Array#pairwise.rb

# patch array for pairwise function
class Array
  def pairwise
    pairs = []
    a = self.slice(1..-1).each_slice(2).to_a
    self.each_slice(2).to_a.each_with_index do |el, idx|
      pairs << el if el.size > 1
      pairs << a[idx] if a[idx] && a[idx].size > 1
    end
    pairs.each do |pair|
      yield(pair[0], pair[1])
    end
  end
end