Gatherers_D1.snippet

// Zipping Gatherer
static <T> Gatherer<T, ?, T> zip(
            Iterable<T> iterable) {

   class State {
      private final Iterator<T> iterator;
      private boolean nextIsElement = true;
      private boolean hasNextElement = false;
      private T element;

      State(Iterator<T> iterator) {
         this.iterator = iterator;
      }

      boolean hasNext() {
         if (nextIsElement) {
            return hasNextElement;
         } else {
            return iterator.hasNext();
         }
      }

      T next() {
         if (nextIsElement) {
            nextIsElement = false;
            hasNextElement = false;
            return element;
         } else {
            nextIsElement = true;
            return iterator.next();
         }
      }

      public void addElement(T element) {
         this.element = element;
         this.hasNextElement = true;
      }
   }

   return Gatherer.ofSequential(
      () -> new State(iterable.iterator()),
      (state, element, downstream) -> {
         state.addElement(element);
         if (state.hasNext()) {
            boolean isRejecting = !downstream.push(state.next());
            if (isRejecting) {
               return false;
            }
         }
         if (state.hasNext()) {
            boolean isRejecting = !downstream.push(state.next());
            if (isRejecting) {
               return false;
            }
         }
         return true;
      }
   );
}

var evens = List.of(0, 2, 4, 6, 8);
var odds  = List.of(1, 3, 5);

var result = evens.stream().gather(zip(odds::iterator)).toList();
System.out.println("result = " + result);