Paginator for Enumerable

paginator.rb

class Paginator

  DEFAULT_PAGE      = 1
  DEFAULT_PER_PAGE  = 5

  attr_reader :page
  attr_reader :per_page

  def paginate(collection, options={})
    setup(options)
    slice(collection)
  end

  private

  def setup(options={})
    @page, @per_page = fetch_options(options)
  end

  def fetch_options(options={})
    _options = default_options.merge(options) do |key, oldval, newval|
      (newval < 0) ? oldval : newval
    end

    [_options[:page], _options[:per_page]]
  end

  def default_options
    @default_options ||= {
      :page     => DEFAULT_PAGE,
      :per_page => DEFAULT_PER_PAGE
    }
  end

  def slice(collection)
    collection[head..tail] if collection.kind_of?(Enumerable)
  end

  def head
    offset - per_page
  end

  def tail
    offset - 1
  end

  def offset
    page * per_page
  end

end

module Enumerable

  def paginate(options={})
    Paginator.new.paginate(self, options)
  end

end

letters = ("a".."z").map { |letter| letter }

puts letters.paginate.inspect                             # => ["a", "b", "c", "d", "e"]
puts letters.paginate(:page => 2).inspect                 # => ["f", "g", "h", "i", "j"]
puts letters.paginate(:per_page => 3).inspect             # => ["a", "b", "c"]
puts letters.paginate(:page => 2, :per_page => 3).inspect # => ["d", "e", "f"]