Modified Frame and FrameSequence so that "future" rolls are referenced without linking through "future" frames

Frame.groovy

package bowling

class Frame 
{
  public add(roll) { rolls << roll }
  public addBonus(roll) { rolls << roll }

  public score(){ rolls.sum() }
  
  private rolls = []
  
  public isStrike(){ roll(1) == 10 }
  public isSpare(){ (roll(1) + roll(2)) == 10 }
  
  private roll(n){ rolls[n-1] }
}

FrameSequence.groovy

package bowling

import java.util.Iterator;

class FrameSequence 
{
  public FrameSequence(n) 
  { 
    n.times{ frames.add( new Frame() ) }
  }

  public populateWith(rolls){ populateWith(rolls.listIterator()) }
  public score() { frames.sum{ it.score() } }

  private populateWith(Iterator rolls)
  {
    frames.each { populate(it, with(rolls)) }
  }
  
  private populate(frame, rolls)
  {
    frame.add(rolls.next())
    if(frame.isStrike())
    {
      def (roll1, roll2) = nextTwo(rolls)
      frame.addBonus(roll1)
      frame.addBonus(roll2)
    }
    else 
    {
      frame.add(rolls.next())
      if (frame.isSpare())
      {
        frame.addBonus(first(rolls))
      }
    }
  }
  
  public first(Iterator rolls) {
    def roll = rolls.next();
    rolls.previous();
    return roll;
  }
  
  public nextTwo(ListIterator rolls) {
    def roll1 = rolls.next();
    def roll2 = rolls.next();
    rolls.previous();
    rolls.previous();
    return [roll1, roll2];
  }
  
  private List frames = [];
  private with(rolls){rolls}
}