[ruby-core:100494] [Ruby master Feature#16851] Ruby hashing algorithm could be improved using Tabulation Hashing

Issue #16851 has been updated by mame (Yusuke Endoh).


> With that it is not allowed to overwrite the hash method for the hashing implementation (which maybe is ok

It is far from okay... Your patch does not call #hash method at all. `make test` does not pass.

```
class Foo
  def hash
    raise
  end
end

h = { Foo.new => 1 } # raises nothing
```

I'm not familiar with the tabulation technique, but I guess it should use the result of `other_func` instead of `rb_obj_id`.

I'm unsure if the performance measurement is affected by the omission of the call to #hash, but could you re-measure the benchmark after `make check` passed?

----------------------------------------
Feature #16851: Ruby hashing algorithm could be improved using Tabulation Hashing
https://bugs.ruby-lang.org/issues/16851#change-88113

* Author: ana06 (Ana Maria Martinez Gomez)
* Status: Open
* Priority: Normal
----------------------------------------
I have implemented Linear Probing and Simple tabulation in Ruby: https://github.com/Ana06/ruby/compare/master...Ana06:tabulation

I tested it using the following code:
https://github.com/Ana06/ruby-tabulation/blob/master/benchmark_tabulation.rb
It benchmarks the insertion of 600000 elements (by hashing their 64 bits ids) in an empty hash 100 times. Below are the times (in seconds) I got executing this code for different versions of the Ruby code:

- master (without Simple Tabulation): 29.68
- master with Linear Probing (without Simple Tabulation): 99.76
- master with Quadratic Probing (without Simple Tabulation): 29.97
- master with Simple Tabulation: 15.76
- master with Linear Probing and Simple Tabulation: 24.23
- **master with Quadratic Probing and Simple Tabulation: 13.73**

`master` refers to ruby 2.8.0dev:
(2020-05-07T16:22:38Z master 7ded8fd) [x86_64-linux].
I tried with 8 x Intel i5-8265U.

Although this is just a proof of concept and not something that could be already use in production, I think it proves the potential of Simple Tabulation to improve current Ruby implementation. It may be worthwhile to explore this idea further. What do you think?

I did this as part of a project for my [Advanced Algorithms course](http://www.cs.columbia.edu/~andoni/advancedS20/index.html). For more details check the project report: 
https://github.com/Ana06/ruby-tabulation/blob/master/latex/RubyTabulation_Project.pdf



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