eriks_array.rb

# [X] length/size
# [X] push
# [X] [] (slice)
# [X] pop
# [X] first
# [X] last
# [X] shift
# [X] unshift
# [X] each
# [X] select/reject/collect/inject
# [ ] inspect

class EriksArray
  include Enumerable

  def initialize
    @size = 0
  end

  def inspect
    "[#{map(&:inspect).join(', ')}]"
  end

  def push(object)
    instance_variable_set("@var#{@size}".to_sym, object)
    @size += 1
    self
  end
  alias :<< push

  def pop
    @size -= 1
    remove_instance_variable("@var#{@size}".to_sym)
  end

  def slice(index)
    instance_variable_get("@var#{index}".to_sym) unless index < 0 # || index >= @size
  end
  alias :[] slice

  def size
    @size
  end
  alias length size

  def last
    # instance_variable_get("@var#{@size - 1}".to_sym)
    # self.slice(@size - 1)
    self[@size - 1]
  end

  def first
    self[0]
  end

  def shift
    # before:  ["a", "b", "c", "d"]
    # shift
    # after: ["b", "c", "d"]
    val = first
    @size -= 1
    @size.times do |i|
      next_val = instance_variable_get("@var#{i + 1}".to_sym)
      instance_variable_set("@var#{i}".to_sym, next_val)
    end
    remove_instance_variable("@var#{@size}".to_sym)
    val
  end

  def unshift(object)
    # before: ["b", "c", "d"]
    # unshift("a")
    # after:  ["a", "b", "c", "d"]
    @size.times do |i|
      val = instance_variable_get("@var#{@size - i - 1}".to_sym)
      instance_variable_set("@var#{@size - i}".to_sym, val)
    end
    instance_variable_set("@var0".to_sym, object)
    @size += 1
    self
  end

  def each(&block)
    @size.times do |i|
      block.yield self[i]
    end
  end

end

require 'rspec'

describe EriksArray do
  
  describe ".new" do
    it "initalizes with a size of zero" do
      array = EriksArray.new
      expect(array.size).to eq 0
    end
  end

  describe "#push" do
    it "pushes an object on to the end of the array" do
      array = EriksArray.new
      array.push(5)
      expect(array.size).to eq 1
      expect(array.slice(0)).to eq 5
      expect(array[0]).to eq 5
    end
  end

  describe "#pop" do
    it "pops an object off the end of the array" do
      array = EriksArray.new
      array.push(5)
      expect(array.pop).to eq 5
      expect(array.length).to eq 0
    end
  end

  describe "#first" do
    it "returns the first element" do
      array = EriksArray.new
      array.push(5)
      array.push(6)
      expect(array.first).to eq 5
    end
  end

  describe "#last" do
    it "returns the last element" do
      array = EriksArray.new
      array.push(5)
      array.push(6)
      expect(array.last).to eq 6
      expect(array.size).to eq 2
    end
  end

  describe "#shift" do
    it "shift the first element off the array" do
      array = EriksArray.new
      array.push(5)
      array.push(6)
      before_size = array.size
      object = array.shift
      expect(object).to eq 5
      expect(array.size).to eq before_size - 1
      expect(array[0]).to eq 6
      expect(array[1]).to be_nil
    end
  end

  describe "#unshift" do
    it "adds an element to the front of the array" do
      array = EriksArray.new
      array.push(5)
      before_size = array.size
      expect(array[0]).to eq 5
      expect(array[1]).to be_nil
      object = array.unshift(4)
      expect(object).to be_an EriksArray
      expect(array.size).to eq before_size + 1
      expect(array[0]).to eq 4
      expect(array[1]).to eq 5
    end
  end

  describe "#each" do
    it "iterates through each element in the array" do
      array = EriksArray.new
      array.push(5)
      array.push(10)
      array.push(15)
      num = 0
      array.each{|i| num += i}
      expect(num).to eq 30
    end
  end

end