missing.rb

def seed
  1 # standing in for a non-numeric object that also responds to #next
end

# missing: Return enough items fill an array of existing items so that it has 10
# (multiple implementations)
# Test: assert_equal([1, 2, 3, 4, 5, 6, 7, 8, 9, 10], missing([]))
# Test: assert_equal([9, 10], missing([1, 2, 3, 4, 5, 6, 7, 8]))
# @param queue: an Array with up to 10 items

# Imperative style
def missing(existing)
  result = []
  while result.size < 10 - existing.size
    result << (result.last.try(:next) || existing.last.try(:next) || seed)
  end
  result
end

# each_with_object over a number range
def missing(existing)
  (existing.size...10).each_with_object([]) do |_, result|
    result << (result.last.try(:next) || existing.last.try(:next) || seed)
  end
end

# recursive
def missing(existing, result = [])
  return result if existing.size + result.size >= 10
  incoming = result.last.try(:next) || existing.last.try(:next) || seed
  missing(existing, result + [incoming])
end

replenish.rb

def seed
  1 # standing in for a non-numeric object that also responds to #next
end

# replenish: Add items to the queue until it has 10
# (multiple implementations)
# Test: assert_equal([1, 2, 3, 4, 5, 6, 7, 8, 9, 10], replenish([]))
# @param queue: an Array with up to 10 items

# Imperative style
def replenish(queue)
  result = queue.dup.presence || [seed]
  while result.size < 10
    result << result.last.next
  end
  result
end

# each_with_object over a number range
def replenish(queue)
  queue = queue.presence || [seed]
  (queue.size...10).each_with_object(queue.dup.presence) do |_, result|
    result << result.last.next
  end
end

# recursive
def replenish(queue)
  return queue if queue.size >= 10
  incoming = queue.last.try(:next) || seed
  replenish(queue + [incoming])
end