gistfile1.txt

FIRST_OPERATORS = Set.new(['+', "-"])
SECOND_OPERATORS = Set.new(['*', '/'])

def is_digit?(char)
    char[0].between?('0', '9')
end

def is_operator?(char)
   op_order(char) > 0
end

def op_order(char)
  if FIRST_OPERATORS.member?(char)
    1
  elsif SECOND_OPERATORS.member?(char)
    2
  else
    0
  end
end

def calculate(s)
  Calculator.new.calculate(s)
end

class Calculator

  def initialize
    @number = 0
    @op_stack = ['+']
    @stack = [0]
  end

  def calculate(str)
    str.chars.each do |char|
      parse(char)
    end

    @stack << @number
    resolve_operations('=')

    @stack.last
  end

  def parse(char)
    if is_digit?(char)
      @number *= 10
      @number += char.to_i
    elsif is_operator?(char)
      @stack << @number
      @number = 0
      resolve_operations(char)
    end
  end

  def resolve_operations(next_operation)
    while !@op_stack.empty? && op_order(next_operation) <= op_order(@op_stack.last)
      right = @stack.pop
      result = operate(@stack.pop, right, @op_stack.pop)
      @stack << result
    end
    @op_stack << next_operation
  end

  def operate(left, right, operator)
      case operator
      when '+'
        left + right
      when '-'
        left - right
      when '*'
        left * right
      when '/'
        left / right
      else
        raise "invalid operator"
      end
  end
  
end