From: "matheusrich (Matheus Richard) via ruby-core" <ruby-core@...>
Date: 2024-02-27T16:42:48+00:00
Subject: [ruby-core:116973] [Ruby master Feature#20300] Hash: set value and get pre-existing value in one call

Issue #20300 has been updated by matheusrich (Matheus Richard).


Rust [calls this method `insert`](https://doc.rust-lang.org/std/collections/struct.HashMap.html#method.insert):

> Inserts a key-value pair into the map.
>
> If the map did not have this key present, None is returned.
>
> If the map did have this key present, the value is updated, and the old value is returned.
>
> ```rs
> let mut map = HashMap::new();
> assert_eq!(map.insert(37, "a"), None);
> assert_eq!(map.is_empty(), false);
> 
> map.insert(37, "b");
> assert_eq!(map.insert(37, "c"), Some("b"));
> assert_eq!(map[&37], "c");
> ```

----------------------------------------
Feature #20300: Hash: set value and get pre-existing value in one call
https://bugs.ruby-lang.org/issues/20300#change-107020

* Author: AMomchilov (Alexander Momchilov)
* Status: Open
----------------------------------------
When using a Hash, sometimes you want to set a new value, **and** see what was already there. Today, you **have** to do this in two steps:

```ruby
h = { k: "old value" }

# 1. Do a look-up for `:k`.
old_value = h[:k]
# 2. Do another look-up for `:k`, even though we just did that!
h[:k] = "new value"

use(old_value)
```

This requires two separate `Hash` look-ups for `:k`. This is fine for symbols, but is expensive if computing `#hash` or `#eql?` is expensive for the key. It's impossible to work around this today from pure Ruby code.

One example use case is `Set#add?`. See https://bugs.ruby-lang.org/issues/20301 for more details.

I propose adding `Hash#update_value`, which has semantics are similar to this Ruby snippet:

```ruby
class Hash
  # Exact method name TBD.
  def update_value(key, new_value)
    old_value = self[key]
    self[key] = new_value
    old_value
  end
end
```

... except it'll be implemented in C, with modifications to `tbl_update` that achieves this with a hash-lookup. 

I'm opening to alternative name suggestions. @nobu came up with `exchange_value`, which I think is great.

Here's a PR with a PoC implementation: https://github.com/ruby/ruby/pull/10092

```ruby
h = { k: "old value" }

# Does only a single hash look-up
old_value = h.update_value(:k, "new value")

use(old_value)
```



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