From: "kyanagi (Kouhei Yanagita) via ruby-core" <ruby-core@...>
Date: 2023-09-28T05:20:18+00:00
Subject: [ruby-core:114908] [Ruby master Feature#18515] Add Range#reverse_each implementation for performance

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


Let's set aside `#pred` for now to avoid going off track.

I have created a new pull request with a simpler implementation and closed the old one.
New PR: https://github.com/ruby/ruby/pull/8525

As benchmark results show, huge memory savings are achieved, especially for large `Range`.
Execution speed is also increased.

```
prelude: |
  rf_1 = 0..1
  rf_1k = 0..1000
  rf_1m = 0..1000000
  big = 2**1000
  rb_1 = big..big+1
  rb_1k = big..big+1000
  rb_1m = big..big+1000000

benchmark:
  "Fixnum 1": rf_1.reverse_each { _1 }
  "Fixnum 1K": rf_1k.reverse_each { _1 }
  "Fixnum 1M": rf_1m.reverse_each { _1 }
  "Bignum 1": rb_1.reverse_each { _1 }
  "Bignum 1K": rb_1k.reverse_each { _1 }
  "Bignum 1M": rb_1m.reverse_each { _1 }
```

### Max resident set size (bytes)

|           |master   |PR      |ratio|
|:----------|--------:|-------:|----:|
|Fixnum 1   |  13.910M| 14.877M|1.069|
|Fixnum 1K  |  14.778M| 14.762M|0.998|
|Fixnum 1M  |  28.770M| 14.025M|0.487|
|Bignum 1   |  14.729M| 14.189M|0.963|
|Bignum 1K  |  14.074M| 13.582M|0.965|
|Bignum 1M  | 224.723M| 15.254M|0.067|

### Iteration per second (i/s)

|           |master |PR      |ratio|
|:----------|------:|-------:|----:|
|Fixnum 1   | 6.653M| 14.511M|2.181|
|Fixnum 1K  |27.866k| 45.527k|1.633|
|Fixnum 1M  | 28.659|  45.667|1.593|
|Bignum 1   | 3.534M|  4.635M|1.311|
|Bignum 1K  | 6.790k|  7.693k|1.132|
|Bignum 1M  |  5.720|   7.532|1.316|


----------------------------------------
Feature #18515: Add Range#reverse_each implementation for performance
https://bugs.ruby-lang.org/issues/18515#change-104776

* 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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