Scala tutorial question

gistfile1.scala

  /* render a tweet using entities: for example
   * val e = Entity(10, 21, """<a href="https://twitter.com/search/?q=%23ScalaIntro">#ScalaIntro</a>""")
   * val entitites = Set(e)
   * render("I love my #ScalaIntro exercises!", entities)
   * should result in
   * """I love my <a href="https://twitter.com/search/?q=%23ScalaIntro">#ScalaIntro</a> exercises!"""
   * The "start" and "end" fields of an Entity refer to the indices in the
   * original string to be substituted with the html field; you may assume
   * that these indices are all non-overlapping.
  */
  case class Entity(start: Int, end: Int, html: String)
  // note: the entities are not guaranteed to be in any particular order
  def render(text: String, entities: Seq[Entity]): CharSequence = {
    var sb = new StringBuilder(text)
    val sortedEntities = entities.sortWith((x: Entity, y: Entity) => x.start < y.start)
    var offset = 0

    for (entity <- sortedEntities) {
      sb.replace(entity.start + offset, entity.end + offset, entity.html)
      offset += entity.html.length - (entity.end - entity.start)
    }

    sb toString
  }