module Pagy::SizeExtra

def series(size: @vars[:size], **_)
  return super unless size.is_a?(Array)
  return [] if size == []
  raise VariableError.new(self, :size, 'to be an Array of 4 Integers or []', size) \
        unless size.is_a?(Array) && size.size == 4 && size.all? { |num| !num.negative? rescue false } # rubocop:disable Style/RescueModifier
  [].tap do |series|
    # This algorithm is up to ~5x faster and ~2.3x lighter than the previous one (pagy < 4.3)
    # However the behavior of the legacy nav bar was taken straight from WillPaginate and Kaminari:
    # it's ill-concieved and complicates the experience of devs and users.
    left_gap_start  = 1 + size[0]
    left_gap_end    = @page - size[1] - 1
    right_gap_start = @page + size[2] + 1
    right_gap_end   = @last - size[3]
    left_gap_end    = right_gap_end  if left_gap_end   > right_gap_end
    right_gap_start = left_gap_start if left_gap_start > right_gap_start
    start           = 1
    if (left_gap_end - left_gap_start).positive?
      series.push(*start...left_gap_start, :gap)
      start = left_gap_end + 1
    end
    if (right_gap_end - right_gap_start).positive?
      series.push(*start...right_gap_start, :gap)
      start = right_gap_end + 1
    end
    series.push(*start..@last)
    series[series.index(@page)] = @page.to_s
  end
end