Challenge 1: A Custom Enumerable Class — Solution

class Playlist
  include Enumerable

  def initialize
    @songs = []
  end

  def add(song)
    @songs << song
    self
  end

  def each
    @songs.each { |song| yield song }
  end
end

playlist = Playlist.new
playlist.add("Bohemian Rhapsody")
         .add("Stairway to Heaven")
         .add("Hotel California")
         .add("Sweet Child O' Mine")
         .add("Purple Rain")

matches = playlist.select { |song| song.include?("Heaven") }
puts matches.inspect
puts playlist.count

=begin
Output:
["Stairway to Heaven"]
5

Notes:
- Playlist only defines each, which yields every element of the internal
  @songs array one at a time.
- .select and .count are never defined anywhere in Playlist — both come
  entirely from include Enumerable, driven by the each method above.
- add returns self at the end, which is what makes chaining
  .add(...).add(...).add(...) possible in the setup code.
=end