calculator.rb

class Calculator
    def calculate(expresion)
        return 0 if expresion.split.size != 3
        parts = expresion.split
        parts[0].to_i.send(parts[1], parts[2].to_i)
    end
end

describe Calculator do
    let(:calculator) { Calculator.new}

    it "adds numbers" do
        calculator.calculate("1 + 2").should == 3
    end

    it "adds big numbers" do
        calculator.calculate("11 + 22").should == 33
    end

    it "subtracts numbers" do
        calculator.calculate("2 - 1").should == 1
    end

    it "multiplies numbers" do
        calculator.calculate("2 * 3").should == 6
    end

    it "divides numbers" do
        calculator.calculate("12 / 3").should == 4
    end

    it "evaluates an empty string as zero" do
        calculator.calculate("").should == 0
    end

    it "evaluates nonsense to zero" do
        calculator.calculate("+ 1 2 3").should == 0
    end
end