aoc2020day5.ex

defmodule Seat do
  defstruct min_row: 0, max_row: 127, min_col: 0, max_col: 7

  def from_string(str) do
    str
    |> String.graphemes
    |> Enum.reduce(%Seat{}, &Day5.process_letter/2)
  end

  def from_tuple({row, col}) do
    %Seat{
      min_row: row,
      max_row: row,
      min_col: col,
      max_col: col,
    }
  end
end

defmodule Day5 do
  def read_input do
    File.read!("input/day5.txt")
    |> String.split("\n", trim: true)
    |> Enum.map(&Seat.from_string/1)
  end

  def process_letter(letter, seat) do
    middle_row = (seat.max_row + seat.min_row) / 2
    middle_col = (seat.max_col + seat.min_col) / 2

    %Seat{
      min_row: (if letter == "B", do: ceil(middle_row), else: seat.min_row),
      max_row: (if letter == "F", do: floor(middle_row), else: seat.max_row),
      min_col: (if letter == "R", do: ceil(middle_col), else: seat.min_col),
      max_col: (if letter == "L", do: floor(middle_col), else: seat.max_col)
    }
  end

  def seat_id(seat) do
    seat.min_row * 8 + seat.min_col
  end

  def part1 do
    read_input()
    |> Enum.map(&seat_id/1)
    |> Enum.max
    |> IO.inspect
  end

  def part2 do
    marked_places = read_input()
                    |> Enum.map(fn seat -> { seat.max_row, seat.max_col } end)
                    |> MapSet.new

    rows = marked_places
           |> Enum.map(fn {row, _col} -> row end)

    max_row = rows |> Enum.max
    min_row = rows |> Enum.min

    possible_places = for row <- min_row+1..max_row-1 , col <- 0..7 do
      { row, col }
    end
    |> MapSet.new

    myplace = MapSet.difference(possible_places, marked_places)
              |> Enum.at(0)
              |> Seat.from_tuple

    IO.inspect(seat_id(myplace))
  end
end