🎄 Year 2024 🔔 Day 10

Setup

input_map =
  File.read!("#{__DIR__}/../../../inputs/2024/day10.txt")
  # """
  # 89010123
  # 78121874
  # 87430965
  # 96549874
  # 45678903
  # 32019012
  # 01329801
  # 10456732
  # """
  |> String.split("\n", trim: true)
  |> Enum.map(fn line ->
    line
    |> String.graphemes()
    |> Enum.map(fn e -> String.to_integer(e) end)
  end)
  |> Enum.with_index(fn line, y ->
    Enum.with_index(line, fn e, x -> {{x, y}, e} end)
  end)
  |> Enum.concat()
  |> Map.new()

{{max_x, max_y}, _} = Enum.max_by(input_map, fn {coords, _} -> coords end)
{max_x, max_y}

Part 1

zeros = Enum.filter(input_map, fn {_, e} -> e == 0 end)

zeros
|> Enum.map(fn start_coord ->
  1..9
  |> Enum.reduce([start_coord], fn i, coords ->
    Enum.flat_map(coords, fn {{x, y}, _} ->
      [
        {x + 1, y},
        {x - 1, y},
        {x, y + 1},
        {x, y - 1}
      ]
    end)
    |> Enum.map(fn coord = {x, y} ->
      if x < 0 || y < 0 || x > max_x || y > max_y do
        {{x, y}, -1}
      else
        e = Map.fetch!(input_map, coord)
        {{x, y}, e}
      end
    end)
    |> Enum.filter(fn {_, e} -> e == i end)
    |> Enum.uniq()
  end)
  |> Enum.count()
end)
|> Enum.sum()

Part 2

scores =
  Enum.filter(input_map, fn {_, e} -> e == 9 end)
  |> Enum.map(fn {coord, 9} -> {coord, 1} end)
  |> Map.new()

scores =
  8..0
  |> Enum.reduce(scores, fn i, scores ->
    new_scores =
      Enum.filter(input_map, fn {_, e} -> e == i end)
      |> Enum.map(fn {{x, y}, _} ->
        score =
          [
            {x + 1, y},
            {x - 1, y},
            {x, y + 1},
            {x, y - 1}
          ]
          |> Enum.map(fn coord = {x, y} ->
            if x < 0 || y < 0 || x > max_x || y > max_y do
              {{x, y}, -1}
            else
              e = Map.fetch!(input_map, coord)
              {{x, y}, e}
            end
          end)
          |> Enum.filter(fn {_, e} -> e == i + 1 end)
          |> Enum.map(fn {coord, _} -> Map.fetch!(scores, coord) end)
          |> Enum.sum()

        {{x, y}, score}
      end)

    Enum.concat(scores, new_scores)
    |> Map.new()
  end)

input_map
|> Enum.filter(fn {_, e} -> e == 0 end)
|> Enum.map(fn {coord, _} -> Map.fetch!(scores, coord) end)
|> Enum.sum()