Day 08

Mix.install([:kino_aoc, :complex])

Kino.configure(inspect: [charlists: :as_lists])

Input

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

parsed = 
  puzzle_input
  |> String.split("\n")
  |> Enum.with_index()
  |> Enum.flat_map(fn {line, row} -> 
    line
    |> String.graphemes()
    |> Enum.with_index()
    |> Enum.map(fn {char, col} -> 
      {Complex.new(row, col), char}  
    end)
  end)
  |> Enum.reject(fn {_coords, char} -> char == "." end)
  |> Enum.group_by(&elem(&1, 1), &elem(&1, 0))

defmodule Day08 do
  def permutations(_list, 0), do: [[]]
  def permutations([], _n), do: [[]]
  def permutations(list, n) do
    for head <- list, rest <- permutations(list--[head], n - 1), do: [head|rest]
  end
end

Part 1

use Complex.Kernel

defmodule Part1 do
  def create_antinodes(antinodes, []), do: antinodes
  def create_antinodes(antinodes, [[p, q] | tail]) do
    antinodes
    |> MapSet.put(p - (q - p))
    |> create_antinodes(tail)
  end
end

Enum.reduce(parsed, MapSet.new(), fn {_freq, coords}, acc -> 
    Part1.create_antinodes(MapSet.new(), Day08.permutations(coords, 2))
    |> MapSet.union(acc)
end)
|> Enum.count(&(trunc(&1.re) in 0..49 and trunc(&1.im) in 0..49))

Part 2

use Complex.Kernel

defmodule Part2 do
  def create_antinodes(antinodes, []), do: antinodes
  def create_antinodes(antinodes, [[p, q] | tail]) do
    d = (q - p)
    antinodes
    |> MapSet.put(p - d)
    |> calc_other_antinodes(p, d)
    |> create_antinodes(tail)
  end

  def calc_other_antinodes(antinodes, p, d) do
    if in_boundaries(p) do
      antinodes
      |> MapSet.put(p)
      |> calc_other_antinodes(p - d, d)
    else
      antinodes
    end
  end

  def in_boundaries(antinode), do: trunc(antinode.re) in 0..49 and trunc(antinode.im) in 0..49
end

Enum.reduce(parsed, MapSet.new(), fn {_freq, coords}, acc -> 
    Part2.create_antinodes(MapSet.new(), Day08.permutations(coords, 2))
    |> MapSet.union(acc)
end)
|> Enum.count(&Part2.in_boundaries/1)