Day 13: Point of Incidence – Advent of Code 2023

input =
  File.read!("/Users/pw/src/weiland/adventofcode/2023/input/13.txt")

# |> Stream.map(&String.trim/1)

test_input = "#.##..##.
..#.##.#.
##......#
##......#
..#.##.#.
..##..##.
#.#.##.#.

#...##..#
#....#..#
..##..###
#####.##.
#####.##.
..##..###
#....#..#"

Parsing

parse = fn input ->
  input
  |> String.split("\n\n")
  |> Enum.map(fn block ->
    String.split(block, "\n", trim: true) |> Enum.map(&String.graphemes/1)
  end)
end

Part One

count = fn lines, limit ->
  r_count = Enum.count(lines)

  0..(r_count - 2)
  |> Enum.reduce_while(nil, fn row, _acc ->
    chars_off =
      Enum.slice(lines, 0..row)
      |> Enum.reverse()
      |> Enum.zip(Enum.slice(lines, (row + 1)..(r_count - 1)))
      |> Enum.reduce(0, fn {lst_a, lst_b}, total_chars_off ->
        count =
          Enum.zip(lst_a, lst_b)
          |> Enum.reduce(0, fn {a, b}, acc ->
            if a == b, do: acc, else: acc + 1
          end)

        count + total_chars_off
      end)

    if chars_off != limit, do: {:cont, false}, else: {:halt, row + 1}
  end)
end

handle_block = fn rows, chars_off ->
  cols = rows |> Enum.zip_with(&Function.identity/1)

  rcols = count.(cols, chars_off)

  if rcols != false do
    rcols
  else
    count.(rows, chars_off) * 100
  end
end

part_one = fn input ->
  input
  |> parse.()
  |> Enum.map(&handle_block.(&1, 0))
  |> Enum.sum()
end

part_one.(test_input) == 405 &&
  part_one.(input)

Part Two

part_two = fn input ->
  input
  |> parse.()
  |> Enum.map(&handle_block.(&1, 1))
  |> Enum.sum()
end

part_two.(test_input) == 400 &&
  part_two.(input)