geotiff: jit packbits_compress (#2049)#2081
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PackBits encode was the last writer-side codec helper still running in pure Python. The kernel matches the shape of the existing lzw_compress path: an @ngjit function operates on preallocated uint8 buffers, and a thin bytes-in / bytes-out wrapper takes care of the conversion. Output sizing uses the TIFF spec bound (2 * src_len + 1). On 1 MB strips the JIT version runs at 0.6-1.7 ms/call versus 30-49 ms/call for the previous Python loop, a 30-50x speedup.
Self-review follow-up to #2081. The original kernel docstring described the worst case as "one literal header per input byte", which is not actually achievable: the encoder packs up to 128 bytes under a single literal header, so the tight upper bound is src_len + ceil(src_len / 128) + 1. The 2 * src_len + 1 allocation in the wrapper is still a safe overestimate; the comments now say so explicitly. Adds three tests: a golden literal-encoding check on the raw kernel, a parametrized buffer-cap invariant over the regime-boundary inputs, and a random sweep that re-checks the cap on 0..4096-byte streams.
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Closes #2049.
Summary
packbits_compressto dispatch into an@ngjitkernel that writes into a preallocatednp.uint8buffer, matching the shape oflzw_compress/_lzw_encode_kernel.2 * src_len + 1(the TIFF spec bound for PackBits: a degenerate stream fills a fresh literal header for every two input bytes).bytesand returnsbytes.Performance
Encode rate on 1 MB strips, single thread, warm JIT cache:
Backend coverage
PackBits is a CPU codec only; no GPU or dask path. The existing
_compression_tagdispatcher in_writer.pyis unchanged.Test plan
TestPackBitsround-trip cases still pass.test_packbits_jit_2049.pycovers regime boundaries: length 0, 1, 2, 128, 129; alternating patterns; runs at start / end; mixed; 4 random seeds; large all-zero buffer; direct kernel checks.Refs #2048 (decode-side companion).