[GitHub] [commons-collections] aherbert commented on pull request #357: Added Hasher.isEmpty() and appropriate tests

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aherbert commented on PR #357:
URL: https://github.com/apache/commons-collections/pull/357#issuecomment-1307849658

   IIUC the use of `isEmpty` is to prevent a client sending an empty something to a server which would waste its time, or use it as a signal. The former seems like an error by the client (or a denial of service attack); the later can be achieved by other means, for example sending empty/signalling bytes before deserialisation on the server side. This would avoid having to create the (empty) HasherCollection altogether.
   
   It seems like quite an edge case scenario. If you have to use an empty Hasher as a signal then it can be done on the server side using:
   ```Java
   static boolean isEmpty(Hasher h) {
       boolean[] flag = {true};
       // Shape can be precomputed for this purpose
       h.indices(Shape.fromKM(1, 1)).forEachIndex(i -> flag[0] = false);
       return flag[0];
   }
   ```
   If you have to deserialise the input bytes to a Hasher then just do it as a customisation of your choice of Hasher with an `isEmpty` method. If you are using an EnhancedDoubleHasher then note that this cannot be "empty" even if the initial and increment are zero given that Shape numberOfHash functions is always `> 0` so at least 1 index will be output.
   
   In general any Hasher should output indices. If you created a hasher that contains nothing, this itself is something that should generate output indices. The Hasher simply outputs indices based on a source, and ideally the output sequence is fairly unique among possible input sources.
   
   


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