perf(java): replace stream+flatMap with pre-sized for-loop in ArrayTransformUtils

[ajzach]

Summary

Seven methods in ArrayTransformUtils used stream().flatMap(entry -> Stream.of(key, value)).toArray() to interleave map entries into a flat array. This pattern allocates one Stream object per entry (via flatMap) plus SpinedBuffer infrastructure since the final array size is unknown to the stream pipeline.

Replaced with a single pre-sized array (map.size() * 2) and a plain for loop over entrySet() — allocates exactly one array, zero intermediate objects.

flattenAllKeysFollowedByAllValues additionally dropped two ArrayList instances, two intermediate toArray() calls, and a concatenateArrays() stream call — replaced with a single-pass index-offset write into one array.

These methods are called on every mset, hset, zadd, xadd, geoadd, and other bulk commands, so the GC pressure reduction scales directly with command throughput.

Benchmark results

JMH, AverageTime, GC profiler, 2 forks × 10 iterations, mapSize = 50:

Method	Latency BEFORE	Latency AFTER	Alloc BEFORE	Alloc AFTER
convertMapToKeyValueGlideStringArray	1029 ns/op	170 ns/op	6976 B/op	416 B/op (-94%)
convertMapToKeyValueStringArray	1207 ns/op	176 ns/op	6976 B/op	416 B/op (-94%)
flattenMapToGlideStringArray	1245 ns/op	181 ns/op	6976 B/op	416 B/op (-94%)
flattenMapToGlideStringArrayValueFirst	992 ns/op	186 ns/op	6976 B/op	416 B/op (-94%)
flattenAllKeysFollowedByAllValues	743 ns/op	190 ns/op	3840 B/op	416 B/op (-89%)
convertMapToValueKeyStringArray	2153 ns/op	968 ns/op	9376 B/op	2816 B/op (-70%)
convertMapToValueKeyStringArrayBinary	3302 ns/op	1533 ns/op	12576 B/op	6016 B/op (-52%)

The 416 B/op residual is exactly the output array (new GlideString[100] with compressed oops = 416 bytes for 50 entries). The remaining allocations in the Value*Key methods are unavoidable Double.toString() / gs() object creation per entry.

Test plan

• Added ArrayTransformUtilsTest with 15 unit tests covering all modified methods (basic, empty, single-entry, size invariant)
• Added ArrayTransformBenchmark via JMH plugin (me.champeau.jmh) under benchmarks/ for future regression tracking — run with ./gradlew :benchmarks:jmh
• All existing unit tests pass

[yeoncheol-jang]

Sorry. This is reviewing here. #5603

[ajzach]

Sorry. This is reviewing here. #5603

This PR applies the same optimization pattern as #5603 (stream+flatMap → pre-sized array + for-loop), but extends it to all methods in ArrayTransformUtils that had the same issue — not just the two covered there:

Method	#5603	This PR
convertMapToKeyValueStringArray	✅	✅
convertMapToKeyValueGlideStringArray	✅	✅
convertMapToValueKeyStringArray	❌	✅
convertMapToValueKeyStringArrayBinary	❌	✅
flattenMapToGlideStringArray	❌	✅
flattenMapToGlideStringArrayValueFirst	❌	✅
flattenAllKeysFollowedByAllValues	❌	✅

flattenAllKeysFollowedByAllValues had the worst allocation profile — it combined two ArrayList instances, two intermediate toArray() calls, and a concatenateArrays() stream on top of the flatMap pattern. Replaced with a single-pass index-offset write into one pre-sized array.

Results were measured with JMH + GC profiler (2 forks × 10 iterations) to have quantitative evidence before and after each change. Allocation reductions range from −52% to −94% depending on the method.

If #5603 merges first, happy to rebase and drop the two overlapping methods.

[jduo]

This seems alot better than concatenating afterwards.

[jduo]

Please address the cases where the key and value are null (technically HashMap allows a null key). Add tests for these scenarios.

[ajzach]

@jduo Addressed in the latest commit. For all 7 methods:

• convertMapToKeyValueStringArray / convertMapToValueKeyStringArray / convertMapToValueKeyStringArrayBinary: null value now propagates as a null element in the result array instead of NPE-ing on .toString()
• convertMapToKeyValueGlideStringArray: null key/value already passed through (direct assignment), now explicitly tested
• flattenMapToGlideStringArray / flattenMapToGlideStringArrayValueFirst / flattenAllKeysFollowedByAllValues: null key/value short-circuits the GlideString.of() call and places null in the result array

Added 8 unit tests covering null key and null value for the representative methods.