Issue #19236 has been updated by Eregon (Benoit Daloze).
If we use `Hash.new(capacity: 4096)` to set the capacity, then `Hash.new({ capacity: 4096
})` should keep the semantics of: `{ capacity: 4096 }` is the default value of the new
Hash.
I think it's a little bit hacky/unclear/source of confusion, but still I'm not
against it.
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Feature #19236: Allow to create hashes with a specific capacity from Ruby
https://bugs.ruby-lang.org/issues/19236#change-101441
* Author: byroot (Jean Boussier)
* Status: Open
* Priority: Normal
* Target version: 3.3
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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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https://bugs.ruby-lang.org/