Advent 2024 - Day 10

Mix.install([
  {:kino, "~> 0.14.2"}
])

Setup

input = Kino.Input.textarea("Please paste your input file")

map = input
  |> Kino.Input.read()
  |> String.split("\n")
  |> Enum.map(fn line ->
    line
    |> String.graphemes()
    |> Enum.map(fn char ->
      case Integer.parse(char) do
        {val, ""} -> val
        :error -> nil
      end
    end)
  end)

defmodule TrailExplorer do
  def get_valid_surronding(map, {x, y}, val) do
    surronding = [
      {x, y - 1},
      {x + 1, y},
      {x, y + 1},
      {x - 1, y}
    ]

    for {x, y} <- surronding do
      if x < 0 or y < 0 or x >= length(map) or y >= length(map) do
        nil
      else
        if map |> Enum.at(y) |> Enum.at(x) == val + 1 do
          {val + 1, x, y}
        else
          nil
        end
      end
    end
    |> Enum.filter(&amp;(&amp;1))
  end

  def navigate(map, position, val) do
    for {val, x, y} <- get_valid_surronding(map, position, val) do
      if val == 9 do
        {x, y}
      else
        navigate(map, {x, y}, val)
      end
    end
  end

  def score(map, trailhead, distinct) do
    endings = navigate(map, trailhead, 0)
    |> List.flatten()

    if distinct do
      endings
      |> MapSet.new()
      |> MapSet.size()
    else
      endings
      |> length()
    end
  end
end

trailheads = for {row, y} <- map |> Enum.with_index() do
  for {val, x} <- row |> Enum.with_index() do
    if val == 0 do
      {x, y}
    end
  end
end
|> List.flatten()
|> Enum.filter(&amp;(&amp;1))

Part 1

for trailhead <- trailheads do
  TrailExplorer.score(map, trailhead, true)
end
|> Enum.sum()

Part 2

for trailhead <- trailheads do
  TrailExplorer.score(map, trailhead, false)
end
|> Enum.sum()