# given an input like the following "five minus ten plus four plus ten times minus six"
# create a function that will perform the calculation (5 - 10 + 4 + 10 * - 6)
# only needs to accept numbers -10..10
# only needs to account for multiplication and addition/subtraction
# needs to follow proper order of operations

defmodule Calculator do
  def operation(:plus, a, b) do
    a + b
  end

  def operation(:minus, a, b) do
    a - b
  end

  def operation(:times, a, b) do
    a * b
  end

  def parse(tokens) do
    tokens = parse(tokens, [], [:times])
    tokens = parse(tokens, [], [:plus, :minus])
    tokens
  end

  def scan(tokens) do
    case tokens do
      [:minus, a, operator, :minus, b | rest] ->
        {:expr, -a, operator, -b, rest}

      [a, operator, :minus, b | rest] ->
        {:expr, a, operator, -b, rest}

      [:minus, a, operator, b | rest] ->
        {:expr, -a, operator, b, rest}

      [a, operator, b | rest] ->
        {:expr, a, operator, b, rest}

      [:minus, a | rest] ->
        {:term, -a, rest}

      [a | rest] ->
        {:term, a, rest}

      [] ->
        :end
    end
  end

  def parse(tokens, accumulator, pass) do
    case scan(tokens) do
      {:expr, a, operator, b, rest} ->
        if operator in pass do
          result = operation(operator, a, b)
          parse([result | rest], accumulator, pass)
        else
          parse(tl(tokens), accumulator ++ [hd(tokens)], pass)
        end

      {:term, a, rest} ->
        parse(rest, accumulator ++ [a], pass)

      :end ->
        accumulator
    end
  end
end