[ruby-core:106291] [Ruby master Feature#16038] Provide a public WeakMap that compares by equality rather than by identity

Issue #16038 has been updated by byroot (Jean Boussier).


>  It doesn't seem like it would allow the deduplication described above.

The subject was enlarged a bit over time.

The initial deduplication scenarios assume both weak keys and weak values, which is currently the case on MRI, but seems complicated to do on Java based implementations.

But I suppose you could do this:

```ruby
REGISTRY = WeakKeysMap.new

if (ref = REGISTRY[instance])
  ref.__getobj__ # would need to deal with dead ref here but whatever
else
  REGISTRY[instance] = WeakRef.new(instance)
end
```

----------------------------------------
Feature #16038: Provide a public WeakMap that compares by equality rather than by identity 
https://bugs.ruby-lang.org/issues/16038#change-94916

* Author: byroot (Jean Boussier)
* Status: Open
* Priority: Normal
----------------------------------------
I know `ObjectSpace::WeakMap` isn't really supposed to be used, and that the blessed interface is `WeakRef`. However, I'd like to make a case for a better public WeakMap.

### Usage

As described in [Feature #16035], `WeakMap` is useful for deduplicating "value objects". A typical use case is as follows:

```ruby
class Position
  REGISTRY = {}
  private_constant :REGISTRY

  class << self
    def new(*)
      instance = super
      REGISTRY[instance] ||= instance
    end
  end

  attr_reader :x, :y, :z

  def initialize(x, y, z)
    @x = x
    @y = y
    @z = z
    freeze
  end

  def hash
    self.class.hash ^
      x.hash >> 1 ^
      y.hash >> 2 ^
      y.hash >> 3
  end

  def ==(other)
    other.is_a?(Position) &&
      other.x == x &&
      other.y == y &&
      other.z == z
  end
  alias_method :eql?, :==
end

p Position.new(1, 2, 3).equal?(Position.new(1, 2, 3))
```

That's pretty much the pattern [I used in Rails to deduplicate database metadata and save lots of memory](https://github.com/rails/rails/blob/f3c68c59ed57302ca54f4dfad0e91dbff426962d/activerecord/lib/active_record/connection_adapters/deduplicable.rb).

The big downside here is that these value objects can't be GCed anymore, so this pattern is not viable in many case.

### Why not use WeakRef

A couple of reasons. First, when using this pattern, the goal is to reduce memory usage, so having one extra `WeakRef` for every single value object is a bit counter productive. 

Then it's a bit annoying to work with, as you have to constantly check wether the reference is still alive, and/or rescue `WeakRef::RefError`.

Often, these two complications make the tradeoff not worth it.

### Ruby 2.7

Since [Feature #13498] `WeakMap` is a bit more usable as you can now use an interned string as the unique key, e.g.

```ruby
class Position
  REGISTRY = ObjectSpace::WeakMap.new
  private_constant :REGISTRY

  class << self
    def new(*)
      instance = super
      REGISTRY[instance.unique_id] ||= instance
    end
  end

  attr_reader :x, :y, :z, :unique_id

  def initialize(x, y, z)
    @x = x
    @y = y
    @z = z
    @unique_id = -"#{self.class}-#{x},#{y},#{z}"
    freeze
  end

  def hash
    self.class.hash ^
      x.hash >> 1 ^
      y.hash >> 2 ^
      y.hash >> 3
  end

  def ==(other)
    other.is_a?(Position) &&
      other.x == x &&
      other.y == y &&
      other.z == z
  end
  alias_method :eql?, :==
end

p Position.new(1, 2, 3).equal?(Position.new(1, 2, 3))
```

That makes the pattern much easier to work with than dealing with `WeakRef`, but there is still that an extra instance.

### Proposal

What would be ideal would be a `WeakMap` that works by equality, so that the first snippet could simply replace `{}` by `WeakMap.new`. 

Changing `ObjectSpace::WeakMap`'s behavior would cause issues, and I see two possibilities:

  - The best IMO would be to have a new top level `::WeakMap` be the equality based map, and have `ObjectSpace::WeakMap` remain as a semi-private interface for backing up `WeakRef`.
  - Or alternatively, `ObjectSpace::WeakMap` could have a `compare_by_equality` method (inverse of `Hash#compare_by_identity`) to change its behavior post instantiation.

I personally prefer the first one.




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