@Test
  public void testHeadTail() {
    List<Integer> ints = IntStreamEx.range(1000).boxed().toList();
    checkShortCircuitCollector("tail(0)", Arrays.asList(), 0, ints::stream, MoreCollectors.tail(0));
    checkCollector("tail(1)", Arrays.asList(999), ints::stream, MoreCollectors.tail(1));
    checkCollector("tail(2)", Arrays.asList(998, 999), ints::stream, MoreCollectors.tail(2));
    checkCollector("tail(500)", ints.subList(500, 1000), ints::stream, MoreCollectors.tail(500));
    checkCollector("tail(999)", ints.subList(1, 1000), ints::stream, MoreCollectors.tail(999));
    checkCollector("tail(1000)", ints, ints::stream, MoreCollectors.tail(1000));
    checkCollector("tail(MAX)", ints, ints::stream, MoreCollectors.tail(Integer.MAX_VALUE));

    checkShortCircuitCollector("head(0)", Arrays.asList(), 0, ints::stream, MoreCollectors.head(0));
    checkShortCircuitCollector(
        "head(1)", Arrays.asList(0), 1, ints::stream, MoreCollectors.head(1));
    checkShortCircuitCollector(
        "head(2)", Arrays.asList(0, 1), 2, ints::stream, MoreCollectors.head(2));
    checkShortCircuitCollector(
        "head(500)", ints.subList(0, 500), 500, ints::stream, MoreCollectors.head(500));
    checkShortCircuitCollector(
        "head(999)", ints.subList(0, 999), 999, ints::stream, MoreCollectors.head(999));
    checkShortCircuitCollector("head(1000)", ints, 1000, ints::stream, MoreCollectors.head(1000));
    checkShortCircuitCollector(
        "head(MAX)", ints, 1000, ints::stream, MoreCollectors.head(Integer.MAX_VALUE));

    checkShortCircuitCollector(
        "head(10000)",
        IntStreamEx.rangeClosed(1, 10000).boxed().toList(),
        10000,
        () -> Stream.iterate(1, x -> x + 1),
        MoreCollectors.head(10000),
        true);
  }
  @Test
  public void testMapping() {
    List<String> input = Arrays.asList("Capital", "lower", "Foo", "bar");
    Collector<String, ?, Map<Boolean, Optional<Integer>>> collector =
        MoreCollectors.partitioningBy(
            str -> Character.isUpperCase(str.charAt(0)),
            MoreCollectors.mapping(String::length, MoreCollectors.first()));
    checkShortCircuitCollector(
        "mapping", new BooleanMap<>(Optional.of(7), Optional.of(5)), 2, input::stream, collector);
    Collector<String, ?, Map<Boolean, Optional<Integer>>> collectorLast =
        MoreCollectors.partitioningBy(
            str -> Character.isUpperCase(str.charAt(0)),
            MoreCollectors.mapping(String::length, MoreCollectors.last()));
    checkCollector(
        "last", new BooleanMap<>(Optional.of(3), Optional.of(3)), input::stream, collectorLast);

    input = Arrays.asList("Abc", "Bac", "Aac", "Abv", "Bbc", "Bgd", "Atc", "Bpv");
    Map<Character, List<String>> expected =
        EntryStream.of('A', Arrays.asList("Abc", "Aac"), 'B', Arrays.asList("Bac", "Bbc")).toMap();
    AtomicInteger cnt = new AtomicInteger();
    Collector<String, ?, Map<Character, List<String>>> groupMap =
        Collectors.groupingBy(
            s -> s.charAt(0),
            MoreCollectors.mapping(
                x -> {
                  cnt.incrementAndGet();
                  return x;
                },
                MoreCollectors.head(2)));
    checkCollector("groupMap", expected, input::stream, groupMap);
    cnt.set(0);
    assertEquals(expected, input.stream().collect(groupMap));
    assertEquals(4, cnt.get());
  }
  @Test
  public void testGroupingByWithDomain() {
    List<String> data = Arrays.asList("a", "foo", "test", "ququq", "bar", "blahblah");
    Collector<String, ?, String> collector =
        MoreCollectors.collectingAndThen(
            MoreCollectors.groupingBy(
                String::length,
                IntStreamEx.range(10).boxed().toSet(),
                TreeMap::new,
                MoreCollectors.first()),
            Object::toString);
    checkShortCircuitCollector(
        "groupingWithDomain",
        "{0=Optional.empty, 1=Optional[a], 2=Optional.empty, 3=Optional[foo], 4=Optional[test], 5=Optional[ququq], "
            + "6=Optional.empty, 7=Optional.empty, 8=Optional[blahblah], 9=Optional.empty}",
        data.size(),
        data::stream,
        collector);

    Map<String, String> name2sex = new LinkedHashMap<>();
    name2sex.put("Mary", "Girl");
    name2sex.put("John", "Boy");
    name2sex.put("James", "Boy");
    name2sex.put("Lucie", "Girl");
    name2sex.put("Fred", "Boy");
    name2sex.put("Thomas", "Boy");
    name2sex.put("Jane", "Girl");
    name2sex.put("Ruth", "Girl");
    name2sex.put("Melanie", "Girl");
    Collector<Entry<String, String>, ?, Map<String, List<String>>> groupingBy =
        MoreCollectors.groupingBy(
            Entry::getValue,
            StreamEx.of("Girl", "Boy").toSet(),
            MoreCollectors.mapping(Entry::getKey, MoreCollectors.head(2)));
    AtomicInteger counter = new AtomicInteger();
    Map<String, List<String>> map =
        EntryStream.of(name2sex).peek(c -> counter.incrementAndGet()).collect(groupingBy);
    assertEquals(Arrays.asList("Mary", "Lucie"), map.get("Girl"));
    assertEquals(Arrays.asList("John", "James"), map.get("Boy"));
    assertEquals(4, counter.get());

    Collector<Entry<String, String>, ?, Map<String, String>> groupingByJoin =
        MoreCollectors.groupingBy(
            Entry::getValue,
            StreamEx.of("Girl", "Boy").toSet(),
            MoreCollectors.mapping(
                Entry::getKey, Joining.with(", ").maxChars(16).cutAfterDelimiter()));
    counter.set(0);
    Map<String, String> mapJoin =
        EntryStream.of(name2sex).peek(c -> counter.incrementAndGet()).collect(groupingByJoin);
    assertEquals("Mary, Lucie, ...", mapJoin.get("Girl"));
    assertEquals("John, James, ...", mapJoin.get("Boy"));
    assertEquals(7, counter.get());
  }
 @Test
 public void testHeadParallel() {
   List<Integer> expected = IntStreamEx.range(0, 2000, 2).boxed().toList();
   List<Integer> expectedShort = Arrays.asList(0, 1);
   for (int i = 0; i < 1000; i++) {
     assertEquals(
         "#" + i,
         expectedShort,
         IntStreamEx.range(1000).boxed().parallel().collect(MoreCollectors.head(2)));
     assertEquals(
         "#" + i,
         expected,
         IntStreamEx.range(10000)
             .boxed()
             .parallel()
             .filter(x -> x % 2 == 0)
             .collect(MoreCollectors.head(1000)));
   }
   assertEquals(
       expectedShort, StreamEx.iterate(0, x -> x + 1).parallel().collect(MoreCollectors.head(2)));
 }
  @Test
  public void testFlatMapping() {
    {
      Map<Integer, List<Integer>> expected =
          IntStreamEx.rangeClosed(1, 100)
              .boxed()
              .toMap(x -> IntStreamEx.rangeClosed(1, x).boxed().toList());
      Collector<Integer, ?, Map<Integer, List<Integer>>> groupingBy =
          Collectors.groupingBy(
              Function.identity(),
              MoreCollectors.flatMapping(
                  x -> IntStream.rangeClosed(1, x).boxed(), Collectors.toList()));
      checkCollector(
          "flatMappingSimple", expected, () -> IntStreamEx.rangeClosed(1, 100).boxed(), groupingBy);
    }

    Function<Entry<String, List<String>>, Stream<String>> valuesStream =
        e -> e.getValue() == null ? null : e.getValue().stream();
    List<Entry<String, List<String>>> list =
        EntryStream.of(
                "a", Arrays.asList("bb", "cc", "dd"), "b", Arrays.asList("ee", "ff"), "c", null)
            .append("c", Arrays.asList("gg"), "b", null, "a", Arrays.asList("hh"))
            .toList();
    {
      Map<String, List<String>> expected =
          EntryStream.of(list.stream())
              .flatMapValues(l -> l == null ? null : l.stream())
              .grouping();
      checkCollector(
          "flatMappingCombine",
          expected,
          list::stream,
          Collectors.groupingBy(
              Entry::getKey, MoreCollectors.flatMapping(valuesStream, Collectors.toList())));
      AtomicInteger openClose = new AtomicInteger();
      Collector<Entry<String, List<String>>, ?, Map<String, List<String>>> groupingBy =
          Collectors.groupingBy(
              Entry::getKey,
              MoreCollectors.flatMapping(
                  valuesStream.andThen(
                      s -> {
                        if (s == null) return null;
                        openClose.incrementAndGet();
                        return s.onClose(openClose::decrementAndGet);
                      }),
                  Collectors.toList()));
      checkCollector(
          "flatMappingCombineClosed",
          expected,
          list::stream,
          MoreCollectors.collectingAndThen(
              groupingBy,
              res -> {
                assertEquals(0, openClose.get());
                return res;
              }));
      boolean catched = false;
      try {
        Collector<Entry<String, List<String>>, ?, Map<String, List<String>>> groupingByException =
            Collectors.groupingBy(
                Entry::getKey,
                MoreCollectors.flatMapping(
                    valuesStream.andThen(
                        s -> {
                          if (s == null) return null;
                          openClose.incrementAndGet();
                          return s.onClose(openClose::decrementAndGet)
                              .peek(
                                  e -> {
                                    if (e.equals("gg")) throw new IllegalArgumentException(e);
                                  });
                        }),
                    Collectors.toList()));
        list.stream()
            .collect(
                MoreCollectors.collectingAndThen(
                    groupingByException,
                    res -> {
                      assertEquals(0, openClose.get());
                      return res;
                    }));
      } catch (IllegalArgumentException e1) {
        assertEquals("gg", e1.getMessage());
        catched = true;
      }
      assertTrue(catched);
    }
    {
      Map<String, List<String>> expected =
          EntryStream.of(
                  "a", Arrays.asList("bb"), "b", Arrays.asList("ee"), "c", Arrays.asList("gg"))
              .toMap();
      Collector<Entry<String, List<String>>, ?, List<String>> headOne =
          MoreCollectors.flatMapping(valuesStream, MoreCollectors.head(1));
      checkCollector(
          "flatMappingSubShort",
          expected,
          list::stream,
          Collectors.groupingBy(Entry::getKey, headOne));
      checkShortCircuitCollector(
          "flatMappingShort",
          expected,
          4,
          list::stream,
          MoreCollectors.groupingBy(Entry::getKey, StreamEx.of("a", "b", "c").toSet(), headOne));
      AtomicInteger cnt = new AtomicInteger();
      Collector<Entry<String, List<String>>, ?, List<String>> headPeek =
          MoreCollectors.flatMapping(
              valuesStream.andThen(s -> s == null ? null : s.peek(x -> cnt.incrementAndGet())),
              MoreCollectors.head(1));
      assertEquals(
          expected, StreamEx.of(list).collect(Collectors.groupingBy(Entry::getKey, headPeek)));
      assertEquals(3, cnt.get());
      cnt.set(0);
      assertEquals(
          expected,
          StreamEx.of(list)
              .collect(
                  MoreCollectors.groupingBy(
                      Entry::getKey, StreamEx.of("a", "b", "c").toSet(), headPeek)));
      assertEquals(3, cnt.get());
    }
    {
      Map<String, List<String>> expected =
          EntryStream.of(
                  "a",
                  Arrays.asList("bb", "cc"),
                  "b",
                  Arrays.asList("ee", "ff"),
                  "c",
                  Arrays.asList("gg"))
              .toMap();
      Collector<Entry<String, List<String>>, ?, List<String>> headTwo =
          MoreCollectors.flatMapping(valuesStream, MoreCollectors.head(2));
      checkCollector(
          "flatMappingSubShort",
          expected,
          list::stream,
          Collectors.groupingBy(Entry::getKey, headTwo));
      AtomicInteger openClose = new AtomicInteger();
      boolean catched = false;
      try {
        Collector<Entry<String, List<String>>, ?, Map<String, List<String>>> groupingByException =
            Collectors.groupingBy(
                Entry::getKey,
                MoreCollectors.flatMapping(
                    valuesStream.andThen(
                        s -> {
                          if (s == null) return null;
                          openClose.incrementAndGet();
                          return s.onClose(openClose::decrementAndGet)
                              .peek(
                                  e -> {
                                    if (e.equals("gg")) throw new IllegalArgumentException(e);
                                  });
                        }),
                    MoreCollectors.head(2)));
        list.stream()
            .collect(
                MoreCollectors.collectingAndThen(
                    groupingByException,
                    res -> {
                      assertEquals(0, openClose.get());
                      return res;
                    }));
      } catch (IllegalArgumentException e1) {
        assertEquals("gg", e1.getMessage());
        catched = true;
      }
      assertTrue(catched);
    }
  }