require 'rubygems'
require 'rspec'
 
module Enumerable
  def inject(*args, &block)
    if(args.size == 2)
      inject_binary_operation(args[0], args[1], self)
    elsif(args.size == 1 && !block_given?)
      inject_binary_operation(self.first, args[0], self.drop(1))
    else
      the_memo = args[0] || self.first
      self.each { |item| the_memo = yield(the_memo, item) if block_given? }
      the_memo
    end
  end
  alias :reduce :inject
 
  private
 
  def inject_binary_operation(the_memo, operator, enum)
    raise TypeError.new("#{operator} is not a symbol") unless operator.is_a?(Symbol)
    enum.each { |item| the_memo = the_memo.send(operator, item) }
    the_memo
  end
end
 
shared_examples_for "an Enumerable object" do
 
  describe "inject" do
 
    describe "given there is a specified memo value" do
 
      it('should return the newly injected "memo" value if using a block') do
        enumerable.inject([3,4,5]) { |memo, enum_item| memo.include?(enum_item) ? memo : memo << enum_item }.should == [3, 4, 5, 1, 2]
      end
 
      describe "when it is a binary operation" do
 
        it('should return a TypeError because the single argument is not a Symbol') do
          lambda { enumerable.inject(2) }.should raise_error(TypeError)
        end
 
        it('should add the values') do
          enumerable.inject(2, :+).should == 8
        end
 
        it('should multiply the values') do
          enumerable.inject(2, :*).should == 12
        end
 
        it('should subtract the values') do
          enumerable.inject(2, :-).should == -4
        end
 
    end
 
    end
 
    describe "given there isn't a specified memo value" do
 
      it('should return the newly injected "memo" value if using a block') do
        enumerable.inject { |memo, enum_item| memo > enum_item ? memo : enum_item }.should == 3
      end
 
      describe "when it is a binary operation" do
 
        it('should add the values') do
          enumerable.inject(:+).should == 6
        end
 
        it('should multiply the values') do
          enumerable.inject(:*).should == 6
        end
 
        it('should subtract the values') do
          enumerable.inject(:-).should == -4
        end
      end
 
    end
  end  
end
 
describe Range do
 
  it_should_behave_like "an Enumerable object" do
    let(:enumerable) { (1..3) }
  end
 
end
 
describe Array do
 
  it_should_behave_like "an Enumerable object" do
    let(:enumerable) { [1,2,3] }
  end
 
end

$ rspec injectable.rb --format documentation
 
Range
  it should behave like an Enumerable object
    inject
      given there is a specified memo value
        should return the newly injected "memo" value if using a block
        when it is a binary operation
          should return a TypeError because the single argument is not a Symbol
          should add the values
          should multiply the values
          should subtract the values
      given there isn't a specified memo value
        should return the newly injected "memo" value if using a block
        when it is a binary operation
          should add the values
          should multiply the values
          should subtract the values
 
Array
  it should behave like an Enumerable object
    inject
      given there is a specified memo value
        should return the newly injected "memo" value if using a block
        when it is a binary operation
          should return a TypeError because the single argument is not a Symbol
          should add the values
          should multiply the values
          should subtract the values
      given there isn't a specified memo value
        should return the newly injected "memo" value if using a block
        when it is a binary operation
          should add the values
          should multiply the values
          should subtract the values
 
Finished in 0.007 seconds
18 examples, 0 failures