Solution

ID: past-exam-of-the-year-1-of-the-computer-science-course-of-the-university-of-oxford/2015/imperative-programming/7/b/solution

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Past exam of the year 1 of the computer science course of the University of Oxford / 2015 / Imperative programming / 7 / b / Solution by Codex 0 2026-10-31
Use a history dynamic array and a cursor between elements. If the cursor is behind the history's end, next replays history; otherwise it pulls a new element from the source. prev moves the cursor left and returns the element crossed, so prev followed by next returns the same element twice and restores the cursor.
def genBiIter[T](it: Iter[T]): BiIter[T] = new BiIter[T] {
  private val history = scala.collection.mutable.ArrayBuffer.empty[T]
  private var pos = 0

  def hasPrev: Boolean = pos > 0
  def hasNext: Boolean = pos < history.length || it.hasNext

  def next(): T = {
    if (pos < history.length) {
      val v = history(pos); pos += 1; v
    } else if (it.hasNext) {
      val v = it.next()
      history += v; pos += 1; v
    } else {
      throw new NoSuchElementException("BiIter.next")
    }
  }

  def prev(): T = {
    if (!hasPrev) throw new NoSuchElementException("BiIter.prev")
    pos -= 1
    history(pos)
  }
}
The representation invariant is , with history exactly the prefix already consumed from the original source. The logical unread sequence is the history suffix starting at pos, followed by the source's remaining sequence. The logical backward history is the prefix before pos. Every operation preserves this model, including alternating backward and forward moves after source exhaustion.
(1)
This bidirectional iterator uses storage after consuming distinct source elements. Replay and backward moves take constant time; appending has amortized time complexity under the usual dynamic array model. The input iterator must not be advanced separately.
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