Elixir

defmodule AdventOfCode.Solution.Year2024.Day04 do
  use AdventOfCode.Solution.SharedParse

  defmodule Map do
    defstruct [:chars, :width, :height]
  end

  @impl true
  def parse(input) do
    chars = String.split(input, "\n", trim: true) |> Enum.map(&String.codepoints/1)
    %Map{chars: chars, width: length(Enum.at(chars, 0)), height: length(chars)}
  end

  def at(%Map{} = map, x, y) do
    cond do
      x < 0 or x >= map.width or y < 0 or y >= map.height -> ""
      true -> map.chars |> Enum.at(y, []) |> Enum.at(x, "")
    end
  end

  def part1(map) do
    dirs = for dx <- -1..1, dy <- -1..1, {dx, dy} != {0, 0}, do: {dx, dy}
    xmas = String.codepoints("XMAS") |> Enum.with_index() |> Enum.drop(1)

    for x <- 0..(map.width - 1),
        y <- 0..(map.height - 1),
        "X" == at(map, x, y),
        {dx, dy} <- dirs,
        xmas
        |> Enum.all?(fn {c, n} -> at(map, x + dx * n, y + dy * n) == c end),
        reduce: 0 do
      t -> t + 1
    end
  end

  def part2(map) do
    for x <- 0..(map.width - 1),
        y <- 0..(map.height - 1),
        "A" == at(map, x, y),
        (at(map, x - 1, y - 1) <> at(map, x + 1, y + 1)) in ["MS", "SM"],
        (at(map, x - 1, y + 1) <> at(map, x + 1, y - 1)) in ["MS", "SM"],
        reduce: 0 do
      t -> t + 1
    end
  end
end