[GitHub] [arrow] bkietz commented on pull request #37418: GH-37378: [C++] Add A Dictionary Compaction Function For DictionaryArray

["bkietz (via GitHub)" <gi...@apache.org>]

bkietz commented on PR #37418:
URL: https://github.com/apache/arrow/pull/37418#issuecomment-1737718644

   > This doesn't need to be a compute function.
   
   I'm sorry I hadn't proposed this possibility already; it's certainly much less complex if it's tenable. Since this issue was opened as a prerequisite of https://github.com/apache/arrow/pull/37100 I was unnecessarily stuck on on handling this problem in the context of compute kernels.
   
   I think providing `DictionaryArray::Compact` will be acceptable. To flesh out the proposal a little more:
   
   ```c++
     /// Return an equivalent dictionary array with no unused dictionary entries.
     ///
     /// For example, consider a dictionary array where only two values of the dictionary
     /// are referenced:
     ///    dict_array = {indices=[3, 5, 3, 5], dictionary=["a", "b", "c", "d", "e", "f"]}
     ///    assert dict_array.Compact() == {indices=[0, 1, 0, 1], dictionary=["d", "f"]}
     Result<std::shared_ptr<DictionaryArray>> DictionaryArray::Compact(MemoryPool*) const;
   
     /// Return transpose mapping from uncompacted indices to compacted indices and a
     ///
     /// For example, consider a dictionary array where only two values of the dictionary
     /// are referenced:
     ///    dict_array = {indices=[3, 5, 3, 5], dictionary=["a", "b", "c", "d", "e", "f"]}
     ///    map = dict_array.CompactTransposeIndices()
     ///    assert map == [-1, -1, -1, 0, -1, 1]
     ///    assert dict_array.Transpose(map) == dict_array.Compact()
     Result<std::shared_ptr<Buffer>> DictionaryArray::CompactTransposeIndices(MemoryPool*) const;
   ```


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