defmodule Day7 do
  def run do
    lines =
      File.read!("data.txt")
      |> String.trim()
      |> String.split("\n")
      |> Enum.map(fn l -> String.split(l, ~r{\D+}) |> Enum.map(&String.to_integer(&1)) end)
      |> Enum.map(fn [head | tail] -> {head, tail} end)

    [
      # part 1's operatons: * and +
      [&*/2, &+/2],
      # part 2's operatons: *, +, and concat
      [&*/2, &+/2, &("#{&1}#{&2}" |> String.to_integer())]
    ]
    |> Enum.map(fn ops ->
      # filter valid lines for parts 1 and 2, grab their targets, and sum them
      Enum.filter(lines, &valid?(&1, ops)) |> Enum.map(&elem(&1, 0)) |> Enum.sum()
    end)
    |> Enum.map(&dbg(&1))
  end

  # base case, when there are 2 operands. returns true if applying any of the ops returns the target
  defp valid?({total, [a, b]}, ops) do
    Enum.any?(ops, fn op -> op.(a, b) == total end)
  end

  # apply each operation to the first two operands, prepend and recurse
  defp valid?({total, [a, b | rest]}, ops) do
    Enum.any?(ops, fn op -> valid?({total, [op.(a, b) | rest]}, ops) end)
  end
end

Day7.run()