#!/usr/bin/env elixir
defmodule Day13 do
  defmodule Machine do
    defstruct [:a, :b, :prize]
  end

  def part1(input) do
    input
    # these are equivalent linear equations
    # know how to do it algebraically, but not algorithmically
    # e.g. for first example
    # 
    # 94a + 22b - 8400 = 34a + 67b - 5400
    # 94a + 22b - 3000 = 34a + 67b
    # 60a + 22b - 3000 = 67b
    # 60a - 3000 = 45b
    # 4a - 200 = 3b
    # a = 3/4b + 50
    # 34(3/4b + 50) + 67b - 5400 = 0
    # 25.5b + 1700 + 67b - 5400 = 0
    # 92.5b = 3700
    # b = 40
    # 34a + 67(40) = 5400
    # 34a + 2680 = 5400
    # 34a = 2720
    # a = 80
  end

  def part2(_input) do
    :ok
  end

  def input do
    with [input_filename] <- System.argv(),
         {:ok, input} <- File.read(input_filename) do
      input
      |> String.split("\n\n")
      |> Enum.map(fn machine ->
        machine
        |> String.split(["+", ",", "=", "\n"], trim: true)
        |> tl()
        |> Enum.take_every(2)
        |> Enum.map(&String.to_integer/1)
      end)
    else
      _ -> :error
    end
  end

  #######################
  # HERE BE BOILERPLATE #
  #######################

  def run do
    case input() do
      :error -> print_usage()
      input -> run_parts_with_timer(input)
    end
  end

  defp run_parts_with_timer(input) do
    run_with_timer(1, fn -> part1(input) end)
    run_with_timer(2, fn -> part2(input) end)
  end

  defp run_with_timer(part, fun) do
    {time, result} = :timer.tc(fun)
    IO.puts("Part #{part} (completed in #{format_time(time)}):\n")
    IO.puts("#{inspect(result)}\n")
  end

  defp format_time(μsec) when μsec < 1_000, do: "#{μsec}μs"
  defp format_time(μsec) when μsec < 1_000_000, do: "#{μsec / 1000}ms"
  defp format_time(μsec), do: "#{μsec / 1_000_000}s"

  defp print_usage do
    IO.puts("Usage: elixir day13.exs input_filename")
  end
end

# Day13.run()