AoC19/lib/day5/day5.ex

defmodule Day5 do
  def read_program do
    File.read!("lib/day5/input.txt")
    |> String.trim()
    |> String.split(",")
    |> Enum.map(&String.to_integer/1)
  end

  def run_program do
    read_program()
    |> run()
  end

  def test do
    run([
      3,
      21,
      1008,
      21,
      8,
      20,
      1005,
      20,
      22,
      107,
      8,
      21,
      20,
      1006,
      20,
      31,
      1106,
      0,
      36,
      98,
      0,
      0,
      1002,
      21,
      125,
      20,
      4,
      20,
      1105,
      1,
      46,
      104,
      999,
      1105,
      1,
      46,
      1101,
      1000,
      1,
      20,
      4,
      20,
      1105,
      1,
      46,
      98,
      99
    ])
  end

  def run(memory, ip \\ 0)

  def run(memory, ip) when ip < length(memory) do
    IO.puts("IP: #{ip}")
    IO.inspect(memory)

    case eval(Enum.drop(memory, ip), memory) do
      {_memory, :halt} ->
        :halt

      {memory, :cont, offset} ->
        run(memory, ip + offset)

      {memory, :jump, ip} ->
        run(memory, ip)
    end

    :ok
  end

  def run(memory, _ip), do: memory

  defmacro opcode(op, expected) do
    quote do
      rem(unquote(op), 100) == unquote(expected)
    end
  end

  def get_param(op, param, val, memory) do
    place = param + 2

    if op |> div(pow(10, place)) |> rem(10) == 1 do
      val
    else
      Enum.at(memory, val)
    end
  end

  def pow(base, 1), do: base

  def pow(base, exp) do
    base * pow(base, exp - 1)
  end

  # halt
  def eval([99 | _], memory) do
    {memory, :halt}
  end

  # add
  def eval([op, a, b, dest | _], memory) when opcode(op, 1) do
    a_val = get_param(op, 0, a, memory)
    b_val = get_param(op, 1, b, memory)

    {
      List.replace_at(memory, dest, a_val + b_val),
      :cont,
      4
    }
  end

  # multiply
  def eval([op, a, b, dest | _], memory) when opcode(op, 2) do
    a_val = get_param(op, 0, a, memory)
    b_val = get_param(op, 1, b, memory)

    {
      List.replace_at(memory, dest, a_val * b_val),
      :cont,
      4
    }
  end

  # input
  def eval([3, addr | _], memory) do
    res = IO.gets("intcode> ") |> String.trim() |> String.to_integer()

    {
      List.replace_at(memory, addr, res),
      :cont,
      2
    }
  end

  # output
  def eval([op, param | _], memory) when opcode(op, 4) do
    out = get_param(op, 0, param, memory)
    IO.puts("Output: #{out}")

    {memory, :cont, 2}
  end

  # jump if non-zero
  def eval([op, condition, ip | _], memory) when opcode(op, 5) do
    case get_param(op, 0, condition, memory) do
      0 ->
        {memory, :cont, 3}

      _ ->
        {memory, :jump, get_param(op, 1, ip, memory)}
    end
  end

  # jump if zero
  def eval([op, condition, ip | _], memory) when opcode(op, 6) do
    case get_param(op, 0, condition, memory) do
      0 ->
        {memory, :jump, get_param(op, 1, ip, memory)}

      _ ->
        {memory, :cont, 3}
    end
  end

  # less than
  def eval([op, a, b, dest | _], memory) when opcode(op, 7) do
    a_val = get_param(op, 0, a, memory)
    b_val = get_param(op, 1, b, memory)

    memory =
      if a_val < b_val do
        List.replace_at(memory, dest, 1)
      else
        List.replace_at(memory, dest, 0)
      end

    {memory, :cont, 4}
  end

  # equals
  def eval([op, a, b, dest | _], memory) when opcode(op, 8) do
    a_val = get_param(op, 0, a, memory)
    b_val = get_param(op, 1, b, memory)

    memory =
      if a_val == b_val do
        List.replace_at(memory, dest, 1)
      else
        List.replace_at(memory, dest, 0)
      end

    {memory, :cont, 4}
  end
end