lights.ex

inputs = ["turn on 499,499 through 500,500"]

res = inputs |> Enum.reduce(%{}, fn (input, acc) ->
  [[_, cmd, x1, y1, x2, y2]] = Regex.scan(~r/(turn (?:on|off)|toggle) (\d+),(\d+) through (\d+),(\d+)/, input)

  x1 = String.to_integer(x1)
  x2 = String.to_integer(x2)
  y1 = String.to_integer(y1)
  y2 = String.to_integer(y2)

  offset = 1_000_000
  
  Enum.reduce(x1..x2, acc, fn (x, x_acc) ->
    Enum.reduce(y1..y2, x_acc, fn (y, y_acc) ->
      Dict.get_and_update(y_acc, x + y * offset, fn (current) ->
        if (current == nil), do: current = 0
        
        case cmd do
          "turn on" ->
            {nil, current + 1}
          "turn off" ->
            if current > 0 do
              {nil, current - 1}
            else
              {nil, 0}
            end
          "toggle" ->
            {nil, current + 2}
        end
      end)
      |> elem(1)
    end)
  end)
end)