Changes between Initial Version and Version 1 of Ticket #10950


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Timestamp:
03/16/11 18:08:08 (10 years ago)
Author:
nthiery
Comment:

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  • Ticket #10950

    • Property Summary changed from The Hash function for matrices has many collisions with permutation matrices: to The Hash function for matrices suffers from many collisions with permutation matrices

  • Ticket #10950 – Description

    initial v1  
    1 The current hash function for generic dense matrices suffers from hard
    2 collisions with permutation matrices. For example: all permutation
    3 matrices of size 4 have hash 0!
     1The current hash function for generic dense matrices suffers from hard collisions with permutation matrices. For example: all permutation matrices of size 4 have hash 0!
    42
    53{{{
    … …  
    3129
    3230I stumbled on this when profiling some code using Weyl groups that
    33 heavily used caching (the hash of a weyl group element is the hash of
    34 the underlying matrix). I gained a speed factor of 10x by just
    35 tweaking the hash of matrices as in the attached patch. Now, I have no
    36 idea if in general that would be a good hash for matrices, so please
    37 some expert write an appropriate patch.
     31heavily used caching (the hash of a weyl group element is the hash of the underlying matrix). I gained a speed factor of 10x by just tweaking the hash of matrices as in the attached patch. Now, I have no idea if in general that would be a good hash for matrices, so please some expert write an appropriate patch.
    3832
    3933Cheers,