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.

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

I got to use some cool Stream functions to solve this one: