[ruby-core:107379] [Ruby master Feature#18515] Add Range#reverse_each implementation for performance

Issue #18515 has been updated by kyanagi (Kouhei Yanagita).


Dan0042 (Daniel DeLorme) wrote in #note-3:
> I agree with both of those statements, but Integer is the only class that has #pred, so in practice "using pred" and "specialized implementation for Integer" are the same thing.

Yes, that is exactly what you said.

If I propose `#pred`-based `Range#reverse_each`, I will propose `Date#pred` too.

To find out my PR implementation (iterating integers directly) is worthwhile even if a `#pred`-based implementation is introduced,
we will need to measure the performance. I'll try it later.

----------------------------------------
Feature #18515: Add Range#reverse_each implementation for performance
https://bugs.ruby-lang.org/issues/18515#change-96281

* Author: kyanagi (Kouhei Yanagita)
* Status: Open
* Priority: Normal
----------------------------------------
PR is https://github.com/ruby/ruby/pull/5489

Current `Range#reverse_each` uses `Enumerable#reverse_each` which is implemented with `#to_a`.
So we are virtually not able to use `reverse_each` for a very large or beginless range, even if few elements are iterated on actually.

```
(1..2**100).reverse_each { |x| p x; break if x.odd? }
(..5).reverse_each { |x| p x; break if x == 0 }
(1..2**32).reverse_each.lazy.select { |x| Prime.prime?(x) }.take(3).to_a
```

This patch, implements `Range#reverse_each` for Integer elements,  enables these examples.

I think `#reverse_each` for an endless range should raise an exception.
This is a different issue, so I'll create another ticket later.
-> posted: https://bugs.ruby-lang.org/issues/18551



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