4.x: Streamable - optimize several operator internals

[akarnokd]

Methodology

• Check for synchronous completion, helper method: StreamableHelper.whenComplete, to avoid allocating the unnecessary CompletableFuture internals.
• Batch count the work-in-progress amount in the trampolines so that one atomic subtraction can be mostly avoided.
• Introduce IndexableSource, EnumerableSource and DeferredEnumerableSource to bypass the CompletionStage dance when the data is synchronously available.

Operators

• concat(Iterable)
• skip
• flattenAsStreamable
• collect
• map
• fromArray
• fromIterable
• last, lastOrError

Scrabble progress

i9 275HX, 32GB LPDDR5 6400MT CL52, Windows 25H2, JDK 26.0.1

Step	Time (ms)	Improvement	vs Baseline
Baseline	42,5119	-	-
Avoid whenComplete usage in collect	37,566	-11,65%	-
IndexableSource in collect	35,815	-4,66%	-15,77%
concatIterable optimizations	31,971	-10,73%	-24,81%
map fusions	30,623	-4,22%	-27,98%
flattenAsFlowable deferred opt	27,999	-8,57%	-34,15%
MutableLong in the bench	26,159	-6,57%	-38,48%
lastOrError opt	25,164	-3,80%	-40,82%
custom Collector summers	24,885	-1,11%	-41,47%
skip optimizations	24,791	-0,38%	-41,69%

[github-actions]

🐷 TruffleHog + Entropy Beauty Scan

Average entropy of changed code: 4.62 bits/char
Verdict: ✅ Mid-4 beauty detected (thoughtful human code!)

Changed files entropy:

src/jmh/java/io/reactivex/rxjava4/streamable/StreamableConcatIterablePerf.java: 4.835
src/jmh/java/io/reactivex/rxjava4/streamable/StreamableLastAsSinglePerf.java: 4.989
src/jmh/java/io/reactivex/rxjava4/streamable/StreamableMapPerf.java: 4.648
src/jmh/java/io/reactivex/rxjava4/streamable/StreamableSkipPerf.java: 4.905
src/main/java/io/reactivex/rxjava4/core/Streamable.java: 4.649
src/main/java/io/reactivex/rxjava4/core/Streamer.java: 4.688
src/main/java/io/reactivex/rxjava4/internal/operators/streamable/StreamableCollector.java: 4.067
src/main/java/io/reactivex/rxjava4/internal/operators/streamable/StreamableConcatIterable.java: 4.100
src/main/java/io/reactivex/rxjava4/internal/operators/streamable/StreamableFromArray.java: 4.552
src/main/java/io/reactivex/rxjava4/internal/operators/streamable/StreamableFromIterable.java: 4.510
src/main/java/io/reactivex/rxjava4/internal/operators/streamable/StreamableHelper.java: 4.420
src/main/java/io/reactivex/rxjava4/internal/operators/streamable/StreamableLastAsSingle.java: 4.302
src/main/java/io/reactivex/rxjava4/internal/operators/streamable/StreamableMap.java: 4.511
src/main/java/io/reactivex/rxjava4/internal/operators/streamable/StreamableSingleFlattenAs.java: 4.510
src/main/java/io/reactivex/rxjava4/internal/operators/streamable/StreamableSkip.java: 4.434
src/main/java/io/reactivex/rxjava4/operators/DeferredEnumerableSource.java: 4.840
src/main/java/io/reactivex/rxjava4/operators/EnumerableSource.java: 4.761
src/test/java/io/reactivex/rxjava4/internal/operators/streamable/StreamableConcatIterableTest.java: 4.459
src/test/java/io/reactivex/rxjava4/internal/operators/streamable/StreamableFromArrayTest.java: 4.867
src/test/java/io/reactivex/rxjava4/internal/operators/streamable/StreamableFromIterableTest.java: 4.804

✅ No secrets or suspicious high-entropy strings found.

Mid-4 beauty heuristic in action — powered by our entropy chats! 😊

[github-actions]

🐷 TruffleHog + Entropy Beauty Scan

Average entropy of changed code: 4.617 bits/char
Verdict: ✅ Mid-4 beauty detected (thoughtful human code!)

Changed files entropy:

src/jmh/java/io/reactivex/rxjava4/streamable/StreamableConcatIterablePerf.java: 4.835
src/jmh/java/io/reactivex/rxjava4/streamable/StreamableLastAsSinglePerf.java: 4.989
src/jmh/java/io/reactivex/rxjava4/streamable/StreamableMapPerf.java: 4.648
src/jmh/java/io/reactivex/rxjava4/streamable/StreamableSkipPerf.java: 4.905
src/main/java/io/reactivex/rxjava4/core/Streamable.java: 4.649
src/main/java/io/reactivex/rxjava4/core/Streamer.java: 4.688
src/main/java/io/reactivex/rxjava4/internal/operators/streamable/StreamableCollector.java: 4.067
src/main/java/io/reactivex/rxjava4/internal/operators/streamable/StreamableConcatIterable.java: 4.098
src/main/java/io/reactivex/rxjava4/internal/operators/streamable/StreamableFromArray.java: 4.552
src/main/java/io/reactivex/rxjava4/internal/operators/streamable/StreamableFromIterable.java: 4.510
src/main/java/io/reactivex/rxjava4/internal/operators/streamable/StreamableHelper.java: 4.420
src/main/java/io/reactivex/rxjava4/internal/operators/streamable/StreamableLastAsSingle.java: 4.302
src/main/java/io/reactivex/rxjava4/internal/operators/streamable/StreamableMap.java: 4.511
src/main/java/io/reactivex/rxjava4/internal/operators/streamable/StreamableSingleFlattenAs.java: 4.510
src/main/java/io/reactivex/rxjava4/internal/operators/streamable/StreamableSkip.java: 4.434
src/main/java/io/reactivex/rxjava4/operators/DeferredEnumerableSource.java: 4.840
src/main/java/io/reactivex/rxjava4/operators/EnumerableSource.java: 4.761
src/test/java/io/reactivex/rxjava4/internal/operators/streamable/StreamableConcatIterableTest.java: 4.405
src/test/java/io/reactivex/rxjava4/internal/operators/streamable/StreamableFromArrayTest.java: 4.867
src/test/java/io/reactivex/rxjava4/internal/operators/streamable/StreamableFromIterableTest.java: 4.804

✅ No secrets or suspicious high-entropy strings found.

Mid-4 beauty heuristic in action — powered by our entropy chats! 😊

[codecov]

Codecov Report

❌ Patch coverage is 81.91489% with 34 lines in your changes missing coverage. Please review.
✅ Project coverage is 98.66%. Comparing base (56dad68) to head (93b84e1).

Files with missing lines	Patch %	Lines
...rnal/operators/streamable/StreamableCollector.java	59.37%	9 Missing and 4 partials ⚠️
...operators/streamable/StreamableConcatIterable.java	82.97%	6 Missing and 2 partials ⚠️
...rnal/operators/streamable/StreamableFromArray.java	68.75%	5 Missing ⚠️
...l/operators/streamable/StreamableLastAsSingle.java	72.72%	1 Missing and 2 partials ⚠️
.../internal/operators/streamable/StreamableSkip.java	70.00%	1 Missing and 2 partials ⚠️
...nternal/operators/streamable/StreamableHelper.java	80.00%	0 Missing and 1 partial ⚠️
...4/internal/operators/streamable/StreamableMap.java	97.67%	0 Missing and 1 partial ⚠️

❌ Your patch check has failed because the patch coverage (81.91%) is below the target coverage (95.00%). You can increase the patch coverage or adjust the target coverage.

Additional details and impacted files

@@             Coverage Diff              @@
##                4.x    #8238      +/-   ##
============================================
- Coverage     98.68%   98.66%   -0.02%     
- Complexity     7148     7155       +7     
============================================
  Files           822      822              
  Lines         50405    50543     +138     
  Branches       6825     6862      +37     
============================================
+ Hits          49740    49869     +129     
- Misses          459      469      +10     
+ Partials        206      205       -1     

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