[SoC] GSoC 2021 - Week 3 Report - Implement pre-sorting methods before GiST index building

[Han Wang]

Hi all,

I am here to share with you my Week3 report. You can also find it at [1]

*Coding Phase *:

   - Refactor the code structure, moving the hash function to the
   inline module
   - Create a pull request and receive suggestions from the community and
   mentors[2]
   - Finish the morton hash function

I used `interleave` to implement a Morton(z-order) hash. What's more, I try
to return the `x` of a `BOX2DF` as the hash order of the geometry object.
What is confusing is that the Hilbert hash, the morton hash and the simple
`x` show little difference in the performance test. So I think it is
necessary to augment our test dataset and use some read world data.

*Plans for next week*:

   - Create larger random data or use real-world data for testing
   - Evaluate the stability and efficiency of hash functions
   - Searching for a more efficient hash function


If you have any questions or suggestions, please let me know.

[1]
https://trac.osgeo.org/postgis/wiki/ImplementSortingMethodsBeforeGistIndexBuilding
[2] https://github.com/postgis/postgis/pull/619

Best regards,
Han
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