String calculator

string_calculator.rb

module StringCalculator

  def add
    return 0 if empty?
    raise "Negatives not allowed: #{ negative_numbers.join(",") }" unless negative_numbers.empty?
    retrieve_numbers.reduce(0) {|total,number|total+number}
  end

  def retrieve_numbers
    @retrieve_numbers ||= split(/[\n#{delimiter}]/).map(&:to_i)
  end

  def negative_numbers
    @negative_numbers ||= retrieve_numbers.select {|number| number < 0}
  end

  def delimiter
    self[0,2] == "//" ? self[2,1] : ","
  end

end

string_calculator_spec.rb

require File.join(File.dirname(__FILE__), "../lib/string_calculator")

describe StringCalculator do

  before :all do 
    class String
       include StringCalculator
    end
  end

  it "returns 0 for empty string" do
    "".add.should == 0
  end

  context "1 number" do
    
    it "returns 0 for 0" do
      "0".add.should == 0
    end

    it "returns 2 for 2" do
      "2".add.should == 2
    end
    
  end

  context "more than 1 number" do 

    it "can sum 2 numbers" do
      "1,2".add.should == 3
    end

    it "can sum 3 numbers" do
      "1,2,3".add.should == 6
    end

    it "can sum 100 numbers" do
      (1..100).to_a.join(",").add.should == 5050
    end

  end


  context "delimiters" do
    
    it "sums numbers separated by commas" do
      "2,3".add.should == 5
    end
    
    it "sums numbers separated by newline" do
      "5\n3".add.should == 8
    end

    it "can have a custom delimiter" do
      "//;\n3;3".add.should == 6
    end

    it "supports mixed delimiters" do
      "2,3\n4".add.should == 9
    end

    it "supports mixed delimiters using a custom delimiter" do
      "//;\n3;3\n2".add.should == 8
    end

    it "supports minus as a custom delimiter" do
      "//-\n2-3-3".add.should == 8
    end
    
  end

  context "negative numbers" do
    
    it "raises an error if there's a negative number" do
      lambda { "-1,2".add }.should raise_error
    end

    it "shows negative numbers in the error message" do
      lambda { "-1,2,-3".add }.should raise_error("Negatives not allowed: -1,-3")
    end

  end

end