[WIP][POC] Pfor encoding#50088
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Implements the PFOR (Patched Frame of Reference) integer compression algorithm as a standalone utility library in arrow/util/pfor/. Includes: - Cost model for optimal bit width selection (histogram-based) - Vector-level encode/decode with FOR + bit-packing + exceptions - Page-level wrapper with header, offset array, and multi-vector layout - Comprehensive unit tests covering edge cases and round-trips
Adds PFOR = 11 to the Encoding enum and wires it into the parquet read/write pipeline: - PforEncoder<DType> in encoder.cc (buffers values, calls PforWrapper::Encode) - PforDecoder<DType> in decoder.cc (decodes all values on first access) - PFOR case in column_reader.cc InitializeDataDecoder - Encoding string mapping in types.cc Supports INT32 and INT64 column types.
Benchmarks encode/decode throughput for int32/int64 across 10 data distributions inspired by Snowflake's NumericComprBenchmark: constant, sequential, small range, high-base-small-range (timestamps), with outliers (exception path), random, TPC-DS date/store/item/quantity keys. Each distribution runs at 1K/10K/100K/1M elements. Reports bytes/s, items/s, and compression ratio.
Load() now returns Result<PforVectorInfo> after the Status/Result refactoring. Use ASSERT_OK_AND_ASSIGN to properly unwrap the result in tests.
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