aoc2020day8.ex

defmodule VM do
  defstruct [:ip, :code, :acc]
end

defmodule Day8 do
  def read_input do
    File.read!("input/day8.txt")
    |> String.split("\n", trim: true)
    |> Enum.map(&(String.split(&1, " ", trim: true)))
    |> Enum.map(fn [action, param] -> { action, String.to_integer(param) } end)
  end

  def run_until_loop(vm, seen \\ MapSet.new()) do
    if MapSet.member?(seen, vm.ip) do
      vm
    else
      run_until_loop(step(vm), MapSet.put(seen, vm.ip))
    end
  end

  def step(%VM{ip: ip, code: code, acc: acc}) do
    case Enum.at(code, ip) do
      { "nop", p } -> %VM{ip: ip + 1, code: code, acc: acc }
      { "acc", p } -> %VM{ip: ip + 1, code: code, acc: acc + p}
      { "jmp", p } -> %VM{ip: ip + p, code: code, acc: acc }
      nil -> IO.puts(acc)
    end
  end

  def swap_instruction(code, index) do
    case Enum.at(code, index) do
      { "nop", param } -> List.replace_at(code, index, { "jmp", param })
      { "jmp", param } -> List.replace_at(code, index, { "nop", param })
      _ -> code
    end
  end

  def part1 do
    code = read_input()
    vm = %VM{ip: 0, code: code, acc: 0}
    result = run_until_loop(vm)
    IO.inspect(result.acc)
  end

  def part2 do
    original_code = read_input()
    lines = Enum.count(original_code)
    vms = for i <- 0..lines do
      %VM{ip: 0, acc: 0, code: swap_instruction(original_code, i)}
    end
    |> Enum.uniq

    vms
    |> Enum.each(fn vm -> run_until_loop(vm) end)
  end
end