Discussion on: Advent of Code 2020 Solution Megathread - Day 8: Handheld Halting

[Frederik 👨‍💻➡️🌐 Creemers]

Here's my solution for part 2 in Elixir. It shares most of its code with part 1, so I don't think it's all that interesting to share both.

defmodule BootLoader do
  def instr("nop " <> _value, acc, ptr) do {acc, ptr + 1} end
  def instr("acc " <> value, acc, ptr) do
    {acc + String.to_integer(value), ptr + 1}
  end
  def instr("jmp " <> value, acc, ptr) do
    {acc, ptr + String.to_integer(value)}
  end

  def execute(lines, seen, acc, ptr) do
    IO.puts("Executing #{ptr}")
    cond do
      ptr in seen ->
        IO.puts("already seen!")
        false
      ptr == length(lines) ->
        IO.puts("final result #{acc}")
        true
      true ->
        {new_acc, new_ptr} = instr(Enum.at(lines, ptr), acc, ptr)
        execute(lines, [ptr | seen], new_acc, new_ptr)
    end
  end

  def get_replacement("nop" <> x) do
    "jmp" <> x
  end

  def get_replacement("jmp" <> x) do
    "nop" <> x
  end

  def get_replacement(foo) do foo end

  def try_replacement(lines, rep_index) do
    rep = get_replacement(Enum.at(lines, rep_index))
    lines2 = List.replace_at(lines, rep_index, rep)
    if not execute(lines2, [], 0, 0) do
      try_replacement(lines, rep_index + 1)
    end
  end

  def main() do
    {:ok, code} = File.read("8.txt")
    lines = String.split(code, "\n")
    lines = List.delete_at(lines, length(lines)-1)
    IO.puts("program size: #{length(lines)}")
    try_replacement(lines, 0)
  end
end

BootLoader.main()