🎄 Year 2023 🔔 Day 03

Setup

filename = "#{__DIR__}/../../../inputs/2023/day03.txt"
file = File.read!(filename)

Part 1

parse_char = fn char ->
  case Integer.parse(char) do
    {int, ""} -> int
    _ -> char
  end
end

is_symbol? = fn
  "." -> false
  char when is_integer(char) -> false
  _ -> true
end

{coords, numbers} =
  String.split(file, "\n", trim: true)
  |> Enum.with_index()
  |> Enum.map(fn {line, y} ->
    line
    |> String.graphemes()
    |> Enum.with_index(fn char, x -> {{x, y}, char} end)
  end)
  |> Enum.reduce({[], []}, fn line, {all_coords, all_numbers} ->
    {coords, numbers} =
      Enum.map_reduce(line, [], fn {{x, y}, char}, acc ->
        parsed_char = parse_char.(char)
        is_symbol = is_symbol?.(parsed_char)
        new_e = {{x, y}, is_symbol}

        case acc do
          _ when not is_integer(parsed_char) ->
            {new_e, acc}

          [] ->
            {new_e, [{[parsed_char], _min_x = x, _max_x = x, y} | acc]}

          [{digits, min_x, max_x, ^y} | acc_tail] when max_x + 1 == x ->
            {new_e, [{[parsed_char | digits], min_x, _new_max_x = x, y} | acc_tail]}

          _ ->
            {new_e, [{[parsed_char], _min_x = x, _max_x = x, y} | acc]}
        end
      end)

    numbers =
      numbers
      |> Enum.reverse()
      |> Enum.map(fn {reverse_digits, min_x, max_x, y} ->
        number = reverse_digits |> Enum.reverse() |> Integer.undigits()
        {number, min_x, max_x, y}
      end)

    {[coords | all_coords], [numbers | all_numbers]}
  end)
  |> then(fn {coords, numbers} ->
    coords = coords |> Enum.reverse() |> Enum.concat() |> Map.new()
    numbers = numbers |> Enum.reverse() |> Enum.concat()
    {coords, numbers}
  end)

Enum.reduce(numbers, 0, fn {number, min_x, max_x, y}, sum ->
  target_coords =
    for(x <- (min_x - 1)..(max_x + 1), y <- [y - 1, y + 1], do: {x, y})
    |> Enum.concat([{min_x - 1, y}, {max_x + 1, y}])

  if Enum.any?(target_coords, fn target_coord -> Map.get(coords, target_coord, false) end) do
    sum + number
  else
    sum
  end
end)

Section

parse_char = fn char ->
  case Integer.parse(char) do
    {int, ""} -> int
    _ -> char
  end
end

is_symbol? = fn
  "." -> false
  char when is_integer(char) -> false
  _ -> true
end

{coords, numbers} =
  String.split(file, "\n", trim: true)
  |> Enum.with_index()
  |> Enum.map(fn {line, y} ->
    line
    |> String.graphemes()
    |> Enum.with_index(fn char, x -> {{x, y}, char} end)
  end)
  |> Enum.reduce({[], []}, fn line, {all_coords, all_numbers} ->
    {coords, numbers} =
      Enum.map_reduce(line, [], fn {{x, y}, char}, acc ->
        parsed_char = parse_char.(char)
        is_symbol = char == "*"
        new_e = {{x, y}, is_symbol}

        case acc do
          _ when not is_integer(parsed_char) ->
            {new_e, acc}

          [] ->
            {new_e, [{[parsed_char], _min_x = x, _max_x = x, y} | acc]}

          [{digits, min_x, max_x, ^y} | acc_tail] when max_x + 1 == x ->
            {new_e, [{[parsed_char | digits], min_x, _new_max_x = x, y} | acc_tail]}

          _ ->
            {new_e, [{[parsed_char], _min_x = x, _max_x = x, y} | acc]}
        end
      end)

    numbers =
      numbers
      |> Enum.reverse()
      |> Enum.map(fn {reverse_digits, min_x, max_x, y} ->
        number = reverse_digits |> Enum.reverse() |> Integer.undigits()
        {number, min_x, max_x, y}
      end)

    {[coords | all_coords], [numbers | all_numbers]}
  end)
  |> then(fn {coords, numbers} ->
    coords =
      coords
      |> Enum.reverse()
      |> Enum.concat()
      |> Enum.filter(fn {{_x, _y}, is_symbol} -> is_symbol end)
      |> Enum.map(&elem(&1, 0))

    numbers = numbers |> Enum.reverse() |> Enum.concat()
    {coords, numbers}
  end)

result =
  Enum.reduce(coords, 0, fn {x, y}, sum ->
    adjacent_numbers =
      Enum.filter(numbers, fn {_number, min_x, max_x, ny} ->
        x >= min_x - 1 && x <= max_x + 1 && ny >= y - 1 && ny <= y + 1
      end)
      |> Enum.map(&elem(&1, 0))

    case adjacent_numbers do
      [n1, n2] -> sum + n1 * n2
      _ -> sum
    end
  end)