[rustyconover]

Stacked on #979. #928 and #968 have merged. Rebased onto the refreshed #979 branch; remains draft until #979 lands.

With #979, ArrowIpcWriterWriteArrayViewWithDictionaries() skips a dictionary that is unchanged since the last batch, but any change still replaces the whole dictionary. A common change is a dictionary that only grows: a producer encodes each batch against a dictionary that it appends new values to. Arrow streams can send just the appended values as a delta (DictionaryBatch.isDelta), which nanoarrow's reader supports since #955.

When a batch's dictionary is longer than the one last written for its ID and starts with the same values, the writer now writes only the appended values, as a delta. To check this, it copies the first n values, where n is the length of the saved copy, and compares that with the saved copy. On a match, it copies the appended values and writes them as a delta, then appends them to the prefix copy, which becomes the new saved copy. Anything else is a replacement, as before.

Deltas are only written to streams. In a file, a dictionary that grew is a replacement and returns ENOTSUP, matching #928, whose writer doesn't write deltas to files. A dictionary that is written again because a nested dictionary changed is written in full.

Cost: the same benchmark as #979, 1000 batches of 2048 rows, dictionary<int32, string>, written to a memory buffer (Release, Linux aarch64; best of 3):

	#968	#979	this PR
10,000 values, growing by 16 a batch	205 ms, 458.3 MB	531 ms, 458.3 MB	346 ms, 9.2 MB
10,000 values, unchanged	126 ms, 258.5 MB	23 ms, 8.6 MB	23 ms, 8.6 MB

Tests (writer_test.cc):

• WriteArrayViewWithDictionariesDeltas: dictionaries [foo, bar], [foo, bar, baz], [foo, bar, baz], [foo, qux] write a full dictionary, a delta, nothing, and a replacement (isDelta 0, 1, 0). Read back with nanoarrow and Arrow C++.
• WriteArrayViewWithDictionariesNullableDelta: int64 values [1, null, 3] growing to [1, null, 3, null, 5], where the delta starts partway through a byte of the validity bitmap.
• WriteArrayViewWithDictionariesNoDeltaInFile: in a file, a dictionary that grew returns ENOTSUP.

ctest passes (Linux aarch64, Debug, 881 tests with NANOARROW_BUILD_TESTS_WITH_ARROW), and 406 tests under ASan/UBSan with clang. The writer tests are clean under valgrind. The changed files are formatted with clang-format 16.0.6, the pre-commit version.

This PR's code and description were written by AI (Claude Code) and reviewed by a human maintainer.

🤖 Generated with Claude Code

Validation after rebasing (October 26): macOS arm64 Debug, 429 CTest cases, no failures; 16 compression cases skipped because compression is disabled in this build.

[codecov-commenter]

Codecov Report

❌ Patch coverage is 79.25532% with 39 lines in your changes missing coverage. Please review.
✅ Project coverage is 78.52%. Comparing base (08d23f6) to head (6bc6f0c).

Files with missing lines	Patch %	Lines
src/nanoarrow/ipc/writer.c	79.25%	8 Missing and 31 partials ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #980      +/-   ##
==========================================
- Coverage   78.52%   78.52%   -0.01%     
==========================================
  Files         106      106              
  Lines       17195    17381     +186     
  Branches     2064     2121      +57     
==========================================
+ Hits        13502    13648     +146     
- Misses       2431     2439       +8     
- Partials     1262     1294      +32     

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