my_flatten.rb

# Putting the method on Array, instead of Enumerable, because I don't want hashes to flatten.
# There are other "flattenable" structures aside from an Array, but I am ignoring that.
class Array
  
  # A structure is defined as "flattenable" if it responds to my_flatten
  def my_flatten
    res = []
    each do |obj|
      if obj.respond_to?(:my_flatten)
        res.concat(obj.my_flatten)
      else
        res << obj
      end
    end
    res
  end
  
  # Another implementation
  # This is a somewhat strange use of inject, since it is using concat and << 
  # (both of which modify the object they are called on) for the fact that 
  # they return that same object. However, I believe using them 
  # is more efficient than the alternative, something like 
  # res + obj.my_flatten_with_inject and res + [obj]
  def my_flatten_with_inject
    inject([]) do |res,obj|
      if obj.respond_to?(:my_flatten_with_inject)
        res.concat(obj.my_flatten_with_inject)
      else
        res << obj
      end
    end
  end
end

require 'rubygems'
require 'spec'

# Not a complete set of specifications, but covers a good amount of cases.
context 'my_flatten' do
  it 'empty' do
    [].my_flatten.should == []
  end
  it 'one element' do
    [1].my_flatten.should == [1]
  end
  it 'one array' do
    [[1]].my_flatten.should == [1]
  end
  it 'one empty array' do
    [[]].my_flatten.should == []
  end
  it 'two elements' do
    [1,2].my_flatten.should == [1,2]
  end
  it 'an element and an array' do
    [1,[2,3]].my_flatten.should == [1,2,3]
  end
  it 'an array within an array' do
    [1,[[2,3],4]].my_flatten.should == [1,2,3,4]
  end
  it 'an empty array mixed with elements and full arrays' do
    [1,2,[3,4],[],5].my_flatten.should == [1,2,3,4,5]
  end
  it 'a hash within the array' do
    [1,2,{3 => 4},5].my_flatten.should == [1,2,{3 => 4},5]
  end
end