Invercargill/src/lib/Modifiers/Filter.vala

namespace Invercargill {

    private class FilterQuery<T> : BaseQuery<T, T> {
        private PredicateDelegate<T> predicate_func;

        public FilterQuery(Enumerable<T> input, owned PredicateDelegate<T> func) {
            this.input = input;
            predicate_func = (owned)func;
        }

        //  public override bool iterate_if (Invercargill.PredicateDelegate<T> handler) {
        //      return input.iterate_if(i => {
        //          if(predicate_func(i)) {
        //              if(!handler(i)) {
        //                  return false;
        //              }
        //          }
        //          return true;
        //      });
        //  }

        public override Tracker<T> get_tracker() {
            var tracker = input.get_tracker();

            return new AdvanceTracker<T>((out obj) => {
                while(tracker.has_next()) {
                    var item = tracker.get_next();
                    if(predicate_func(item)){
                        obj = item;
                        return true;
                    }
                }
                obj = null;
                return false;
            });
        }
    }

}