[ruby-core:85578] [Ruby trunk Feature#14476] Adding same_all? for checking whether all items in an Array are same

Issue #14476 has been updated by duerst (Martin D端rst).


Four points.

First, re. uniq?:

mrkn (Kenta Murata) wrote:

> We don't have Array#uniq?.
> Do you suggest adding such a new method?

I remember having suggested a method named `uniq?`. Unfortunately, I didn't find the issue (yet). But the meaning was different. The method checked whether all elements were unique, not whether all elements were the same. In other words, it was:

```ruby
def uniq?
  uniq.length == length
end
```

In an efficient implementation, it would return `false` as soon as it detected two elements that were the same, and `true` otherwise.

Second, about naming:
`same_all?` doesn't sound good to me. The two words 'same' and 'all' don't appear in this order in English. What about `all_same?`, or better even, `all_equal?`?

Third, about need:
It's clear that when you implement something in C, it should get faster. If that were the only criterion for accepting new methods into Ruby, we'd have many more methods. The way you describe it, my understanding is that you only just today had a need for such functionality, not before. I think there should be some indication that this is needed regularly in order to accept it.

Fourth, about alternative ways to write it. What about

```ruby
ary.each_cons(2).all? { |a, b| a == b }
```

----------------------------------------
Feature #14476: Adding same_all? for checking whether all items in an Array are same
https://bugs.ruby-lang.org/issues/14476#change-70371

* Author: mrkn (Kenta Murata)
* Status: Assigned
* Priority: Normal
* Assignee: matz (Yukihiro Matsumoto)
* Target version: 2.6
----------------------------------------
In this issue, I propose to introduce `same_all?` instance method of `Array` class.
This new method checks whether the all items in the receiver are same.

Today, I needed to write the code to judge whether all items in an Array are same.
I wanted to make the following expression, that I've written at first, more efficiently.

```ruby
ary.all? {|e| e.foo == ary[0].foo }
```

I thought the following another simpler case, too.

```ruby
ary.all? {|e| e == ary[0] }
```

As I discussed with some CRuby committers and my colleagues at Speee, Inc., I found that both cases have the following efficient expressions:

```ruby
# for the 1st case
ary.empty? || ary[0].foo.yield_self {|e0| ary[1..-1].all? {|e| e.foo == e0 } }窶ィ

# for the 2nd case
ary.empty? || ary[0..-2] == ary[1..-1]
```

Both expressions aren't easy to understand the original purpose that is checking whether all the items are same.

I want to give this feature a clear name to make the code readable.
And I think it should be provided as a core feature because it can be more efficient by implementing in C language.

The benchmark script is: https://gist.github.com/mrkn/26a0fcfc431a45fe809fbbef95aceaf5
I used it to find the efficient expressions.
The example result of this benchmark on my MacBook Pro is here.

```
$ ruby -v bench.rb
ruby 2.6.0dev (2018-02-14 trunk 62402) [x86_64-darwin16]
----------------------------------------
Benchmark case: shuffle
                 user     system      total        real
all?-method-1  0.001525   0.000088   0.001613 (  0.001632)
all?-method-2  0.000489   0.000031   0.000520 (  0.000565)
all?-method-3  0.000444   0.000039   0.000483 (  0.000482)
all?-item    0.000325   0.000078   0.000403 (  0.000402)
opt-method   0.655959   0.033814   0.689773 (  0.708515)
opt-item     0.000316   0.000001   0.000317 (  0.000317)

----------------------------------------
Benchmark case: tail0
                 user     system      total        real
all?-method-1  9.412810   0.231126   9.643936 (  9.681118)
all?-method-2  5.375075   0.137908   5.512983 (  5.754550)
all?-method-3  5.226132   0.167640   5.393772 (  5.507031)
all?-item    0.873700   0.007545   0.881245 (  0.917210)
opt-method   5.319648   0.172547   5.492195 (  5.633140)
opt-item     0.174349   0.001974   0.176323 (  0.183002)

----------------------------------------
Benchmark case: head0
                 user     system      total        real
all?-method-1  0.002421   0.000068   0.002489 (  0.002489)
all?-method-2  0.002169   0.000213   0.002382 (  0.002382)
all?-method-3  0.001624   0.000026   0.001650 (  0.001651)
all?-item    0.000623   0.000001   0.000624 (  0.000624)
opt-method   4.779120   0.146312   4.925432 (  4.951167)
opt-item     0.000629   0.000001   0.000630 (  0.000629)

----------------------------------------
Benchmark case: all1
                 user     system      total        real
all?-method-1  9.379650   0.255865   9.635515 (  9.683078)
all?-method-2  4.950280   0.150659   5.100939 (  5.140174)
all?-method-3  4.857898   0.129125   4.987023 (  5.003142)
all?-item    0.694113   0.001295   0.695408 (  0.702370)
opt-method   5.032373   0.121708   5.154081 (  5.189599)
opt-item     0.170540   0.002069   0.172609 (  0.180343)

```



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