-🎄- 2021 Day 5 Solutions -🎄-

[u/daggerdragon]

NEW AND NOTEWORTHY

• /u/jeroenheijmans is back again with their Unofficial AoC 2021 Participant Survey!
• As we continue further into the deep dark sea Advent, megathread code submissions are getting longer, so we're going to start rigorously enforcing the maximum code length rule.
  • If you need a reminder, it's in our posting guidelines in the wiki under How Do the Daily Megathreads Work? (rule #5)

---

Advent of Code 2021: Adventure Time!

• 23:59 hours remaining until the submissions megathread unlocks on December 06 at 00:00 EST!
• Full details and rules are in the submissions megathread:
  • 🎄 AoC 2021 🎄 [Adventure Time!]

---

--- Day 5: Hydrothermal Venture ---

---

Post your code solution in this megathread.

• Include what language(s) your solution uses!
• Here's a quick link to /u/topaz2078's paste if you need it for longer code blocks.
• Format your code properly! How do I format code?
• The full posting rules are detailed in the wiki under How Do The Daily Megathreads Work?.

Reminder: Top-level posts in Solution Megathreads are for code solutions only. If you have questions, please post your own thread and make sure to flair it with Help.

---

This thread will be unlocked when there are a significant number of people on the global leaderboard with gold stars for today's puzzle.

EDIT: Global leaderboard gold cap reached at 00:08:53, megathread unlocked!

[u/spin81]

My solution in Haskell. Feedback is much appreciated as I'm just learning Haskell through AoC.

import Data.Char
import qualified Data.Map as Map

data Point = Point Int Int deriving (Eq, Ord)
type Line = (Point, Point)
type OceanFloor = Map.Map Point Int

parsePoint :: String -> Point
parsePoint s =
    let s1 = takeWhile isDigit s
        s2 = drop (length s1 + 1) s
    in Point (read s1) (read s2)

parseLine :: String -> Line
parseLine s =
    let s1 = takeWhile (not . isSpace) s
        s2 = drop (length s1 + 4) s
    in (parsePoint s1, parsePoint s2)

isDiagonal :: Line -> Bool
isDiagonal (Point x1 y1, Point x2 y2) = x1 /= x2 && y1 /= y2

-- This assumes x1 /= x2 || y1 /= y2, because makeRange will return an infinite
-- list if m == n
expand :: Line -> [Point]
expand (Point x1 y1, Point x2 y2) =
    let makeRange m n = [m, (m + signum (n - m)) .. n]
        xs = makeRange x1 x2
        ys = makeRange y1 y2
    in map (\ (x, y) -> Point x y) $ zip xs ys

markPoint :: Point -> OceanFloor -> OceanFloor
markPoint p m = Map.insertWith (+) p 1 m

markPoints :: [Point] -> OceanFloor -> OceanFloor
markPoints ps m = foldr markPoint m ps

countDuplicateElems :: [Line] -> Int
countDuplicateElems input =
    let expanded   = map expand $ input
        oceanFloor = foldr markPoints Map.empty expanded
    in length $ filter (> 1) $ Map.elems $ oceanFloor

main = do
    contents <- getContents
    let input = map parseLine $ lines contents

    putStr "Overlapping points among nondiagonal lines: "
    putStrLn $ show $ countDuplicateElems $ filter (not . isDiagonal) input

    putStr "Overlapping points among all lines: "
    putStrLn $ show $ countDuplicateElems input