require 'active_record'

# This module extends ActiveRecord::Base to provide the
# .first_after_created_at(date) and .first_after_id(id) functions
module FirstAfterCreatedAt
  def first_after_created_at(date)
    first = select(:id, :created_at).first
    last = select(:id, :created_at).last
    jump_size = (last.id - first.id) / 2
    best = _best(last, _best(first, nil, date), date)
    center = _first_after_id(first.id + jump_size)

    while jump_size != 0
      jump_size /= 2
      direction = center.created_at < date ? 1 : -1 # moving forward or backward
      new_center = _first_after_id(center.id + jump_size * direction)

      best = _best(new_center, best, date)

      center = new_center
    end

    find(best.id) if best
  end

  def _best(center, best, date)
    distance = center.created_at - date
    if distance >= 0 && (!best || distance < (best.created_at - date))
      center
    else
      best
    end
  end

  def _first_after_id(id)
    select(:id, :created_at).where('id >= ?', id).order('id asc').first
  end
end

ActiveRecord::Base.send(:extend, FirstAfterCreatedAt)