Mull It Over

Mix.install([{:kino, "~> 0.14.2"}])

Input

input = Kino.Input.textarea("Input:")

input = Kino.Input.read(input)

Part 1

muls = Regex.scan(~r"mul\(\d+,\d+\)", input)

instructions =
  Enum.map(muls, fn [m] ->
    [_, a, b] = Regex.run(~r"mul\((\d+),(\d+)\)", m)
    {String.to_integer(a), String.to_integer(b)}
  end)

Enum.reduce(instructions, 0, fn {a, b}, c -> a * b + c end)

Part 2

instructions =
  Regex.scan(~r"mul\(\d+,\d+\)|don't|do", input)
  |> Enum.map(fn
    ["mul" <> args] ->
      [_, a, b] = Regex.run(~r"\((\d+),(\d+)\)", args)
      {:mul, {String.to_integer(a), String.to_integer(b)}}

    ["don't"] ->
      :disable

    ["do"] ->
      :enable
  end)

interpret = fn
  {:mul, {a, b}}, {:enabled, acc} -> {:enabled, acc + a * b}
  {:mul, _}, {:disabled, acc} -> {:disabled, acc}
  :enable, {_, acc} -> {:enabled, acc}
  :disable, {_, acc} -> {:disabled, acc}
end

Enum.reduce(instructions, {:enabled, 0}, interpret)