Token based calculator

calc.rb

Inst = Struct.new(:op, :a, :b) do
  def comp
    raise "Missing Operand: a" unless a
    raise "Missing Operand: b" unless b
    return a.to_i + b.to_i if op == '+'
    return a.to_i - b.to_i if op == '-'
    raise "Invalid Op: #{op}"
  end
end

i1 = "((1+(4+5+2)-3))+(6+8)"

# temp: 
# c: 
# inst: (23)
# stack: () 

def calc(input)
  stack = []
  inst = Inst.new
  input.each_char do |c|
    if c == '('
      stack.push(inst)
      inst = Inst.new
    elsif c == ')'
      temp = inst.a
      inst = stack.pop

      inst.b = temp if inst.a
      inst.a = temp unless inst.a
      if inst.b
        inst = Inst.new(nil, inst.comp, nil)
      end
    elsif c == '+'
      inst.op = '+'
    elsif c == '-'
      inst.op = '-'
    else # c is digit
      inst.b = c if inst.a
      inst.a = c unless inst.a
      if inst.b
        inst = Inst.new(nil, inst.comp, nil)
      end
    end
  end
  return inst.a
end

puts(calc(i1))