Day8

Mix.install([
  {:kino_aoc, git: "https://github.com/ljgago/kino_aoc"}
])

Setup

{:ok, data} = KinoAOC.download_puzzle("2024", "8", System.fetch_env!("LB_AOC_SECRET"))

Solve

defmodule Day8 do
  defguard in_grid(rm, cm, r, c) when r in (0..rm-1//1) and c in (0..cm-1//1)

  def parse(data) do
    data
    |> split_with(fn r -> String.split(r, "", trim: true) end)
  end

  def split_with(str, fun) do
    str
    |> String.split("\n", trim: true)
    |> Enum.map(&fun.(&1))
  end

  def to_grid(data) do
    rm = length(data)
    cm = data |> List.first() |> length()

    g = data
      |> Enum.with_index()
      |> Enum.reduce(%{}, fn {row, ri}, g ->
        row
        |> Enum.with_index()
        |> Enum.reduce(g, fn {c, ci}, g -> Map.put(g, {ri,ci}, c) end)
      end)
    {{rm, cm}, g}
  end

  def plot(grid, {mr, mc}) do
    Enum.map(0..mr-1, fn r ->
      Enum.reduce(0..mc-1, "", fn c, acc -> acc <> grid[{r, c}] end)
    end)
    |> Enum.join("\n")
    |> IO.puts()
    grid
  end

  def solve(data) do
    {max, g} = data |> parse() |> to_grid()
    gr = g |> Enum.filter(fn {_p, v} -> v != "." end) |> Enum.group_by(fn {_p, v} -> v end)

    {r1, r2} = Enum.reduce(gr, {MapSet.new(), MapSet.new()}, fn {_k, v}, {t1, t2} ->
      {r1, r2} = antinodes(max, v)
      {MapSet.union(t1, r1), MapSet.union(t2, r2)}
    end)

    # debug
    # Enum.reduce(r2, g, fn an, g ->
    #   if g[an] == "." do
    #     Map.put(g, an, "#")
    #   else
    #     g
    #   end
    # end)
    # |> plot(max)

    {MapSet.size(r1), MapSet.size(r2)}
  end

  def antinodes(max, gr) do
    pos = Enum.map(gr, fn {p, _v} -> p end)

    for a <- pos, b <- pos, a != b do
      {ra, ca} = a
      {rb, cb} = b
      dr = ra - rb
      dc = ca - cb

      {
        nodes(max, ra, ca, dr, dc, MapSet.new(), :t1),
        nodes(max, rb, cb, dr, dc, dr, dc, MapSet.new(), :t2)
      }
    end
    |> Enum.reduce({MapSet.new(), MapSet.new()}, fn {r1, r2}, {t1, t2} ->
      {MapSet.union(t1, r1), MapSet.union(t2, r2)}
    end)
  end

  def nodes({rm, cm}, r, c, dr, dc, ddr, ddc, acc, :t2) when in_grid(rm, cm, r+ddr, c+ddc) do
    acc = MapSet.put(acc, {r+ddr, c+ddc})
    nodes({rm, cm}, r, c, dr, dc, ddr+dr, ddc+dc, acc, :t2)
  end

  def nodes(_max, _r, _c, _dr, _dc, _ddr, _ddc, acc, :t2), do: acc

  def nodes({rm, cm}, r, c, dr, dc, acc, :t1) when in_grid(rm, cm, r+dr, c+dc) do
    MapSet.put(acc, {r+dr, c+dc})
  end

  def nodes(_max, _r, _c, _dr, _dc, acc, :t1), do: acc
end

tdata = """
............
........0...
.....0......
.......0....
....0.......
......A.....
............
............
........A...
.........A..
............
............
"""

tdata2 = """
T.........
...T......
.T........
..........
..........
..........
..........
..........
..........
..........
"""

Day8.solve(tdata) |> IO.inspect()
Day8.solve(data) # {252, 839}