slotmap VS too-many-lists

It looks like .get_disjoint_mut() from slotmap failed under stacked borrows, but seems to pass under tree borrows

I think slotmap is meant to solve this exact issue. Basically when you insert into a collection you get an Id:Version tuple as key. When you reuse a slot, next time the key will be Id:Version+1 and when you try to access the removed value by using Id:Version, it will return None. You can think about it as delayed invalidation.

Dunno about existing implementations, but it looks like it's a feature they'd accept: https://github.com/orlp/slotmap/issues/73

It's actually quite easy.

You can use either slot map or slab to side step rust borrow checker. Example https://github.com/orlp/slotmap/blob/master/examples/rand_meld_heap.rs

I don't know, only have some theories.

1. The name isn't particularly catchy or descriptive. It is the correct name for the data structure, but not too many people know the data structure.

2. People don't even know what they're missing. It's not a very Google-able problem to begin with. Slotmap provides an interesting solution to (circular) ownership and safe allocator / weak pointer design problems, but people don't recognize that they're having them or that slotmap could help.

As an example of this, the doubly linked list example (https://github.com/orlp/slotmap/blob/master/examples/doubly_...) can safely remove nodes from the linked list given their handle, in O(1), even from the middle, completely safely and correctly, even in the presence of double deletions or ABA memory re-use. You can't replicate this with just pointers, without introducing heavy refcounting solutions.

Nope, it's perfectly safe: https://github.com/orlp/slotmap/blob/master/examples/doubly_linked_list.rs.

I think keeping the relationship between child and parent elements in the node graph might be better accommodating better via a psuedo-ECS system, see https://www.reddit.com/r/rust/comments/cnjhup/idiomatic_way_to_reference_parent_struct/. The https://github.com/orlp/slotmap crate looks promising. I think I'm just going to ditch the global shared HashMap in favor of something that can better accommodate child/parent relations.

I think the slotmap crate is similar to what you're trying to write: https://github.com/orlp/slotmap

You seem to have a preset opinion, and I'm not sure you are interested in re-evaluating it. So this is not written to change your mind.

I've developed production code in C, C++, Rust, and several other languages. And while like pretty much everything, there are situations where it's not a good fit, I find that the solutions tend to be the most robust and require the least post release debugging in Rust. That's my personal experience. It's not hard data. And yes occasionally it's annoying to please the compiler, and if there were no trait constraints or borrow rules, those instances would be easier. But way more often in my experience the compiler complained because my initial solution had problems I didn't realize before. So for me, these situations have been about going from building it the way I wanted to -> compiler tells me I didn't consider an edge case -> changing the implementation and or design to account for that edge case. Also using one example, where is Rust is notoriously hard and or un-ergonomic to use, and dismissing the entire language seems premature to me. For those that insist on learning Rust by implementing a linked list there is https://rust-unofficial.github.io/too-many-lists/.

Advent of Code was okay until I encounterd a problem that required a graph, tree or linked list to solve, where I hit a wall. Most coding exercises are similar--those requiring arrays and hashmaps and sets are okay, but complex data structures are a PITA. (There is an online course dedicated to linked lists in Rust but I couldn't grok it either). IMO you should simply skip problems that you can't solve with your current knowledge level and move on.

I feel obligated to point to the original cannon literature: https://rust-unofficial.github.io/too-many-lists/

I started learning Rust and like how the compiler is fussy about things. My plan was to implement the data structures I knew, but I got stuck at the singly linked list's remove() method. I've read the book as well, but I have no clue how to simplify this further:

My impression from the article is that Zig provides several different hashtables and not all of them are broken in this way.

This reminds me of Aria's comment in her Rust tutorial https://rust-unofficial.github.io/too-many-lists/ about failing to kill LinkedList. One philosophy (and the one Rust chose) for a stdlib is that this is only where things should live when they're so commonly needed that essentially everybody needs them either directly or to talk about. So, HashTable is needed by so much otherwise unrelated software that qualifies, BloomFilter, while it's real useful for some people, not so much. Aria cleaned out Rust's set of standard library containers before Rust 1.0, trying to keep only those most people would need. LinkedList isn't a good general purpose data structure, but, it was too popular and Aria was not able to remove it.

Having multiple hash tables feels like a win (they're optimized for different purposes) but may cost too much in terms of the necessary testing to ensure they all hit the quality you want.

> Cyclic references can be dealt with runtime safety checks too - like Rc and Weak.

Indeed. Starting out with code sprinkled with Rc, Weak, RefCell, etc is perfectly fine and performance will probably not be worse than in any other safe languages. And if you do this, Rust is pretty close to those languages in ease of use for what are otherwise complex topics in Rust.

A good reference for different approaches is Learn Rust With Entirely Too Many Linked Lists https://rust-unofficial.github.io/too-many-lists/

Second, once you've finished something introductory like The Book, read Learning Rust With Entirely Too Many Linked Lists. It really helped me to understand what ownership and borrowing actually mean in practical terms. If you don't mind paying for learning materials, a lot of people recommend Programming Rust, Second Edition by Blandy, Orendorff, and Tindall as either a complement, follow-up, or alternative to The Book.

Learning Rust With Entirely Too Many Linked Lists which highlights a lot of the differences with how you need to structure your code in Rust compared to other languages.