Abhinav's Notes
2020-12-05 Edit

Advent of Code 2020 ā€” Week 1

Iā€™m solving the Advent of Code 2020 in the Haskell REPL (GHCi). You can copy the code and paste it in GHCi to play with it. Here are my solutions for week 1 (Dec 1ā€“5):

Problem: https://adventofcode.com/2020/day/1

Solution:

entries <- map read . lines <$> readFile "/tmp/input1" :: IO [Int]
head [i * j | i <- entries, j <- entries, i + j == 2020] -- part 1
head [i * j * k | i <- entries, j <- entries, k <- entries , i + j + k == 2020] -- part 2

Day 2

Problem: https://adventofcode.com/2020/day/2

Solution:

import Data.List.Split (splitOn)
data Policy = Policy Char Int Int
data Entry = Entry Policy String
:{
parse s = let
    [po, pa] = splitOn ": " s
    [pol, poc] = splitOn " " po
    [pon, pox] = splitOn "-" pol
  in Entry (Policy (head poc) (read pon) (read pox)) pa
:}
entries <- map parse . lines <$> readFile "/tmp/input2"
:{
validate (Entry (Policy c min max) pass) =
  let count = length (filter (== c) pass) in count >= min && count <= max
:}
length $ filter validate entries -- part 1
xor a b = if a then not b else b
:{
validate (Entry (Policy c p1 p2) pass) =
  (c == pass !! (p1-1)) `xor` (c == pass !! (p2-1))
:}
length $ filter validate entries -- part 2

Day 3

Problem: https://adventofcode.com/2020/day/3

Solution:

data Z = Z Int String [String]
toZ (x:xs) = Z 0 x xs
goRight (Z l (x:xs) b) = Z (l+1) xs b
goDown (Z l r (x:xs)) = Z l (drop l x) xs
atBottom (Z _ _ b) = null b
zAt (Z _ (x:_) _) = x
import Data.List (unfoldr)
:{
travel move = unfoldr $ \g ->
  if atBottom g then Nothing else let g' = move g in Just (zAt g', g')
:}
treeCount grid = length . filter (== '#') . flip travel grid
f `times` n = (!! n) . iterate f
grid <- toZ . map cycle . lines <$> readFile "/tmp/input3"
import Control.Arrow ((>>>))
treeCount grid (goRight `times` 3 >>> goDown) -- part 1
-- part 2
:{
product $ map (treeCount grid) [
  goRight `times` 3 >>> goDown
, goRight >>> goDown
, goRight `times` 5 >>> goDown
, goRight `times` 7 >>> goDown
, goRight >>> goDown `times` 2]
:}

Day 4

Problem: https://adventofcode.com/2020/day/4

Solution:

import Data.List.Split (splitOn)
splitPair pair = let [a,b] = splitOn ":" pair in (a, b)
:{
passports <-
  map (map splitPair . words) . splitOn "\n\n" <$> readFile "/tmp/input4"
:}
reqFields = ["hcl", "iyr", "eyr", "ecl", "pid", "byr", "hgt"]
:{
fieldsPresent passport =
  let fields = map fst passport in all (`elem` fields) reqFields
:}
length $ filter fieldsPresent passports -- part 1
safeRead s = case reads s of { [(x, "")] -> Just x; _ -> Nothing }
checkLimits min max = maybe False (\y -> y >= min && y <= max) . safeRead
validByr = checkLimits 1920 2002
validIyr = checkLimits 2010 2020
validEyr = checkLimits 2020 2030
import Data.List.Split (split, dropFinalBlank, onSublist)
endBy d = split (dropFinalBlank $ onSublist d)
:{
validHgt hgt
  | [h, "cm"] <- endBy "cm" hgt = checkLimits 150 193 h
  | [h, "in"] <- endBy "in" hgt = checkLimits 59 76 h
  | otherwise = False
:}
import Data.Char (isHexDigit)
validHcl hcl = case hcl of { '#':cs -> all isHexDigit cs; _ -> False }
validEcl = flip elem ["amb", "blu", "brn", "gry", "grn", "hzl", "oth"]
import Data.Char (isDigit)
validPid pid = length pid == 9 && all isDigit pid
:{
validators = [("hcl", validHcl), ("iyr", validIyr), ("eyr", validEyr),
  ("ecl", validEcl), ("pid", validPid), ("byr", validByr), ("hgt", validHgt)]
:}
fieldsValid = all $ \(f, v) -> maybe True ($ v) $ lookup f validators
valid passport = fieldsPresent passport && fieldsValid passport
length $ filter valid passports -- part 2

Day 5

Problem: https://adventofcode.com/2020/day/5

Solution:

:{
partition lower upper (min, max) c
  | c == lower = (min, (min+max) `div` 2)
  | c == upper = ((min+max) `div` 2 + 1, max)
:}
locateRow = fst . foldl (partition 'F' 'B') (0, 127)
locateCol = fst . foldl (partition 'L' 'R') (0, 7)
seatId code = locateRow (take 7 code) * 8 + locateCol (drop 7 code)
codes <- lines <$> readFile "/tmp/input5"
seatIds = map seatId codes
maximum seatIds -- part 1
import Data.List ((\\))
missingSeatIds = [8..1015] \\ seatIds
-- part 2
filter (\id -> (id-1) `elem` seatIds && (id+1) `elem` seatIds) missingSeatIds