For http://stackoverflow.com/q/35234128/4856258

GroupRunsTest.java

package test;

import java.util.concurrent.TimeUnit;
import one.util.streamex.*;
import java.util.stream.*;
import java.util.function.*;
import java.util.*;

import org.openjdk.jmh.infra.Blackhole;
import org.openjdk.jmh.annotations.*;
 
@Warmup(iterations = 5, time = 500, timeUnit = TimeUnit.MILLISECONDS)
@Measurement(iterations = 10, time = 500, timeUnit = TimeUnit.MILLISECONDS)
@BenchmarkMode(Mode.AverageTime)
@OutputTimeUnit(TimeUnit.MICROSECONDS)
@Fork(3)
@State(Scope.Benchmark)
public class GroupRunsTest {
  @Param({"1000", "100000", "1000000"})
  int n;

  @Param({"true", "false"})
  boolean serial;

  int[] input;

  IntUnaryOperator next;

  @Setup
  public void setup() {
    if(serial) {
      // input contains many series
      input = new Random(1).ints(0, 10).flatMap(x -> IntStream.range(0, x)).limit(n).toArray();
    } else {
      // input is random: series are only accidental
      input = new Random(1).ints(n, 0, 10).toArray();
    }
    next = i -> i + 1;
  }

  @Benchmark
  public Deque<Deque<Integer>> plain() {
    IntStream seq = IntStream.of(input);
    Deque<Deque<Integer>> r = new ArrayDeque<>(Collections.singleton(new ArrayDeque<>()));

    seq.filter(i -> !r.getLast().isEmpty() && r.getLast().getLast() + 1 != i || !r.getLast().add(i))
            .forEach(i -> r.add(new ArrayDeque<>(Collections.singleton(i))));
    return r;
  }

  @Benchmark
  public List<List<Integer>> streamEx() {
    return IntStreamEx.of(input).boxed().groupRuns((i1, i2) -> next.applyAsInt(i1) == i2).toList();
  }

  @Benchmark
  public List<List<Integer>> streamExParallel() {
    return IntStreamEx.of(input).parallel().boxed().groupRuns((i1, i2) -> next.applyAsInt(i1) == i2).toList();
  }
}

results.txt

# JMH 1.11.2 (released 100 days ago, please consider updating!)
# VM version: JDK 1.8.0_45, VM 25.45-b02
...
Benchmark                           (n)  (serial)  Mode  Cnt      Score      Error  Units
GroupRunsTest.plain                1000      true  avgt   30     17,622 ±    0,080  us/op
GroupRunsTest.plain                1000     false  avgt   30     18,533 ±    0,205  us/op
GroupRunsTest.plain              100000      true  avgt   30   1979,649 ±   30,602  us/op
GroupRunsTest.plain              100000     false  avgt   30   2063,066 ±    7,643  us/op
GroupRunsTest.plain             1000000      true  avgt   30  19904,397 ±  187,035  us/op
GroupRunsTest.plain             1000000     false  avgt   30  24862,988 ± 1670,014  us/op
GroupRunsTest.streamEx             1000      true  avgt   30     17,010 ±    0,209  us/op
GroupRunsTest.streamEx             1000     false  avgt   30     17,066 ±    0,705  us/op
GroupRunsTest.streamEx           100000      true  avgt   30   2026,392 ±  163,430  us/op
GroupRunsTest.streamEx           100000     false  avgt   30   1622,729 ±   15,577  us/op
GroupRunsTest.streamEx          1000000      true  avgt   30  20064,932 ±  203,330  us/op
GroupRunsTest.streamEx          1000000     false  avgt   30  19449,919 ±  897,298  us/op
GroupRunsTest.streamExParallel     1000      true  avgt   30     28,201 ±    0,463  us/op
GroupRunsTest.streamExParallel     1000     false  avgt   30     31,578 ±    0,685  us/op
GroupRunsTest.streamExParallel   100000      true  avgt   30   1247,100 ±   41,272  us/op
GroupRunsTest.streamExParallel   100000     false  avgt   30   1270,670 ±   25,329  us/op
GroupRunsTest.streamExParallel  1000000      true  avgt   30  12501,340 ±  813,384  us/op
GroupRunsTest.streamExParallel  1000000     false  avgt   30  13350,011 ±  450,967  us/op