Past exam of the year 1 of the computer science course of the University of Oxford / 2015 / Imperative programming (resit) / 6 / a / Solution

class DocImpl extends Doc {
  private var chars: List[Char] = Nil
  private var length: Int = 0

  def size: Int = length

  def insert(x: Char, pos: Int): Unit = {
    require(0 <= pos && pos <= length)
    val parts = chars.splitAt(pos)
    chars = parts._1 ::: (x :: parts._2)
    length += 1
  }

  def delete(pos: Int): Char = {
    require(0 <= pos && pos < length)
    val parts = chars.splitAt(pos)
    val old = parts._2.head
    chars = parts._1 ::: parts._2.tail
    length -= 1
    old
  }

  def read(pos: Int): Char = {
    require(0 <= pos && pos < length)
    chars(pos)
  }
}