array.rb

class Array
  def promote_element(e)
    i = self.index(e)
    return self if i == 0 or i == nil
    result = self.slice(0, i-1)
    result << e
    result << self[i-1]
    result.concat(self.slice(i+1, self.length))
  end

  def demote_element(e)
    i = self.index(e)
    return self if i == self.length - 1 or i == nil
    result = self.slice(0, i)
    result << self[i+1]
    result << e
    result.concat(self.slice(i+2, self.length))
  end

  def remove_element(e)
    i = self.index(e)
    return self if i == nil
    result = self.slice(0, i)
    result.concat(self.slice(i+1, self.length))
  end

end

array_spec.rb

require 'spec_helper'

describe Array do

  before do
    @list = [:one, :two, :three]
  end

  describe "promote_element" do

    it "promotes the element" do
      @list.promote_element(:two).should == [:two, :one, :three]
      @list.promote_element(:three).should == [:one, :three, :two]
    end

    it "returns self when promoting the first" do
      @list.promote_element(:one).should == @list
    end

    it "returns self when promoting non-element" do
      @list.promote_element(:four).should == @list
    end

  end

  describe "demote_element" do

    it "demotes the element" do
      @list.demote_element(:two).should == [:one, :three, :two]
      @list.demote_element(:one).should == [:two, :one, :three]
    end

    it "returns self when demoting the last" do
      @list.demote_element(:three).should == @list
    end

    it "returns self when demoting non-element" do
      @list.demote_element(:four).should == @list
    end

  end

  describe "remove element" do

    it "removes the element" do
      @list.remove_element(:one).should == [:two, :three]
      @list.remove_element(:two).should == [:one, :three]
      @list.remove_element(:three).should == [:one, :two]
    end

    it "returns self when removing non-element" do
      @list.remove_element(:four).should == @list
    end

  end

end