aoc2018/day3.exs

defmodule DayThree do
  def parse_claim(claim) do
    pattern = ~r/#(\d+) @ (\d+),(\d+): (\d+)x(\d+)/
    [[_ | match]] = Regex.scan(pattern, claim)
    [id, x, y, xwidth, ywidth] = Enum.map(match, &String.to_integer(&1))

    {id, x, y, xwidth, ywidth}
  end

  def part_one(input) do
    claims =
      String.split(input, "\n")
      |> Enum.filter(&(&1 != ""))
      |> Enum.map(&DayThree.parse_claim/1)

    dupes = new_doubles(claims)

    area_with =
      dupes
      |> Enum.map(fn {_, {_, _, dx, dy}} -> dx * dy end)
      |> List.foldl(0, &+/2)

    area_with
  end

  def new_doubles(claims) do
    Enum.map(claims, fn {id1, x1, y1, dx1, dy1} ->
      Enum.map(claims, fn {id2, x2, y2, dx2, dy2} ->
        {{:id, id1, id2}, intersect({id1, x1, y1, dx1, dy1}, {id2, x2, y2, dx2, dy2})}
      end)
      |> Enum.filter(fn {_, square} -> square != nil end)
    end)
    |> List.flatten()
    |> Enum.filter(fn {_, square} -> square != nil end)
  end

  def intersect({id1, x1, y1, dx1, dy1}, {id2, x2, y2, dx2, dy2}) do
    if id1 == id2 do
      nil
    else
      intersect({x1, y1, dx1, dy1}, {x2, y2, dx2, dy2}, true)
    end
  end

  def intersect({x1, y1, dx1, dy1}, {x2, y2, dx2, dy2}, ignore \\ false) do
    if {x1, y1, dx1, dy1} == {x2, y2, dx2, dy2} && not ignore do
      nil
    else
      xe1 = x1 + dx1
      xe2 = x2 + dx2
      ye1 = y1 + dy1

      if x2 <= xe1 && y2 <= ye1 && x2 >= x1 && y2 >= y1 do
        {y2, x2, xe1 - x2, ye1 - y2}
      else
        if xe2 >= x1 && y2 <= ye1 && x2 <= x1 && y2 >= y1 do
          {x1, y2, xe2 - x1, ye1 - y2}
        else
          nil
        end
      end
    end
  end

  def part_two(input) do
    claims =
      String.split(input, "\n")
      |> Enum.filter(&(&1 != ""))
      |> Enum.map(&DayThree.parse_claim/1)

    dupes =
      new_doubles(claims)
      |> Enum.dedup()
      |> Enum.map(fn {{:id, id1, id2}, _} -> [id1, id2] end)
      |> List.flatten()

    claim_id = Enum.map(claims, fn {id, _, _, _, _} -> id end)
    [id] = Enum.filter(claim_id, &(not Enum.member?(dupes, &1)))
    id
  end
end

{:ok, content} = File.read("input3.txt")

IO.puts("Part 1 (most likely incorrect): #{DayThree.part_one(content)}")
IO.puts("Part 2: #{DayThree.part_two(content)}")