[ruby-core:113408] [Ruby master Feature#19236] Allow to create hashes with a specific capacity from Ruby

["Dan0042 (Daniel DeLorme) via ruby-core" <ruby-core@ml.ruby-lang.org>]

Issue #19236 has been updated by Dan0042 (Daniel DeLorme).


Previousy, a `capacity` reader/writer was suggested by @byroot in #18683#note-2

I would like to see this idea considered more seriously because

1. It doesn't need to change anything to the initialize arguments of Array/Hash/String, which are already quite complex enough
2. The same API can be used for any class; it's nicely consistent and easy to remember
3. It's more versatile, as it can be used more than once after object creation, ex:

```ruby
buffer = String.new #this example is with String, but the same could apply to Hash/Array
while line = gets
  #increase buffer capacity by chunks of 10k
  buffer.capacity += 10000 if buffer.capacity < buffer.bytesize + line.bytesize
  buffer << line
end
buffer.capacity = 0 #trim buffer to minimal size (aka "right-size")
buffer.capacity == buffer.bytesize #=> true
```


----------------------------------------
Feature #19236: Allow to create hashes with a specific capacity from Ruby
https://bugs.ruby-lang.org/issues/19236#change-102979

* Author: byroot (Jean Boussier)
* Status: Open
* Priority: Normal
* Target version: 3.3
----------------------------------------
Followup on [Feature #18683] which added a C-API for this purpose.

Various protocol parsers such as Redis `RESP3` or `msgpack`, have to create hashes, and they know the size in advance.
For efficiency, it would be preferable if they could directly allocate a Hash of the necessary size, so that large hashes wouldn't cause many re-alloccations and re-hash.

`String` and `Array` both already offer similar APIs:

```ruby
String.new(capacity: XXX)
Array.new(XX) / rb_ary_new_capa(long)
```

However there's no such public API for Hashes in Ruby land.

### Proposal

I think `Hash` should have a way to create a new hash with a `capacity` parameter.


The logical signature of `Hash.new(capacity: 1000)` was deemed too incompatible in [Feature #18683].


@Eregon proposed to add `Hash.create(capacity: 1000)`.





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