path: root/src/day08.exs

defmodule Utils do
  def comb(acc \\ [], l, 2) do
    case l do
      [h | t] ->
        Enum.reduce(t, acc, fn x, acc -> [{h, x} | acc] end)
        |> comb(t, 2)
      _ ->
        acc
    end
  end
end

defmodule Day08 do
  defmodule Grid do
    defstruct [:grid, :height, :width]

    def from_data(data) do
      grid = data
      |> String.split("\n", trim: true)
      |> Enum.map(& &1 |> String.codepoints() |> :array.from_list())
      |> :array.from_list()

      height = :array.size(grid)

      width = :array.size(:array.get(0, grid))

      %Grid{grid: grid, height: height, width: width}
    end

    def at(grid, i, j) do
      :array.get(j, :array.get(i, grid.grid))
    end

    def in?(grid, i, j) do
      i >= 0 and i < grid.height and j >= 0 and j < grid.width
    end

    def find_elements(grid, regex) do
      for i <- 0..grid.height-1,
          j <- 0..grid.width-1,
          v = at(grid, i, j) do
        if String.match?(v, regex) do
          {v, i, j}
        end
      end
      |> Enum.filter(& &1 != nil)
    end
  end

  def part1(data) do
    grid = data
    |> Grid.from_data()

    grid
    |> Grid.find_elements(~r/[[:alnum:]]/)
    |> Enum.group_by(fn {k, _, _} -> k end, fn {_, i, j} -> {i, j} end)
    |> Enum.flat_map(fn {_, antennas} -> antinodes1(antennas, grid) end)
    |> Enum.uniq()
    |> Enum.count()
  end

  defp antinodes1(antennas, grid) do
    antennas
    |> Utils.comb(2)
    |> Enum.flat_map(fn {{a, b}, {c, d}} -> [{a+a-c, b+b-d}, {c+c-a, d+d-b}] end)
    |> Enum.filter(fn {a, b} -> Grid.in?(grid, a, b) end)
  end

  def part2(data) do
    grid = data
    |> Grid.from_data()

    grid
    |> Grid.find_elements(~r/[[:alnum:]]/)
    |> Enum.group_by(fn {k, _, _} -> k end, fn {_, i, j} -> {i, j} end)
    |> Enum.flat_map(fn {_, a} -> antinodes2(a, grid) end)
    |> Enum.uniq()
    |> Enum.count()
  end

  defp antinodes2(antennas, grid) do
    antennas
    |> Utils.comb(2)
    |> Enum.flat_map(fn {a, b} -> antinodes2(a, b, grid) end)
  end

  defp antinodes2({ai, aj}, {bi, bj}, grid) do
    di = abs(ai - bi)
    dj = abs(aj - bj)

    cond do
      di == 0 ->
        0..grid.width-1
      dj == 0 ->
        0..grid.height-1
      true ->
        g = Integer.gcd(di, dj)
        di = div(di, g)
        dj = div(dj, g)
        antinodes2(ai, aj, -di, if aj < bj do -dj else dj end, grid)
        |> antinodes2(ai, aj, di, if aj < bj do dj else -dj end, grid)
    end
  end

  defp antinodes2(acc \\ [], i, j, di, dj, grid) do
    if Grid.in?(grid, i, j) do
      antinodes2([{i, j} | acc], i+di, j+dj, di, dj, grid)
    else
      acc
    end
  end
end

data = IO.read(:stdio, :eof)

{time1 , ans1} = :timer.tc(fn -> Day08.part1(data) end)
IO.puts("Time  : #{time1 / 1000000}")
IO.puts("Answer: #{ans1}")

{time2 , ans2} = :timer.tc(fn -> Day08.part2(data) end)
IO.puts("Time  : #{time2 / 1000000}")
IO.puts("Answer: #{ans2}")