Futhark

I translated my Haskell solution to Futhark, basically. It runs abysmally faster.

The syntax highlighting is likely very off, because the closest language highlighter I could find was ocaml.

def fst 'a 'b ((a, _b): (a, b)): a = a
def snd 'a 'b ((_a, b): (a, b)): b = b
def (>>>) 'a 'b 'c (f: a -> b) (g: b -> c) (x: a): c = g (f x)
def (|) '^a 'b (f: a -> b) (x: a): b = f x -- $ is not allowed
def even (x: i64): bool = x % 2 == 0

def digitCount (x: i64): i64
  = snd | 
      loop (i, len) = (x, 0)
      while i != 0
      do (i / 10, len + 1)

def digitAt (n: i64) (i: i64): i64 = (n / 10 ** i) % 10

def keepTrue (p: i64 -> bool) (x: i64): i64
  = if p x
      then x
      else 0

def tup2RangeArray ((start, end): (i64, i64)): []i64
  = (start ... end)

def sumInvalidIds (p: i64 -> bool) (rangeTup: (i64, i64)): i64 
  = let range = tup2RangeArray rangeTup in
  reduce (+) 0 (map (keepTrue p) range)

def tup2FromArray 'a (as: [2]a): (a, a) = (as[0], as[1])

def impl (p: i64 -> bool) (ranges: [](i64, i64)): i64 
  = reduce (+) 0 (map (sumInvalidIds p) ranges)

def withValidRepeatOffsets (nDigits: i64) (f: i64 -> bool): bool
  = match nDigits
    case 2  -> map f >>> or | [1]
    case 3  -> map f >>> or | [1]
    case 4  -> map f >>> or | [1, 2]
    case 5  -> map f >>> or | [1]
    case 6  -> map f >>> or | [1, 2, 3]
    case 7  -> map f >>> or | [1]
    case 8  -> map f >>> or | [1, 2, 4]
    case 9  -> map f >>> or | [1, 3]
    case 10 -> map f >>> or | [1, 2, 5]
    case 11 -> map f >>> or | [1]
    case 12 -> map f >>> or | [1, 2, 3, 4, 6]
    case _ -> false

def isValid2 (x: i64): bool = 
  let len = digitCount x in
  let lookupDigit = digitAt x in
  withValidRepeatOffsets len | \ repeatOffset ->
    let repeatCount = len / repeatOffset in
    let digitIndices = (0..< repeatOffset) in
    let repeatIndices = (0..<repeatCount) in
    and | 
      map (\ digitIndex -> 
        and |
          map (\ repeatIndex -> 
            let expectedDigit = lookupDigit digitIndex in
            let actualDigit   = lookupDigit | repeatIndex * repeatOffset + digitIndex in
            expectedDigit == actualDigit
          ) 
          repeatIndices
      ) digitIndices

def part2 : [](i64, i64) -> i64 = impl isValid2 

def isValid1 (x: i64): bool = 
  let len = digitCount x in
  let halfLength = len / 2 in
  let first = x / 10 ** halfLength in
  let second = x % 10 ** halfLength in
  even len && first == second

def part1 : [](i64, i64) -> i64 = impl isValid1 

def main (rangeArrays: [][2]i64) 
  = let rangeTuples = map tup2FromArray rangeArrays in
    (part1 rangeTuples, part2 rangeTuples)

i [
s/\([0-9]\+\)-\([0-9]\+\)/\[\1, \2]/g
a ]

Futhark

I am on my way to re-do all previous days in Futhark and complete the Rest of AoC, hopefully.

def hole: u8 = 0
def zipIndices 'a (xs: []a): [](i64, a) = zip (indices xs) xs
def foldMin (xs: []u8): (i64, u8) = 
  let indexedXs = tail (zipIndices xs) in
  let start = (0, head xs) in
  foldl (\ (ci, cv) (ni, nv) -> if nv > cv then (ni, nv) else (ci, cv)) start indexedXs

def slice 'a (xs: []a) (start: i64) (end: i64) = drop start (take end xs)

def pickBattery (bank: []u8) (reserved: i64): (i64, u8) = 
  let batteries = slice bank 0 (length bank - reserved) in
  foldMin batteries

def pickNBatteries (n: i8) (banks: []u8): u64 =
  let (_, result) =
    loop (batteries, sum) = (banks, 0)
    for i in reverse (0...n-1)
    do
      let (offset, battery) = pickBattery batteries (i64.i8 i) in
      (drop (offset + 1) batteries, sum * 10 + u64.u8 battery)
  in result

def part1 (banks: [][]u8): u64 = reduce (+) 0 (map (pickNBatteries 2) banks)

def part2 (banks: [][]u8): u64 = reduce (+) 0 (map (pickNBatteries 12) banks)

def main (banks: [][]u8) = (part1 banks, part2 banks)

{-# OPTIONS_GHC -Wall #-}
{-# LANGUAGE OverloadedStrings #-}
import qualified Data.Text.IO as TextIO
import Control.Monad ((<$!>))
import qualified Data.Array.Unboxed as Array
import qualified Data.Text as Text
import qualified Data.Char as Char
import Data.Array.Unboxed (UArray)
import qualified Data.List as List
import qualified Data.ByteString as ByteString
import Data.Word (byteSwap64, Word64)
import GHC.ByteOrder (ByteOrder(..), targetByteOrder)
import qualified Data.Bits as Bits

parse :: Text.Text -> UArray (Int, Int) Int
parse t = let
    banks = init $ Text.lines t
    bankSize = maybe 0 pred $ Text.findIndex (== '\n') t
    bankCount = Text.count "\n" t - 2
  in Array.listArray ((0, 0), (bankCount, bankSize)) $ List.concatMap (fmap Char.digitToInt . Text.unpack) banks

rowsOf :: UArray (Int, Int) Int -> Int
rowsOf = fst . snd . Array.bounds

colsOf :: UArray (Int, Int) Int -> Int
colsOf = snd . snd . Array.bounds

byteStringLeWord64 :: Word64 -> ByteString.ByteString
byteStringLeWord64 word = let
    leWord = case targetByteOrder of
      BigEndian -> byteSwap64 word
      LittleEndian -> word
  in ByteString.pack . map (fromIntegral . (leWord `Bits.shiftR`)) $ [0,8..56]

main :: IO ()
main = do
  batteryBanks <- parse <$!> TextIO.getContents
  putChar 'b'
  ByteString.putStr (ByteString.singleton 2) -- version
  ByteString.putStr (ByteString.singleton 2) -- dimensions
  TextIO.putStr "  u8" -- type
  ByteString.putStr (byteStringLeWord64 . fromIntegral . succ . rowsOf $ batteryBanks) -- outer dim
  ByteString.putStr (byteStringLeWord64 . fromIntegral . succ . colsOf $ batteryBanks) -- inner dim
  ByteString.putStr . ByteString.pack . fmap fromIntegral . Array.elems $ batteryBanks -- elements

Futhark

Only part 1 so far, I want to do part 2 later too.

This is my first ever futhark program. I have not yet figured out whether string parsing is possible or intended with this language. I used a combination of sed and vim to bring the input into a form futhark can read.

def neighbors (x: i32, y: i32): [8](i32, i32) = [(x+1, y+1), (x+1, y), (x+1, y-1), (x, y+1), (x, y-1), (x-1, y+1), (x-1, y), (x-1, y-1)]

def count 't (p: t -> bool) (xs: []t) : i32 = reduce (+) 0 (map (\ x -> i32.bool (p x)) xs)
def count2 't (p: t -> bool) (xs: [][]t) : i32 = reduce (+) 0 (map (count p) xs)

def zipIndices [n] 't (xs: [n]t): [n](i32, t) = zip (map i32.i64 (indices xs)) xs
def zipIndices2 [n][m] 't (xs: [m][n]t): [m][n]((i32, i32), t) = 
  let innerIndices = map zipIndices xs in
  let innerAndOuterIndices = zipIndices innerIndices in
  map (\ (r, a) -> map (\ (c, x) -> ((r, c), x)) a) innerAndOuterIndices

def countIndexed2 't (p: (i32, i32) -> t -> bool) (xs: [][]t): i32 = 
  let withIndices = zipIndices2 xs in
  count2 (\ (i, x) -> p i x) withIndices

type option 't
  = #single t
  | #empty

def safeIndex 't (xs: []t) (i: i32): option t = if i32.i64 (length xs) > i && i >= 0
  then #single xs[i]
  else #empty

def safeIndex2 't (xs: [][]t) ((r, c): (i32, i32)): option t = 
  match safeIndex xs r
    case #single a -> safeIndex a c
    case #empty -> #empty

def orElse 't (o: option t) (d: t): t =
  match o
    case #single x -> x
    case #empty    -> d

def isAccessible (grid: [][]bool) (p: (i32, i32)) (x:bool): bool =
  let neighborsOptions = map (safeIndex2 grid) (neighbors p) in
  let neighborsFilled = map (`orElse` false) neighborsOptions in
  x && count id neighborsFilled < 4

def mapIndexed2 'a 'b (f: (i32, i32) -> a -> b) (xs: [][]a): [][]b =
  let withIndices = zipIndices2 xs in
  map (map (\ (i, x) -> f i x)) withIndices

def removeAccessibles (grid: [][]bool): [][]bool = mapIndexed2 (\ p x -> x && not (isAccessible grid p x)) grid

def part1 (grid: [][]bool): i32 = countIndexed2 (isAccessible grid) grid
def part2 (grid: [][]bool): i32 =
  let (reducedGrid, _) = 
    loop (current, last) = (removeAccessibles grid, grid)
    while current != last
    do 
      let current' = removeAccessibles current in
      let last'    = copy current in
      (current', last')
  in count2 id grid - count2 id reducedGrid

def main (grid: [][]bool) = (part1 grid, part2 grid)

The highlighting is a bit off because I used ocaml as the language. There is no futhark highlighter (at least in Web UI) yet.
Edit: Part2

Also, it runs blazingly fast 🚀 :O, even in sequential C mode

Haskell

I tried rewriting part 2 to use a MutableArray, but it only made everything slower. So I left it at this. I saw somebody do a 1-second-challenge last year and I feel like that will be very hard unless I up my performance game.

{-# LANGUAGE OverloadedStrings #-}
{-# OPTIONS_GHC -Wall #-}
module Main (main) where
import qualified Data.Text as Text
import Data.Array.Unboxed (UArray)
import qualified Data.Array.IArray as Array
import qualified Data.List as List
import Control.Monad ((<$!>), guard)
import qualified Data.Text.IO as TextIO
import Data.Maybe (fromMaybe)
import Control.Arrow ((&&&))

parse :: Text.Text -> UArray (Int, Int) Bool
parse t = let
    gridLines = init $ Text.lines t
    lineSize = maybe 0 pred $ Text.findIndex (== '\n') t
    lineCount = Text.count "\n" t - 2
  in Array.listArray ((0, 0), (lineCount, lineSize)) $ List.concatMap (fmap (== '@') . Text.unpack) gridLines

neighbors8 :: (Int, Int) -> [(Int, Int)]
neighbors8 p@(x, y) = do
  x' <- [pred x .. succ x]
  y' <- [pred y .. succ y]
  let p' = (x', y')
  guard (p /= p')
  pure p'

main :: IO ()
main = do
  grid <- parse <$!> TextIO.getContents
  print $ part1 grid
  print $ part2 grid

part2 :: UArray (Int, Int) Bool -> Int
part2 grid = case accessiblePositions grid of
  [] -> 0
  xs -> List.length xs + part2 (grid Array.// fmap (id &&& const False) xs)

part1 :: UArray (Int, Int) Bool -> Int
part1 = List.length . accessiblePositions

accessiblePositions :: UArray (Int, Int) Bool -> [(Int, Int)]
accessiblePositions grid = let
     lookupPosition = fromMaybe False . (grid Array.!?)
     positions = Array.indices grid
     paperRollPositions = List.filter lookupPosition positions
     isPositionAccessible = (< 4) . List.length . List.filter lookupPosition . neighbors8
   in List.filter isPositionAccessible paperRollPositions

Haskell

Usually, I get up for AoC, way earlier than I normally would. But today I had to get up at exactly AoC time. I ended up postponing the puzzles until now:

{-# OPTIONS_GHC -Wall #-}
import qualified Data.Text.IO as TextIO
import Control.Monad ((<$!>))
import qualified Data.Array.Unboxed as Array
import qualified Data.Text as Text
import qualified Data.Char as Char
import Data.Array.Unboxed (UArray)
import qualified Data.Foldable as Foldable
import Control.Arrow ((&&&))
import qualified Data.List as List

parse :: Text.Text -> UArray (Int, Int) Int
parse t = let
    banks = init $ Text.lines t
    bankSize = maybe 0 pred $ Text.findIndex (== '\n') t
    bankCount = Text.count "\n" t - 2
  in Array.listArray ((0, 0), (bankCount, bankSize)) $ List.concatMap (fmap Char.digitToInt . Text.unpack) banks

rowsOf :: UArray (Int, Int) Int -> Int
rowsOf = fst . snd . Array.bounds

colsOf :: UArray (Int, Int) Int -> Int
colsOf = snd . snd . Array.bounds

main :: IO ()
main = do
  batteryBanks <- parse <$!> TextIO.getContents
  print $ part1 batteryBanks
  print $ part2 batteryBanks

part1 :: UArray (Int, Int) Int -> Int
part1 batteryBanks = Foldable.sum $ pickBatteries 2 batteryBanks <$> [0.. rowsOf batteryBanks]

part2 :: UArray (Int, Int) Int -> Int
part2 banks = Foldable.sum $ pickBatteries 12 banks <$> [0.. rowsOf banks]

pickBatteries :: Int -> UArray (Int, Int) Int -> Int -> Int
pickBatteries batteryCount banks row = let
    width = colsOf banks
    getBattery col = banks Array.! (row, col)

    go acc 0 _      = acc
    go acc n offset = let
        effectiveEnd = width - pred n
        availableIndices = [offset .. effectiveEnd]
        batteryWithIndices = (id &&& getBattery) <$> availableIndices
        (offset', selectedBattery) = maximumOn snd batteryWithIndices
      in go (acc * 10 + selectedBattery) (pred n) (succ offset')
  in go 0 batteryCount 0

maximumOn :: (Foldable t, Ord b) => (a -> b) -> t a -> a
maximumOn f collection = case Foldable.toList collection of
  [] -> error "maximumOn: empty foldable"
  (x:xs) -> List.foldl selectMax x xs
  where
    selectMax a b = if f a < f b then b else a

Easy one to get through, no edge-cases biting me this time.

I learned this year again: running in interpreted mode can cause significant slowdowns. Later, I’ll hopefully find the time clean it up, this solution feels ugly. Reading everyone else did it also like this or with regex makes me feel better about it though.

Haskell

module Main (main) where
import qualified Text.ParserCombinators.ReadP as ReadP
import Numeric.Natural (Natural)
import Control.Monad ((<$!>), guard)
import qualified Data.List as List
import Control.Arrow ((>>>))
import qualified Data.Text as Text
import qualified Data.Foldable as Foldable

newtype Range = Range { getRange :: (Natural, Natural) }
  deriving Show

parseRange :: ReadP.ReadP Range
parseRange = do
  n1 <- ReadP.readS_to_P reads
  _ <- ReadP.char '-'
  n2 <- ReadP.readS_to_P reads
  pure . Range $ (n1, n2)

parseLine :: ReadP.ReadP [Range]
parseLine = parseRange `ReadP.sepBy` ReadP.char ','

main :: IO ()
main = do
  ranges <- fst . last . ReadP.readP_to_S parseLine <$!> getContents
  print $ part1 ranges
  print $ part2 ranges

part1 :: [Range] -> Natural
part1 = List.concatMap (uncurry enumFromTo . getRange)
  >>> List.filter isDoublePattern
  >>> Foldable.sum

part2 :: [Range] -> Natural
part2 = List.concatMap (uncurry enumFromTo . getRange)
  >>> List.filter isMultiplePattern
  >>> Foldable.sum

isMultiplePattern :: Natural -> Bool
isMultiplePattern n = let
    textN = Text.show n
    textLength = Text.length textN
  in flip any (divisorsOf textLength) $ \ divisor -> let
      patternLength = textLength `div` divisor
      patternPart = Text.take (fromIntegral patternLength) textN
    in Text.replicate (fromIntegral divisor) patternPart == textN

isDoublePattern :: Natural -> Bool
isDoublePattern n = let
  textN = Text.show n
  evenLength = even (Text.length textN)
  (first, second) = Text.splitAt (Text.length textN `div` 2) textN
  in evenLength && first == second

divisorsOf :: Integral b => b -> [b]
divisorsOf n = do
  x <- [2..n]
  guard ((n `mod` x) == 0)
  pure x

Using the interpreter, this solution made me wait for two minutes until I could submit. x.x After testing it again in compiled mode, it only takes four seconds.

Thank you for the excellent question. This made me reflect on my coding style and why I actually chose this. Maybe you have noticed, my usage of LambdaCase is inconsistent: I didn’t use it in the definition of foldRotation. Which happened with some refactorings (You couldn’t know that, I didn’t tell anywhere), but still.

After going through some ‘old’ code I found that I didn’t start using it until early this year. (For context: I started doing Haskell in September 2024) But that may just coincide with me installing HLS.

Anyway, back to the topic: I actually think it’s very elegant because it saves re-typing the function name and/or other parameters. It also easily allows me to add further arguments to the function (but only before the last one). In my mind, this is where LambdaCase shines.

Sometimes I end up refactoring functions because it’s very hard to match on multiple arguments using LambdaCase. I also try to avoid adding arguments in the back, which might bite me later and limits flexibility a lot.

I think that’s a really cool usage of the Writer Monad. I thought about the State Monad but didn’t end up using it. Was too hectic for me. Thanks for showing and sharing this!

How could I run this code? Do you use some kind of framework for AoC?

The struggled with a counting solution for a long time. I submitted with a simple enumerative solution in the end but managed to get it right after some pause time:

Haskell

{-# LANGUAGE LambdaCase #-}
{-# LANGUAGE OrPatterns #-}
module Main (main) where

import Control.Monad ( (<$!>) )
import qualified Data.List as List

main :: IO ()
main = do
  rotations <- (fmap parseRotation . init . lines) <$!> getContents
  print $ part1 rotations
  print $ part2 rotations

part2 :: [Either Int Int] -> Int
part2 rotations = let

    foldRotation (position, zeroCount) operation = case operation of
      Left y -> let
        (zeroPasses, y') = y `divMod` 100
        position' = (position - y') `mod` 100
        zeroCount' = zeroPasses + zeroCount + if position <= y' then fromEnum $ position /= 0 else 0
        in (position', zeroCount')
      Right y -> let
        (zeroPasses, y') = y `divMod` 100
        position' = (position + y') `mod` 100
        zeroCount' = zeroPasses + zeroCount + if y' + position >= 100 then 1 else 0
        in (position', zeroCount')

  in snd $ List.foldl' foldRotation (50, 0) rotations

part1 :: [Either Int Int] -> Int
part1 rotations = let
    positions = List.scanl applyRotation 50 rotations
  in List.length . filter (== 0) $ positions

applyRotation :: Int -> Either Int Int -> Int
applyRotation x = \case
  Left y -> (x - y) `mod` 100
  Right y -> (x + y) `mod` 100

parseRotation :: String -> Either Int Int
parseRotation = \case
  'R':rest -> Right $ read rest
  'L':rest -> Left $ read rest
  bad -> error $ "invalid rotation operation: " ++ bad

-- | Old solution enumerating all the numbers

part2' :: [Either Int Int] -> Int
part2' rotations = let
  intermediatePositions _ [] = []
  intermediatePositions x (op:ops) = case op of
    Left 0; Right 0 -> intermediatePositions x ops
    Left y -> let x' = pred x `mod` 100 in x' : intermediatePositions x' (Left (pred y) : ops)
    Right y -> let x' = succ x `mod` 100 in x' : intermediatePositions x' (Right (pred y) : ops)
  in List.length . List.filter (== 0) . intermediatePositions 50 $ rotations

Nice Setup and Picture and also the lighting!

Seriously though, how do you take such pictures? I tried and failed multiple times already. Too dark, bad angles, blinding lights, you name it.

Thank you for this update. Now that problwm and solution fit, I can understand whats going on in your code :]

I was scared of a hard combinatorial puzzle day, but this was a breeze.

{-# LANGUAGE TupleSections #-}
module Main (main) where
import Control.Monad ((<$!>))
import qualified Data.Text.IO as TextIO
import Data.Text (Text)
import qualified Data.Text as Text
import qualified Data.IntSet as IntSet
import Control.Arrow ((>>>))
import qualified Data.List as List
import qualified Data.IntMap as IntMap

part1 :: [IntSet.Key] -> IntSet.Key
part1 = IntSet.fromList
  >>> IntSet.foldl (+) 0

part2 :: [IntSet.Key] -> IntSet.Key
part2 = IntSet.fromList
  >>> IntSet.toAscList
  >>> take 20
  >>> sum

part3 :: [IntMap.Key] -> Int
part3 = List.map (, 1)
  >>> IntMap.fromListWith (+)
  >>> IntMap.toList
  >>> List.map snd
  >>> maximum

main :: IO ()
main = do
  sizes <- map (read . Text.unpack) . Text.split (== ',') <$!> TextIO.getLine
  print $ part1 sizes
  print $ part2 sizes
  print $ part3 sizes

I struggled for a long time because I had nearly the correct results. I had to switch div with quot.

This puzzle was fun. If you have a visualization, it’s even cooler. (It’s a fractal)

{-# LANGUAGE LambdaCase #-}
{-# LANGUAGE PatternSynonyms #-}
{-# OPTIONS_GHC -Wall #-}
module Main (main) where
import Text.Read (ReadPrec, Read (readPrec))
import Data.Functor ((<&>))
import Data.Text (pattern (:<), Text)
import qualified Data.Text as Text
import qualified Data.Text.IO as TextIO
import Control.Monad ((<$!>))
import Control.Arrow ((<<<))

newtype Complex = Complex (Int, Int)

instance Read Complex where
  readPrec :: ReadPrec Complex
  readPrec = readPrec <&> \case
    [a, b] -> Complex (a, b)
    _ -> undefined

instance Show Complex where
  show :: Complex -> String
  show (Complex (a, b))= show [a, b]

readAEquals :: Text -> Complex
readAEquals ('A' :< '=':< rest) = read $ Text.unpack rest
readAEquals _ = undefined


-- >>> Complex (1, 1) `add` Complex (2, 2)
-- [3,3]

add :: Complex -> Complex -> Complex
(Complex (x1, y1)) `add` (Complex (x2, y2)) = Complex (x1 + x2, y1 + y2)

-- >>> Complex (2, 5) `times` Complex (5, 7)
-- [-25,-11]

times :: Complex -> Complex -> Complex
(Complex (x1, y1)) `times` (Complex (x2, y2)) = Complex (x1 * x2 - y1 * y2, x1 * y2 + x2 * y1)

dividedBy :: Complex -> Complex -> Complex
(Complex (x1, y1)) `dividedBy` (Complex (x2, y2)) = Complex (x1 `quot` x2, y1 `quot` y2)

step :: Complex -> Complex -> Complex
step a r = let
 r1 = r `times` r
 r2 = r1 `dividedBy` Complex (10, 10)
 r3 = r2 `add` a
 in r3

zero :: Complex
zero = Complex (0, 0)

part1 :: Complex -> Complex
part1 a = iterate (step a) (Complex (0, 0)) !! 3

shouldBeEngraved :: Complex -> Bool
shouldBeEngraved complexPoint = let

  cycleStep :: Complex -> Complex -> Complex
  cycleStep point r = let
    r2 = r `times` r
    r3 = r2 `dividedBy` Complex (100000, 100000)
    in point `add` r3

  inRange x = x <= 1000000 && x >= -1000000


  in all (\ (Complex (x, y)) -> inRange x && inRange y)
    <<< take 101
    <<< iterate (cycleStep complexPoint)
    $ zero

-- >>> shouldBeEngraved $ Complex (35630,-64880)
-- True
-- >>> shouldBeEngraved $ Complex (35460, -64910)
-- False
-- >>> shouldBeEngraved $ Complex (35630, -64830)
-- False

part2 :: Complex -> Int
part2 (Complex (xA, yA)) = let

    xB = xA + 1000
    yB = yA + 1000

  in length . filter shouldBeEngraved $ do
    x <- [xA, xA+10.. xB]
    y <- [yA, yA+10.. yB]
    pure $ Complex (x, y)

part3 :: Complex -> Int
part3 (Complex (xA, yA)) = length . filter shouldBeEngraved $ do
  x <- [xA..xA+1000]
  y <- [yA..yA+1000]
  pure $ Complex (x, y)

-- >>> [0, 10..100]
-- [0,10,20,30,40,50,60,70,80,90,100]

main :: IO ()
main = do
  a <- readAEquals <$!> TextIO.getContents
  print $ part1 a
  print $ part2 a
  print $ part3 a

My girlfriend is learning python, we are taking on the challenges together, today I may upload her solution:

A=[-3344,68783]
R = [0, 0]
B= [A[0]+1000, A[1]+1000]
pointsengraved = 0
cycleright = 0


for i in range(A[1], B[1]+1):
    for j in range(A[0], B[0]+1):
        for k in range(100):
            R = [int(R[0] * R[0] - R[1] * R[1]), int(R[0] * R[1] + R[1] * R[0])]
            R = [int(R[0] / 100000), int(R[1] / 100000)]
            R = [int(R[0] + j), int(R[1] + i)]
            if -1000000>R[0] or R[0]>1000000 or -1000000>R[1] or R[1]>1000000:
                #print(".", end="")
                break
            cycleright += 1
        if cycleright == 100:
            pointsengraved += 1
            #print("+", end="")
        cycleright = 0
        R = [0, 0]
    #print()

print(pointsengraved)

The commented out print statements produce an ascii map of the set, which can be cool to view at the right font size.

I’m very curious about F# since I’ve never used it or seen it anywhere before but I’m afraid I’m too tired to read it right now. Thank you for posting, I hope I’ll remember to come back tomorrow.

I coded this along with my girlfriend who’s learning python, but not motivated to share her solution. The program reads from stdin, because I usually invoke it like so: runhaskell Main.hs < input or runhaskell Main.hs < example. I think this is quite handy because I don’t have to change the source code to check the example input again.

I struggled with Part 3, where I suddenly forgot I could’ve simply used mod, which I ended up doing anyway. I immediately recognized that Part 3 needs Mutable Arrays if I care to avoid Index hell, which is not what I wanted to with Haskell but oh well.

{-# OPTIONS_GHC -Wall #-}
{-# LANGUAGE PatternSynonyms #-}
module Main (main) where

import qualified Data.Text as Text
import qualified Data.Text.IO as TextIO
import Control.Monad ((<$!>), forM_)
import Data.Text (Text, pattern (:<))
import qualified Data.List as List
import qualified Data.Array.MArray as MutableArray
import Control.Monad.ST (runST, ST)
import Data.Array.ST (STArray)

commaSepLine :: IO [Text.Text]
commaSepLine = Text.split (== ',') <$!> TextIO.getLine

readInstruction :: Text -> Int
readInstruction ('R' :< n) = read . Text.unpack $ n
readInstruction ('L' :< n) = negate . read . Text.unpack $ n
readInstruction _ = undefined

myName :: (Foldable t, Ord b, Enum b, Num b) => b -> t b -> b
myName maxPosition = List.foldl' (\ pos offset -> min (pred maxPosition) . max 0 $ pos + offset) 0

parentName1 :: [Int] -> Int
parentName1 = List.sum

newSTArray :: [e] -> ST s (STArray s Int e)
newSTArray xs = MutableArray.newListArray (0, length xs - 1) xs

swap :: (MutableArray.MArray a e m, MutableArray.Ix i) => a i e -> i -> i -> m ()
swap array i0 i1 = do
  e0 <- MutableArray.readArray array i0
  e1 <- MutableArray.readArray array i1
  MutableArray.writeArray array i0 e1
  MutableArray.writeArray array i1 e0

parentName2 :: [Text] -> [Int] -> Text
parentName2 nameList instructions = runST $ do
  names <- newSTArray nameList
  arrayLength <- succ . snd <$> MutableArray.getBounds names
  forM_ instructions $ \ offset -> do
    let arrayOffset = offset `mod` arrayLength
    swap names 0 arrayOffset
  MutableArray.readArray names 0

main :: IO ()
main = do
  names <- commaSepLine
  _ <- TextIO.getLine
  instructions <- fmap readInstruction <$> commaSepLine

  let namesLength = length names
  print $ names !! myName namesLength instructions
  print . (names !!) . (`mod` namesLength) $ parentName1 instructions
  print $ parentName2 names instructions

Yes exactly that. It was previously called Coq, maybe you know it under that name?

https://rocq-prover.org/

Somewhat, I’m quite motivated but it’s part of my studies.

Briesiges Wetter ist die beste Wetter-App, ändere meinen Verstand.

I think this is what they are referring to: https://en.wikipedia.org/wiki/Broughton_Suspension_Bridge.

TL;DR: The bridge collapsed because soldiers marching on it created force they hadn’t anticipated, soldiers breaking step supposedly don’t have as much of an impact.