Day 13

Mix.install([
  {:req, "~> 0.4.5"},
  {:kino, "~> 0.11.3"},
  {:nx, "~> 0.6.4"}
])

Input

input =
  Req.get!(
    "https://adventofcode.com/2023/day/13/input",
    headers: [{"Cookie", ~s"session=#{System.fetch_env!("LB_AOC_SESSION")}"}]
  ).body

Kino.Text.new(input, terminal: true)

Part 1

defmodule Part1 do
  def parse(input) do
    for pattern <- String.split(input, "\n\n") do
      for line <- String.split(pattern, "\n", trim: true) do
        String.graphemes(line)
        |> Enum.map(fn c -> if c == "#", do: 1, else: 0 end)
      end
      |> Nx.tensor(type: :u8, names: [:y, :x])
    end
  end

  def summarize(patterns) do
    for pattern <- patterns do
      {axis, {index, _}} =
        reflect(pattern)

      left = index + 1
      if axis == :x, do: left, else: 100 * left
    end
    |> Enum.sum()
  end

  def reflect(pattern) do
    [:x, :y]
    |> Enum.map(fn axis -> {axis, reflect_along(pattern, axis)} end)
    |> Enum.max_by(fn {_, {_, size}} -> size end)
  end

  def reflect_along(pattern, axis) do
    0..Nx.axis_size(pattern, axis)
    |> Enum.reduce({-1, -1}, fn index, {_, prev_size} = prev ->
      {_, next_size} = next = reflect_at(pattern, axis, index)
      if next_size > prev_size, do: next, else: prev
    end)
  end

  def reflect_at(pattern, axis, index), do: reflect_at(pattern, axis, index, index + 1)

  def reflect_at(pattern, axis, left, right) do
    if left < 0 or right >= Nx.axis_size(pattern, axis) do
      reflection(left + 1, right - 1)
    else
      left_tensor = pattern[Keyword.new([{axis, left}])]
      right_tensor = pattern[Keyword.new([{axis, right}])]

      if left_tensor != right_tensor do
        {-1, -1}
      else
        reflect_at(pattern, axis, left - 1, right + 1)
      end
    end
  end

  def reflection(left, right) when left > right, do: {-1, -1}

  def reflection(left, right) do
    size = div(right - left, 2)
    {left + size, size}
  end
end

input
|> Part1.parse()
|> Part1.summarize()

Part 2

defmodule Part2 do
  def summarize(patterns) do
    for pattern <- patterns do
      {axis, {index, _}} =
        reflect(pattern)

      left = index + 1
      if axis == :x, do: left, else: 100 * left
    end
    |> Enum.sum()
  end

  def reflect(pattern) do
    [:x, :y]
    |> Enum.map(fn axis -> {axis, reflect_along(pattern, axis)} end)
    |> Enum.max_by(fn {_, {_, size}} -> size end)
  end

  def reflect_along(pattern, axis) do
    0..Nx.axis_size(pattern, axis)
    |> Enum.reduce({-1, -1}, fn index, prev ->
      {start, size, smudge} = reflect_at(pattern, axis, index)
      if smudge, do: {start, size}, else: prev
    end)
  end

  def reflect_at(pattern, axis, index), do: reflect_at(pattern, axis, index, index + 1, false)

  def reflect_at(pattern, axis, left, right, smudge) do
    if left < 0 or right >= Nx.axis_size(pattern, axis) do
      reflection(left + 1, right - 1, smudge)
    else
      left_tensor = pattern[Keyword.new([{axis, left}])]
      right_tensor = pattern[Keyword.new([{axis, right}])]

      if left_tensor != right_tensor do
        if smudge or
             Nx.equal(left_tensor, right_tensor)
             |> Nx.logical_not()
             |> Nx.sum() != Nx.tensor(1, type: :u64) do
          {-1, -1, false}
        else
          reflect_at(pattern, axis, left - 1, right + 1, true)
        end
      else
        reflect_at(pattern, axis, left - 1, right + 1, smudge)
      end
    end
  end

  def reflection(left, right, _) when left > right, do: {-1, -1, false}

  def reflection(left, right, smudge) do
    size = div(right - left, 2)
    {left + size, size, smudge}
  end
end

input
|> Part1.parse()
|> Part2.summarize()