           , ".#...##..#."
           , "..#.##....."
           , ".#.#.#....#"
           , ".#........#"
           , "#.##...#..."
           , "#...##....#"
           , ".#..#...#.#"
           ]

data Position = Position { x_::Int, y_::Int }

instance Show (Position)
  where
    show p = "("<>show (x_ p)<>","<>show (y_ p)<>")"

-- Returns a half-line starting on (0,0) and following the (x*a,x*b) eq:
line :: Int -> Int -> [Position]
line a b = Position 0 0 : [ Position (x*a) (x*b) | x <- [1..] ]

-- Starting at the top-left corner of your map and following a slope of right 3
-- and down 1, how many trees would you encounter?

-- Requirements for this first task:
-- - datastructure supporting random-access (or similar) interface (lists would work)
-- - function generating a list of (x,y) coordinate to check
-- - must have 2D datastructure, since it's infinite on 1D (x axis)
-- - in this version at least, there's no need to modify the datastructure, so
--   it can be read-only, or even just a function taking a position and returning
--   SquareSortEmpty or SquareSortTree

data SquareSort = SquareSortTree | SquareSortEmpty
  deriving Eq

instance Show SquareSort
  where
    show SquareSortTree   = "#"
    show SquareSortEmpty  = "_"

char2ss :: Char -> Maybe SquareSort
char2ss '#' = Just SquareSortTree
char2ss '.' = Just SquareSortEmpty
char2ss _ = Nothing

str2sss :: String -> Maybe [SquareSort]
str2sss = sequence . (map char2ss)

newtype Grid = Grid [[SquareSort]]

instance Show (Grid)
  where
    show (Grid x) = concat $ intersperse "\n" $ map show x

parseInput :: [String] -> Maybe Grid
parseInput x = Grid <$> sequence ( (map str2sss) x )

getSquareSortAtPosition :: Grid -> Position -> SquareSort
getSquareSortAtPosition (Grid grid) (Position x y) =
  grid !! y !! (x `mod` n)
  where
    n = length (grid !! 0)

solveDay3Part1 :: Grid -> (Int,Int) -> Int
solveDay3Part1 grid@(Grid lx) (x',y')=
  length $ filter (== SquareSortTree) $ map (getSquareSortAtPosition grid) px
  where
    px = take n $ line x' y'
    n = (length lx) `div` y'

-- Determine the number of trees you would encounter if, for each of the
-- following slopes, you start at the top-left corner and traverse the map all
-- the way to the bottom:
--
--   - Right 1, down 1.
--   - Right 3, down 1. (This is the slope you already checked.)
--   - Right 5, down 1.
--   - Right 7, down 1.
--   - Right 1, down 2.
--
-- What do you get if you multiply together the number of trees encountered on
-- each of the listed slopes?

solveDay3Part2 :: Grid -> Int
solveDay3Part2 grid =
  foldl (*) 1 [ solveDay3Part1 grid (1,1)
              , solveDay3Part1 grid (3,1)
              , solveDay3Part1 grid (5,1)
              , solveDay3Part1 grid (7,1)
              , solveDay3Part1 grid (1,2)
              ]

main :: IO ()
main = do
  putStrLn "Day 3 - Part 1"
  inputData <- readFile "day3/input"
  print $ take 5 $ line 3 1
  print $ take 5 $ line 7 1
  let (Just parsedTestData) = parseInput testData
  print parsedTestData
  print $ getSquareSortAtPosition parsedTestData (Position 10 10)
  let (Just parsedInputData) = parseInput (lines inputData)
  putStrLn "Part 1:"
  print $ solveDay3Part1 parsedTestData (3,1)
  print $ solveDay3Part1 parsedInputData (3,1)
  putStrLn "Part 2:"
  print $ solveDay3Part2 parsedTestData  -- This gives the right answer, but
  print $ solveDay3Part2 parsedInputData -- This gives the wrong answer :s