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ID: past-exam-of-the-year-1-of-the-computer-science-course-of-the-university-of-oxford/2019/imperative-programming-resit/6/c/i/solution

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Past exam of the year 1 of the computer science course of the University of Oxford / 2019 / Imperative programming resit / 6 / c / i / Solution by

Codex  0 2026-10-25

Replace edit-specific inverse information by before/after snapshots implementing the Memento pattern. The same Change and History interfaces still work. A complete alternative suite is:

class SnapshotSlideSuite(initial: Sequence[Slide]) {
  private var slides = initial

  private def changeTo(after: Sequence[Slide]): Change = {
    val before = slides
    slides = after
    new Change {
      def undo(): Unit = { slides = before }
      def redo(): Unit = { slides = after }
    }
  }

  def insert(i: Int, x: Slide): Change =
    changeTo(slides.insert(i, x))

  def delete(i: Int): Change =
    changeTo(slides.delete(i))

  def move(from: Int, to: Int): Change = {
    val x = slides.elementAt(from)
    changeTo(slides.delete(from).insert(to, x))
  }
}

By-value evaluation computes after without changing slides, then changeTo captures the still-current before and installs the new value. The val bindings inside each call are stable snapshots, rather than later reads of a mutable variable. Under the same chronological history preconditions, undo and redo are constant-time reference assignments. This is the Memento pattern applied to an immutable sequence. Retaining a reference to an in-place-mutated sequence would not save its old state. The snapshot covers slide order and membership; it does not promise to undo unrelated mutations inside a Slide object.

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