defmodule BalancedOctopus do
  @opening_to_closing %{"(" => ")", "[" => "]", "{" => "}"}
  @closing_to_opening %{")" => "(", "]" => "[", "}" => "{"}

  def balanced?(text, stack \\ [])
  def balanced?("", []), do: true
  def balanced?(_text, :error), do: false
  def balanced?("", _pending_stack), do: false

  def balanced?(text, stack) do
    {current_char, next_text} = String.split_at(text, 1)
    updated_stack = maybe_stack(stack, current_char)
    balanced?(next_text, updated_stack)
  end

  defp maybe_stack(stack, char) do
    cond do
      Map.has_key?(@opening_to_closing, char) ->
        stack_push(stack, char)

      Map.has_key?(@closing_to_opening, char) ->
        {last_opening, rest} = stack_pop(stack)
        required_last_opening = @closing_to_opening[char]
        if last_opening == required_last_opening, do: rest, else: :error

      true ->
        stack
    end
  end

  defp stack_push(stack, inserting), do: [inserting | stack]

  defp stack_pop([]), do: {nil, []}
  defp stack_pop([popped | rest] = _stack), do: {popped, rest}
end

[
  ["abcd", true],
  ["(abcd", false],
  ["(abcd)", true],
  ["(ab[cd)ef]", false],
  ["(abc[%def])", true],
  ["$(abc[de]fg{hi}jk)%//", true],
  ["(({abc})", false]
]
|> Enum.each(fn [text, expectation] ->
  IO.write("Testing #{text} as #{expectation}... ")

  if BalancedOctopus.balanced?(text) == expectation do
    IO.puts("✅")
  else
    IO.puts("🔴")
  end
end)