[GitHub] [orc] dirtysalt edited a comment on pull request #670: Update ORCv1.md: Fix documentation about bitset in bloom filter section

[GitBox <gi...@apache.org>]

dirtysalt edited a comment on pull request #670:
URL: https://github.com/apache/orc/pull/670#issuecomment-808692529


   @dongjoon-hyun  Oh. I never know that Java bit shift `<<` is sort of like a circular bit shift.  That's to say `1L << 129` equals `1L << (129 % 64)`. So that explains why `1L << position` works. 
   
   If I do that in C++, the compiler will give a warning, and the result is undefined.
   ```
   /// c++ code
   int main() {
       unsigned long long x = 0;
       x |= (1ULL << 129);
       printf("%llu\n", x);
       return 0;
   }
   
   /// shell execution
   mac :: ~/Public » g++ test.cpp
   test.cpp:14:16: warning: shift count >= width of type [-Wshift-count-overflow]
       x |= (1ULL << 129);
                  ^  ~~~
   1 warning generated.
   mac :: ~/Public » ./a.out
   4526632784
   mac :: ~/Public » ./a.out
   4345032528
   ```
   
   But I do suggest modifying documentation as `1L << (position % 64)` which is less confused to C++ guys. 


-- 
This is an automated message from the Apache Git Service.
To respond to the message, please log on to GitHub and use the
URL above to go to the specific comment.

For queries about this service, please contact Infrastructure at:
users@infra.apache.org