From: "bughit (bug hit) via ruby-core" <ruby-core@...>
Date: 2023-04-27T20:28:44+00:00
Subject: [ruby-core:113368] [Ruby master Feature#8707] Hash#reverse_each

Issue #8707 has been updated by bughit (bug hit).


> Do we really need it? What is use-cases?

When you have an ordered collection it seems self evident that you may need to iterate in reverse. This would for example back a performant `last(n = 1)` method (#12165)

Would you demand to know why one may want the last n elements of an array? These are both ordered collections, if `last(n = 1)` makes sense for one, it makes sense for the other.

Why is this closed with a status of "Feedback"?

----------------------------------------
Feature #8707: Hash#reverse_each
https://bugs.ruby-lang.org/issues/8707#change-102934

* Author: Glass_saga (Masaki Matsushita)
* Status: Feedback
* Priority: Normal
* Assignee: matz (Yukihiro Matsumoto)
----------------------------------------
Currently, {}.reverse_each calls Enumerable#reverse_each.
It will make array and its size can be large.
I made Hash#reverse_each to avoid array creation and performance improvement.

benchmark:

require "benchmark"

Size = 10000
HASH = Hash[*Array.new(Size) {|i| [i, true] }.flatten]

Benchmark.bmbm do |x|
  x.report("Hash#reverse_each") do
    300.times do
      HASH.reverse_each {|a, b|}
    end
  end
end

result:

trunk(r42256):
Rehearsal -----------------------------------------------------
Hash#reverse_each   1.210000   0.000000   1.210000 (  1.207964)
-------------------------------------------- total: 1.210000sec

                        user     system      total        real
Hash#reverse_each   0.950000   0.000000   0.950000 (  0.951069)

proposal: 
Rehearsal -----------------------------------------------------
Hash#reverse_each   0.600000   0.000000   0.600000 (  0.600242)
-------------------------------------------- total: 0.600000sec

                        user     system      total        real
Hash#reverse_each   0.450000   0.000000   0.450000 (  0.459006)

---Files--------------------------------
patch.diff (7.55 KB)


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