Advent of Code 2024 - Day 04

Mix.install([{:kino, github: "livebook-dev/kino"}])

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

Part 1

input = Kino.Input.read(kino_input)

directions = [
  {0, -1},  # up
  {0, 1},   # down
  {1, 0},   # right
  {-1, 0},  # left
  {1, -1},  # up-right
  {1, 1},   # down-right,
  {-1, -1}, # up-left,
  {-1, 1},  # down-left
]

grid = String.split(input, "\n", trim: true)
  |> Enum.map(&String.graphemes/1)

traverse = fn grid, x, y, {dx, dy}, length ->
  0..(length - 1)
  |> Enum.map(fn i ->
    nx = x + i * dx
    ny = y + i * dy

    if ny in 0..(length(grid) - 1) && nx in 0..(length(List.first(grid)) - 1) do
      Enum.at(Enum.at(grid, ny), nx)
    else
      nil
    end
  end)
end

directions
|> Enum.flat_map(fn direction ->
  for y <- 0..(length(grid) - 1),
      x <- 0..(length(List.first(grid)) - 1),
      Enum.at(Enum.at(grid, y), x) == "X",
      do: traverse.(grid, x, y, direction, 4)
end)
|> Enum.count(fn list -> list == String.graphemes("XMAS") end)

Part 2

input = Kino.Input.read(kino_input)

directions = [
  [ {1, -1}, {-1, 1} ],   # up-right, down-left
  [ {-1, -1}, {1, 1} ],   # up-left, down-right
]

grid = String.split(input, "\n", trim: true)
  |> Enum.map(&String.graphemes/1)

valid_position = fn grid, x, y ->
  y >= 0 &amp;&amp; y < length(grid) &amp;&amp; x >= 0 &amp;&amp; x < length(List.first(grid))
end

match_pattern = fn grid, x, y ->
  directions
  |> Enum.all?(fn [{dx1, dy1}, {dx2, dy2}] ->
    nx1 = x + dx1
    ny1 = y + dy1
    nx2 = x + dx2
    ny2 = y + dy2

    if valid_position.(grid, nx1, ny1) &amp;&amp; valid_position.(grid, nx2, ny2) do
      char1 = Enum.at(Enum.at(grid, ny1), nx1)
      char2 = Enum.at(Enum.at(grid, ny2), nx2)

      (char1 == "M" &amp;&amp; char2 == "S") || (char1 == "S" &amp;&amp; char2 == "M")
    else
      false
    end
  end)
end

matches = for y <- 1..(length(grid) - 2),
    x <- 1..(length(List.first(grid)) - 2),
    Enum.at(Enum.at(grid, y), x) == "A",
    match_pattern.(grid, x, y),
    do: {x, y}

Enum.count(matches)