Haskell

A nice easy one today: didn’t even have to hit this with the optimization hammer.

import Data.Char
import Data.List
import Data.Map (Map)
import Data.Map qualified as Map

readInput :: String -> Map (Int, Int) Int
readInput s =
  Map.fromList
    [ ((i, j), digitToInt c)
      | (i, l) <- zip [0 ..] (lines s),
        (j, c) <- zip [0 ..] l
    ]

findTrails :: Map (Int, Int) Int -> [[[(Int, Int)]]]
findTrails input =
  Map.elems . Map.map (filter ((== 10) . length)) $
    Map.restrictKeys accessible starts
  where
    starts = Map.keysSet . Map.filter (== 0) $ input
    accessible = Map.mapWithKey getAccessible input
    getAccessible (i, j) h
      | h == 9 = [[(i, j)]]
      | otherwise =
          [ (i, j) : path
            | (di, dj) <- [(-1, 0), (0, 1), (1, 0), (0, -1)],
              let p = (i + di, j + dj),
              input Map.!? p == Just (succ h),
              path <- accessible Map.! p
          ]

main = do
  trails <- findTrails . readInput <$> readFile "input10"
  mapM_
    (print . sum . (`map` trails))
    [length . nub . map last, length]

Haskell

Cool task, nothing to optimize

import Control.Arrow

import Data.Array.Unboxed (UArray)
import Data.Set (Set)

import qualified Data.Char as Char
import qualified Data.List as List
import qualified Data.Set as Set
import qualified Data.Array.Unboxed as UArray

parse :: String -> UArray (Int, Int) Int
parse s = UArray.listArray ((1, 1), (n, m)) . map Char.digitToInt . filter (/= '\n') $ s
        where
                n = takeWhile (/= '\n') >>> length $ s
                m = filter (== '\n') >>> length >>> pred $ s

reachableNeighbors :: (Int, Int) -> UArray (Int, Int) Int -> [(Int, Int)]
reachableNeighbors p@(py, px) a = List.filter (UArray.inRange (UArray.bounds a))
        >>> List.filter ((a UArray.!) >>> pred >>> (== (a UArray.! p)))
        $ [(py-1, px), (py+1, px), (py, px-1), (py, px+1)]

distinctTrails :: (Int, Int) -> UArray (Int, Int) Int -> Int
distinctTrails p a
        | a UArray.! p == 9 = 1
        | otherwise = flip reachableNeighbors a
                >>> List.map (flip distinctTrails a)
                >>> sum
                $ p

reachableNines :: (Int, Int) -> UArray (Int, Int) Int -> Set (Int, Int)
reachableNines p a
        | a UArray.! p == 9 = Set.singleton p
        | otherwise = flip reachableNeighbors a
                >>> List.map (flip reachableNines a)
                >>> Set.unions
                $ p

findZeros = UArray.assocs
        >>> filter (snd >>> (== 0))
        >>> map fst

part1 a = findZeros
        >>> map (flip reachableNines a)
        >>> map Set.size
        >>> sum
        $ a
part2 a = findZeros
        >>> map (flip distinctTrails a)
        >>> sum
        $ a

main = getContents
        >>= print
        . (part1 &&& part2)
        . parse

Nim

As many others today, I’ve solved part 2 first and then fixed a ‘bug’ to solve part 1. =)

type Vec2 = tuple[x,y:int]
const Adjacent = [(x:1,y:0),(-1,0),(0,1),(0,-1)]

proc path(start: Vec2, grid: seq[string]): tuple[ends, trails: int] =
  var queue = @[@[start]]
  var endNodes: HashSet[Vec2]
  while queue.len > 0:
    let path = queue.pop()
    let head = path[^1]
    let c = grid[head.y][head.x]

    if c == '9':
      inc result.trails
      endNodes.incl head
      continue

    for d in Adjacent:
      let nd = (x:head.x + d.x, y:head.y + d.y)
      if nd.x < 0 or nd.y < 0 or nd.x > grid[0].high or nd.y > grid.high:
        continue
      if grid[nd.y][nd.x].ord - c.ord != 1: continue
      queue.add path & nd
  result.ends = endNodes.len

proc solve(input: string): AOCSolution[int, int] =
  let grid = input.splitLines()
  var trailstarts: seq[Vec2]

  for y, line in grid:
    for x, c in line:
      if c == '0':
        trailstarts.add (x,y)

  for start in trailstarts:
    let (ends, trails) = start.path(grid)
    result.part1 += ends
    result.part2 += trails

Codeberg Repo

Nice to have a really simple one for a change, both my day 1 and 2 solutions worked on their very first attempts.
I rewrote the code to combine the two though, since the implementations were almost identical for both solutions, and also to replace the recursion with a search list instead.

int[] heights = new int[0];
(int, int) size = (0, 0);

public void Input(IEnumerable<string> lines)
{
  size = (lines.First().Length, lines.Count());
  heights = string.Concat(lines).Select(c => int.Parse(c.ToString())).ToArray();
}

int trails = 0, trailheads = 0;
public void PreCalc()
{
  for (int y = 0; y < size.Item2; ++y)
    for (int x = 0; x < size.Item1; ++x)
      if (heights[y * size.Item1 + x] == 0)
      {
        var unique = new HashSet<(int, int)>();
        trails += CountTrails((x, y), unique);
        trailheads += unique.Count;
      }
}

public void Part1()
{
  Console.WriteLine($"Trailheads: {trailheads}");
}
public void Part2()
{
  Console.WriteLine($"Trails: {trails}");
}

int CountTrails((int, int) from, HashSet<(int,int)> unique)
{
  int found = 0;

  List<(int,int)> toSearch = new List<(int, int)>();
  toSearch.Add(from);

  while (toSearch.Any())
  {
    var cur = toSearch.First();
    toSearch.RemoveAt(0);

    int height = heights[cur.Item2 * size.Item1 + cur.Item1];
    for (int y = -1; y <= 1; ++y)
      for (int x = -1; x <= 1; ++x)
      {
        if ((y != 0 && x != 0) || (y == 0 && x == 0))
          continue;

        var newAt = (cur.Item1 + x, cur.Item2 + y);
        if (newAt.Item1 < 0 || newAt.Item1 >= size.Item1 || newAt.Item2 < 0 || newAt.Item2 >= size.Item2)
          continue;

        int newHeight = heights[newAt.Item2 * size.Item1 + newAt.Item1];
        if (newHeight - height != 1)
          continue;

        if (newHeight == 9)
        {
          unique.Add(newAt);
          found++;
          continue;
        }

        toSearch.Add(newAt);
      }
  }

  return found;
}

Rust

Definitely a nice and easy one, I accidentally solved part 2 first, because I skimmed the challenge and missed the unique part.

#[cfg(test)]
mod tests {

    const DIR_ORDER: [(i8, i8); 4] = [(-1, 0), (0, 1), (1, 0), (0, -1)];

    fn walk_trail(board: &Vec<Vec<i8>>, level: i8, i: i8, j: i8) -> Vec<(i8, i8)> {
        let mut paths = vec![];
        if i < 0 || j < 0 {
            return paths;
        }
        let actual_level = match board.get(i as usize) {
            None => return paths,
            Some(line) => match line.get(j as usize) {
                None => return paths,
                Some(c) => c,
            },
        };
        if *actual_level != level {
            return paths;
        }
        if *actual_level == 9 {
            return vec![(i, j)];
        }

        for dir in DIR_ORDER.iter() {
            paths.extend(walk_trail(board, level + 1, i + dir.0, j + dir.1));
        }
        paths
    }

    fn count_unique(p0: &Vec<(i8, i8)>) -> u32 {
        let mut dedup = vec![];
        for p in p0.iter() {
            if !dedup.contains(p) {
                dedup.push(*p);
            }
        }
        dedup.len() as u32
    }

    #[test]
    fn day10_part1_test() {
        let input = std::fs::read_to_string("src/input/day_10.txt").unwrap();

        let board = input
            .trim()
            .split('\n')
            .map(|line| {
                line.chars()
                    .map(|c| {
                        if c == '.' {
                            -1
                        } else {
                            c.to_digit(10).unwrap() as i8
                        }
                    })
                    .collect::<Vec<i8>>()
            })
            .collect::<Vec<Vec<i8>>>();

        let mut total = 0;

        for (i, row) in board.iter().enumerate() {
            for (j, pos) in row.iter().enumerate() {
                if *pos == 0 {
                    let all_trails = walk_trail(&board, 0, i as i8, j as i8);
                    total += count_unique(&all_trails);
                }
            }
        }

        println!("{}", total);
    }
    #[test]
    fn day10_part2_test() {
        let input = std::fs::read_to_string("src/input/day_10.txt").unwrap();

        let board = input
            .trim()
            .split('\n')
            .map(|line| {
                line.chars()
                    .map(|c| {
                        if c == '.' {
                            -1
                        } else {
                            c.to_digit(10).unwrap() as i8
                        }
                    })
                    .collect::<Vec<i8>>()
            })
            .collect::<Vec<Vec<i8>>>();

        let mut total = 0;

        for (i, row) in board.iter().enumerate() {
            for (j, pos) in row.iter().enumerate() {
                if *pos == 0 {
                    total += walk_trail(&board, 0, i as i8, j as i8).len();
                }
            }
        }

        println!("{}", total);
    }
}