Advent of Code 2023 - Day 13

day13.livemd

Steve Marx

@smarx

aoc2023

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Advent of Code 2023 - Day 13

Sample

sample = """
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#.#.##.#.

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"""

"#.##..##.\n..#.##.#.\n##......#\n##......#\n..#.##.#.\n..##..##.\n#.#.##.#.\n\n#...##..#\n#....#..#\n..##..###\n#####.##.\n#####.##.\n..##..###\n#....#..#\n"

Input

input = """
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Part 1

defmodule Part1 do
  defp parse_one(input) do
    rows = String.split(input, "\n", trim: true)
    chars = rows |> Enum.map(&amp;String.graphemes/1)

    cols =
      0..(length(List.first(chars)) - 1)
      |> Enum.map(fn n ->
        chars
        |> Enum.map(&amp;Enum.at(&amp;1, n))
        |> Enum.join("")
      end)

    {rows, cols}
  end

  def parse(input) do
    String.split(input, "\n\n", trim: true)
    |> Enum.map(&amp;parse_one/1)
  end

  def is_mirror_at?(rows, n) when n > length(rows) / 2 do
    is_mirror_at?(Enum.reverse(rows), length(rows) - n)
  end

  def is_mirror_at?(rows, n) do
    Enum.slice(rows, 0..(n - 1)) == Enum.reverse(Enum.slice(rows, n..(2 * n - 1)))
  end

  defp find_reflection(items) do
    1..(length(items) - 1)
    |> Enum.filter(&amp;is_mirror_at?(items, &amp;1))
    |> List.first(0)
  end

  def find_reflections(rows, cols) do
    find_reflection(rows) * 100 + find_reflection(cols)
  end
end

Part1.parse(input)
|> Enum.map(fn {rows, cols} ->
  Part1.find_reflections(rows, cols)
end)
|> Enum.sum()

Part 2

defmodule Part2 do
  defp parse_one(input) do
    rows = String.split(input, "\n", trim: true)
    chars = rows |> Enum.map(&amp;String.graphemes/1)

    cols =
      0..(length(List.first(chars)) - 1)
      |> Enum.map(fn n ->
        chars
        |> Enum.map(&amp;Enum.at(&amp;1, n))
      end)

    {Enum.map(chars, &amp;to_int/1), Enum.map(cols, &amp;to_int/1)}
  end

  def to_int(string) do
    string
    |> Enum.with_index()
    |> Enum.map(fn {c, i} ->
      if c == "#" do
        2 ** (length(string) - i - 1)
      else
        0
      end
    end)
    |> Enum.sum()
  end

  def count_bits(n) do
    for(<>, do: bit) |> Enum.sum()
  end

  def parse(input) do
    String.split(input, "\n\n", trim: true)
    |> Enum.map(&amp;parse_one/1)
  end

  def is_mirror_at?(items, n) when n > length(items) / 2 do
    is_mirror_at?(Enum.reverse(items), length(items) - n)
  end

  def is_mirror_at?(items, n) do
    diffs =
      Enum.zip(Enum.slice(items, 0..(n - 1)), Enum.reverse(Enum.slice(items, n..(2 * n - 1))))
      |> Enum.map(fn {a, b} ->
        count_bits(Bitwise.bxor(a, b))
      end)

    Enum.all?(diffs, &amp;(&amp;1 < 2)) and Enum.count(diffs, &amp;(&amp;1 == 1)) == 1
  end

  defp find_reflection(items) do
    1..(length(items) - 1)
    |> Enum.filter(&amp;is_mirror_at?(items, &amp;1))
    |> List.first(0)
  end

  def find_reflections(rows, cols) do
    find_reflection(rows) * 100 + find_reflection(cols)
  end
end

Part2.parse(input)
|> Enum.map(fn {rows, cols} ->
  Part2.find_reflections(rows, cols)
end)
|> Enum.sum()

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