triples.rb

class Foo
  attr_reader :start, :stop
  def initialize(start, stop)
    @start, @stop = start, stop
  end
  def each
    return enum_for(:each) unless block_given?
    start.upto(stop) do |count|
      yield count
    end
  end

  def each_triple
    return enum_for(:each_triple) unless block_given?
    p = c = n = nil
    each do |foo|
      p, c, n = c, n, foo
      next if p.nil? && c.nil?
      yield [p, c, n]
    end
    if n.nil?
      yield []
    else
      yield [c, n, nil]
    end
  end
end


# example
foos = Foo.new(0, 7)
foos.each_triple do |t|
  puts t.inspect
end

# [nil, 0, 1]
# [0, 1, 2]
# [1, 2, 3]
# [2, 3, 4]
# [3, 4, 5]
# [4, 5, 6]
# [5, 6, 7]
# [6, 7, nil]