# liste de base
a = [10, 11, 15, 16, 17, 18, 21, 33 ]

# critère de sélection
x = 3

prev = nil
liste = Array.new

a.find_all{ |item|
  if not prev
    prev = item
    next
  end
  if item - 1 == prev
    if not liste.include? prev
      liste << prev
    end
    liste << item
  else
    liste = Array.new
  end
  if liste.length == x
    break
  end
  prev = item
  }
puts liste