Loved part 2 today.

My solution, not clean but working. Everytime a branch fails (returns false) it tries another branch interpreting jmp as nop or vice versa – but only once tracked by the changed bool - finishes as soon as a valid branch has been found:

#!/usr/bin/env elixir

defmodule Day8 do
  def reduce(instr, ptr, state, changed) do
    case Map.pop(instr, ptr) do
      {{"nop", n}, instr} -> reduce(instr, ptr + 1, state, changed) || (unless changed, do: reduce(instr, ptr + n, state, true))
      {{"jmp", n}, instr} -> reduce(instr, ptr + n, state, changed) || (unless changed, do: reduce(instr, ptr + 1, state, true))
      {{"acc", n}, instr} -> reduce(instr, ptr + 1, state + n, changed)
      {nil, _instr} -> false
      {:fin, _instr} -> state
    end
  end
end

instr = File.read!("8.csv")
|> String.split("\n", trim: true)
|> Enum.with_index()
|> Enum.map(fn {row, idx} ->
  [op, num] = String.split(row)
  num = String.to_integer(num)
  {idx, {op, num}}
end)
|> Map.new()

instr = Map.put(instr, map_size(instr), :fin)
Day8.reduce(instr, 0, 0, false)
|> IO.inspect()

I build a struct + proper API for the bootloader today, before even trying to get to the answers – the goal being someone should be able to understand the code even without knowing the problem. This approach made part two quite simple because all I needed to add was brute-forcing the intended instruction changes and attempting to run the bootloader for each attempt like for part 1.

https://github.com/LostKobrakai/aoc2020/commit/586e5ef8b3bf00697ddbc5f563c936d9e1506600

This one is really clever, it completes in 38 microseconds instead of 28ms which my brute force method does