GH-3522: Reuse intermediate buffers in RunLengthBitPackingHybridDecoder PACKED path (~22% throughput on dictionary-id decode)

[iemejia]

Summary

Closes #3522.

RunLengthBitPackingHybridDecoder allocates a new int[] and byte[] on every PACKED run during decode. The code itself flagged this with a // TODO: reuse a buffer comment. This PR resolves the TODO by reusing the buffers across runs within the same decoder instance, growing them lazily only when a larger run is encountered.

Also adds a currentBufferLength field to track the logical active-region length in packedValuesBuffer (since packedValuesBuffer.length may now exceed the current run's size after a prior larger run grew it).

Benchmark

RleDictionaryIndexDecodingBenchmark (added in #3512) isolates the RLE/bit-packed dictionary-id decode path. 100k INT32 dictionary IDs, BIT_WIDTH=10, JMH -wi 5 -i 10 -f 2 (20 measurement iterations):

Pattern	master (ops/s)	optimized (ops/s)	Improvement
SEQUENTIAL	93,061,521	113,856,860	+22.3%
RANDOM	92,929,824	114,238,638	+22.9%
LOW_CARDINALITY	92,813,229	115,271,347	+24.2%

End-to-end FileReadBenchmark sees a much smaller ~2% improvement because RLE decoding is only one of many pipeline stages; the isolated micro-benchmark shows the true magnitude on the affected code path.

Validation

• parquet-column: 573 tests pass
• TestRunLengthBitPackingHybridEncoder: 9 tests pass (these round-trip values through the decoder)
• Built with -Dspotless.check.skip=true -Drat.skip=true -Djapicmp.skip=true

Scope

17 LOC change to a single file. Self-contained and obviously correct (resolves the existing TODO).

Related

Part of the focused performance PR series from https://github.com/iemejia/parquet-perf. The companion ByteStreamSplit writer/reader changes from the same source commit (ba52f82c3) have already been submitted as #3504 and #3506.

How to reproduce

The benchmark is added in #3512. Once that lands, reproduce with:

./mvnw clean package -pl parquet-benchmarks -DskipTests \
    -Dspotless.check.skip=true -Drat.skip=true -Djapicmp.skip=true
java -jar parquet-benchmarks/target/parquet-benchmarks.jar \
    'RleDictionaryIndexDecodingBenchmark' -wi 5 -i 10 -f 2

[arouel]

Have you seen the PR #3467 which addresses the same issue? I would prefer if we help each other in reviewing instead of creating more PRs of the same kind.