# Dijkstra's Two-Stack Algorithm for Expression Evaluation,
# see examples in test/expressions_test.rb
module Expressions
  def self.evaluate(expression)
    operators = []
    values = []

    expression.each_char do |ch|
      next if ch == '('

      if ch == ')'
        operator = operators.shift
        value2 = values.shift
        value1 = values.shift
        result = value1.send(operator, value2)
        values.unshift(result)
      elsif '+-*/'.include?(ch)
        operators.unshift(ch)
      else
        values.unshift(ch.to_i)
      end
    end

    values.shift
  end
end