[claude] fastlanes: allow signed integers in Delta encoding

[joseph-isaacs]

Summary

Lifts the is_unsigned_int gate on DeltaArray so i8 / i16 / i32 / i64 columns can be delta-encoded.

The upstream FastLanes Delta::delta / Transpose::transpose kernels are bounded on T: FastLanes: Unsigned, so signed inputs are processed by reinterpret-casting the underlying buffer to the same-width unsigned counterpart, running the existing kernel, then reinterpret-casting back. wrapping_sub / wrapping_add are bit-identical for signed and unsigned operands under two's-complement, so the round-trip is exact.

Also:

• Delta::cast now bails on signed sources. Value-preserving casts of signed deltas (e.g. -1i8 → 4294967295u32) break the wrapping-add invariant during decompression — the slow decompress-and-re-encode path handles those.
• Added encodings/fastlanes/src/delta/FUSED_DECODE.md describing a triple-fused unpack + add-reference + undelta kernel for future work. Today decoding a Delta(FoR(BitPacked)) makes three passes over the buffer; a fused kernel cuts that to one.

Measured impact

Synthetic workloads (32 KiB raw, i32, 8 × 1024 elements each):

Workload	Δ range	Wnaive	Wffor	ratio
monotone i32 (0..N)	[0, 1]	1	1	15.97x
sensor i32 in [-100, 100]	[-196, 199]	32	9	3.20x
offset i32 base=-1e9	[0, 1]	1	1	15.97x
near-monotone i32 (5% backtrack)	[-2, 1]	32	2	10.65x

Wnaive = 32 on the two workloads with negative deltas confirms the bit-packing collapse on raw signed deltas (any negative two's-complement delta sets the high bits and the OR mask forces W = T). FFoR brings them to 9 and 2 bits.

Test plan

• cargo build -p vortex-fastlanes (clean)
• cargo nextest run -p vortex-fastlanes — 256/256 pass, including 7 new signed rstest cases (i8_full_range, i32_crossing_zero, i32_all_negative, i16_crossing_zero, i64_large_negative, nullable_i32_crossing, i32_non_negative)
• cargo clippy -p vortex-fastlanes --all-targets --all-features — no warnings
• cargo fmt --all -- --check — clean
• ./scripts/public-api.sh — not run locally (no public API surface added; unsigned_counterpart is pub(crate)). Worth verifying in CI.

---

Generated by Claude Code

[codspeed-hq]

Merging this PR will degrade performance by 32.11%

⚠️ Unknown Walltime execution environment detected

Using the Walltime instrument on standard Hosted Runners will lead to inconsistent data.

For the most accurate results, we recommend using CodSpeed Macro Runners: bare-metal machines fine-tuned for performance measurement consistency.

❌ 4 regressed benchmarks
✅ 1206 untouched benchmarks

Warning

Please fix the performance issues or acknowledge them on CodSpeed.

Performance Changes

	Mode	Benchmark	BASE	HEAD	Efficiency
❌	Simulation	decompress_rd[f32, (100000, 0.01)]	413.1 µs	585.8 µs	-29.48%
❌	Simulation	decompress_rd[f64, (100000, 0.01)]	668.8 µs	1,023.4 µs	-34.65%
❌	Simulation	decompress_rd[f32, (100000, 0.1)]	413.2 µs	585.8 µs	-29.47%
❌	Simulation	decompress_rd[f64, (100000, 0.1)]	668.8 µs	1,023.4 µs	-34.65%

Tip

Investigate this regression by commenting @codspeedbot fix this regression on this PR, or directly use the CodSpeed MCP with your agent.

---

_{Comparing claude/vortex-delta-negative-values-yQh1m (48d5602) with develop (da19bca)}