Invercargill/src/lib/Queries/Position.vala

namespace Invercargill {

    private class PositionQuery<T> : BaseQuery<T, PositionItemPair<T>> {

        public PositionQuery(Enumerable<T> input) {
            this.input = input;
        }

        public override Tracker<PositionItemPair<T>> get_tracker() {
            return new PositionTracker<T>(input);
        }

        private class PositionTracker<T> : Tracker<PositionItemPair<T>> {

            private Tracker<T> base_tracker;
            private int position;
            public PositionTracker(Enumerable<T> input) {
                base_tracker = input.get_tracker();
                position = -1;
            }

            public override bool has_next() {
                return base_tracker.has_next();
            }
            public override PositionItemPair<T> get_next() {
                return new PositionItemPair<T>(++position, base_tracker.get_next());
            }
            
        }
    }

}