Reverse Polish Notation Calculator

rpn_spec.rb

require 'rspec'

module RPN
  def calculate(input="")
    do_calculate(input.split(' ').map{|t|t == "#{t.to_i}" ? t.to_i : t.to_sym})
  end

  def do_calculate(tokens=[])
    if tokens.length == 3
      x,y,opp = tokens
      [:+, :-, :*, :/].include?(opp) ? x.send(opp,y) : "Invalid Input"
    elsif tokens.length > 3
      x,y,opp  = tokens[0], do_calculate(tokens[1..-2]), tokens[-1]
      do_calculate([x,y,opp])
    else
      "Invalid Input"
    end
  end

  module_function :calculate, :do_calculate
end

samples = [
  ["",               "Invalid Input"    ],
  ["1 1",            "Invalid Input"    ],
  ["1 1 ?",          "Invalid Input"    ],
  ["1 1 +",          2    ],
  ["5 4 -",          1    ],
  ["99 100 *",       9900 ],
  ["10 2 /",         5    ],
  ["6 2 3 + -",      1    ],
  ["5 6 2 3 + - *",  5    ]
]

describe RPN do
  samples.each do |sample|
    it "interprets '#{sample[0]}' and returns #{sample[1]}" do
      expect(RPN.calculate(sample[0])).to eq(sample[1])
    end
  end
end



# Below is a sample run of these tests:
#
# $ rspec -fd rpn.rb
# RPN
#   interprets '' and returns Invalid Input
#   interprets '1 1' and returns Invalid Input
#   interprets '1 1 ?' and returns Invalid Input
#   interprets '1 1 +' and returns 2
#   interprets '5 4 -' and returns 1
#   interprets '99 100 *' and returns 9900
#   interprets '10 2 /' and returns 5
#   interprets '6 2 3 + -' and returns 1
#   interprets '5 6 2 3 + - *' and returns 5
#
# Finished in 0.00232 seconds (files took 0.15198 seconds to load)
# 9 examples, 0 failures