🎄 Year 2024 🔔 Day 08

Setup

input_string =
  File.read!("#{__DIR__}/../../../inputs/2024/day08.txt")

input =
  input_string
  |> String.split("\n", trim: true)
  |> Enum.with_index(fn line, y ->
    line
    |> String.graphemes()
    |> Enum.with_index(fn e, x ->
      {{x, y}, e}
    end)
  end)
  |> Enum.concat()
  |> Enum.filter(fn {_, e} -> e != "." end)
  |> Enum.filter(fn {_, e} -> e != "#" end)
  |> Enum.group_by(&elem(&1, 1), &elem(&1, 0))

max_x =
  input_string
  |> String.split("\n", trim: true)
  |> Enum.at(0)
  |> String.length()
  |> then(&(&1 - 1))

max_y =
  input_string
  |> String.split("\n", trim: true)
  |> Enum.count()
  |> then(&(&1 - 1))

{max_x, max_y}

Part 1

input
|> Enum.flat_map(fn {_, coords} ->
  coords
  |> then(fn list ->
    0..(Enum.count(list) - 1)
    |> Enum.flat_map_reduce(list, fn _, [head | tail] ->
      {Enum.map(tail, fn e -> {head, e} end), tail}
    end)
    |> elem(0)
  end)
end)
|> Enum.flat_map(fn {{x1, y1}, {x2, y2}} ->
  xd = abs(x1 - x2)
  yd = abs(y1 - y2)

  [
    {
      if(x1 < x2, do: x1 - xd, else: x1 + xd),
      if(y1 < y2, do: y1 - yd, else: y1 + yd)
    },
    {
      if(x2 < x1, do: x2 - xd, else: x2 + xd),
      if(y2 < y1, do: y2 - yd, else: y2 + yd)
    }
  ]
end)
|> Enum.filter(fn {x, y} ->
  x >= 0 && x <= max_x && y >= 0 && y <= max_y
end)
|> Enum.uniq()
|> Enum.count()

Part 2

input
|> Enum.flat_map(fn {_, coords} ->
  coords
  |> then(fn list ->
    0..(Enum.count(list) - 1)
    |> Enum.flat_map_reduce(list, fn _, [head | tail] ->
      {Enum.map(tail, fn e -> {head, e} end), tail}
    end)
    |> elem(0)
  end)
end)
|> Enum.flat_map(fn {{x1, y1}, {x2, y2}} ->
  xd = abs(x1 - x2)
  yd = abs(y1 - y2)
  max_iterations = if xd <= yd, do: div(max_x, xd), else: div(max_y, yd)

  0..max_iterations
  |> Enum.flat_map(fn i ->
    [
      {
        if(x1 < x2, do: x1 - xd * i, else: x1 + xd * i),
        if(y1 < y2, do: y1 - yd * i, else: y1 + yd * i)
      },
      {
        if(x2 < x1, do: x2 - xd * i, else: x2 + xd * i),
        if(y2 < y1, do: y2 - yd * i, else: y2 + yd * i)
      }
    ]
  end)
end)
|> Enum.filter(fn {x, y} ->
  x >= 0 && x <= max_x && y >= 0 && y <= max_y
end)
|> Enum.uniq()
|> Enum.count()