[GitHub] [arrow] maartenbreddels commented on pull request #8621: ARROW-9128: [C++] Implement string space trimming kernels: trim, ltrim, and rtrim

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maartenbreddels commented on pull request #8621:
URL: https://github.com/apache/arrow/pull/8621#issuecomment-725345905


   The `std::vector<bool>` was a good idea, and indeed because of it's bit usage, the memory usage for Unicode isn't that heavy (most extreme: `0x10FFFF bits = 140kb` in case of a contiguous array implementation).
   
   Benchmarks:
   ```
   set:
   TrimManyAscii_median   28346892 ns   28345125 ns         25   558.956MB/s   35.2794M items/s
   TrimManyUtf8_median    28302644 ns   28294883 ns         25   559.949MB/s   35.3421M items/s
   
   unordered_set:
   TrimManyAscii_median   32017530 ns   32014024 ns         22   494.898MB/s   31.2363M items/s
   TrimManyUtf8_median (not run)
   
   vector<bool>
   TrimManyAscii_median   14911543 ns   14910620 ns         47   1062.58MB/s   67.0663M items/s
   TrimManyUtf8_median    16148001 ns   16146053 ns         44   981.273MB/s   61.9346M items/s
   
   bitset<256>
   TrimManyAscii_median   14304925 ns   14304010 ns         49   1107.64MB/s   69.9105M items/s
   ```
   
   
   `vector<bool>` is good enough I think, the bitset is consistently faster (5%), but I'd rather have similar code for both solutions.
   
   


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