Ruby inject method implementation

injectable.rb

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

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