Gatherers_B17.snippet

// Custom Equal Collecting Gatherer
static <T, R, RR> Gatherer<T, ?, RR> customEqualCollecting(
   Function<? super T, ? extends R> equalMapper,
   Collector<? super T, ?, ? extends RR> collector) {

   return Gatherer.ofSequential(
      () -> new HashMap<R, List<T>>(),
         (map, element, _) -> {
         var key = equalMapper.apply(element);
         map.computeIfAbsent(key, _ -> new ArrayList<>()).add(element);
         return true;
      },
      (map, downstream) -> {
         map.values().stream()
            .takeWhile(_ -> !downstream.isRejecting())
            .map(values -> values.stream().collect(collector))
            .forEach(downstream::push);
      }
   );
}


var stream = Stream.of("one", "One", "ONE", "Two", "two", "tWo", "Three", "three");
var result =
   stream
      .gather(
         customEqualCollecting(String::toLowerCase, Collectors.toList()))
      .toList();
System.out.println("result = " + result);