Stabby RPN

rpn_calc.rb

class RPNCalculator
  
  def initialize
    @special_cases = [->(op, arr) {arr.include?('0') && op == '*' && '0'},
                      ->(op, arr) {arr.first == ('0') && op == '/' && '0'},
                      ->(op, arr) {arr.include?('1') && op == '*' && (arr-['1']).first},
                      ->(op, arr) {arr.include?('0') && op =~ /[+-]/ && (arr-['0']).first}]
  end

  def evaluate(*inputs)
    symbol_to_value = inputs[1..-1].last || {}
    @stack = []
    inputs.first.split.each do |element|
      element = (symbol_to_value[element.to_sym] || element).to_s
      if element =~ /[+-\/*]/
        special_case = @special_cases.find {|spc| spc.call element, @stack[-2..-1]}
        if special_case
          @stack[-1] = special_case.call element, [@stack[-2], @stack.pop]
        else
          two_numbers_on_stack? ? compute(element) : @stack << element
        end
      else
        @stack << element
      end
    end
    @stack.join(' ')
  end

  def two_numbers_on_stack?
    @stack[-1] =~ /\d+/ && @stack[-2] =~ /\d+/
  end

  def compute(element)
    @stack[-1] = eval("#{@stack[-2]} #{element} #{@stack.pop}").to_s
  end

end

calc = RPNCalculator.new

puts calc.evaluate('3 4 +')                           == '7'
puts calc.evaluate('y x +')                           == 'y x +'
puts calc.evaluate('3 1 x * +')                       == '3 x +'
puts calc.evaluate('0 x +')                           == 'x'
puts calc.evaluate('x y +', :x => 1, :y => 2)         == '3'
puts calc.evaluate('x y +', :x => 1)                  == '1 y +'
puts calc.evaluate('x y +', :y => 10)                 == 'x 10 +'
puts calc.evaluate('w z *', :z => 1)                  == 'w'
puts calc.evaluate('z x y + *', :z => 10)             == '10 x y + *'
puts calc.evaluate('z x y + *', :x => 1, :y => 2)     == 'z 3 *'
puts calc.evaluate('z x y + *', :z => 10, :x => 3)    == '10 3 y + *'


p calc.evaluate('b 8 5 a * * *')