defmodule Aoc2023.Day15 do
  use Aoc2023

  def parse(input) do
    input |> String.split(",")
  end

  defp hash(str) do
    for <<char <- str>>, reduce: 0 do
      acc -> Bitwise.band((acc + char) * 17, 255) 
    end
  end

  def part1(input) do
    input |> Enum.map(&hash/1) |> Enum.sum
  end

  def part2(input) do
    for instr <- input, reduce: 0..255 |> Enum.map(&{&1, []}) |> Enum.into(%{}) do
      boxes ->
        [_, label, op, lens] = Regex.run(~r/([a-z]+)([-=])(\d*)/, instr)
        box = hash(label)
        label = String.to_atom(label)
        case op do
          "=" -> boxes |> Map.update!(box, &Keyword.update(&1, label, String.to_integer(lens), fn _ -> String.to_integer(lens) end))
          "-" -> boxes |> Map.update!(box, &Keyword.delete(&1, label))
        end
    end
    |> Enum.flat_map(fn {box, lenses} -> lenses |> Enum.with_index(1) |> Enum.map(fn {{_, focus}, i} -> focus * (box + 1) * i end) end)
    |> Enum.sum
  end
end