peritext VS reference-crdts

This doesn’t appear to support rich text formatting ranges like bold, italic, etc - unless I’m missing something in the API. AFAIK Peritext is still the state of the art in rich text CRDT algorithms https://www.inkandswitch.com/peritext/

I’d love to see this build the rich text stuff from the Peritext algorithm.

The work Ink & Switch (unaffiliated) do has been an inspiration to my with regard to local-first and decentralized software: https://www.inkandswitch.com

They have a quasi-manifesto on local-first (https://www.inkandswitch.com/local-first/) and have published the best rich text CRDT around, Peritext: https://www.inkandswitch.com/peritext/

Lots of interesting work happening in this space.

Take a look at https://automerge.org/ and the stack those folks are building. You're exactly right that it's a difficult balance (specifically the trick is proving commutativity for the domain-specific data of your application). But automerge (and then https://github.com/inkandswitch/peritext) show it's at least possible. Good stuff.

Yes - The BFT problem only matters when you have Byzantine actors. But I think users deserve and expect the system to be reasonably well behaved and predictable in all situations. Anything publically writable, for example, needs BFT resilience. Or any video game.

As for the prosemirror problem, I assume you’re talking about weird merges from users putting markdown in a text crdt? You’re totally right - this is a problem. Text CRDTs treat documents as a simple sequence of characters. And that confuses a lot of structured formats. For example, if two users concurrently bold the same word, the system should see that users agree that it should be bolded. But if that “bold” intent is translated into “insert double asterisks here and here”, you end up with 4 asterisks before and after the text, and that confused markdown parsers. The problem is that a text crdt doesn’t understand markdown.

JSON editing has similar problems. I’ve heard of plenty of people over the years putting json text into a text crdt, only to find that when concurrent edits happen, the json grows parse errors. Eg if two users concurrently insert “a” and “b” into an empty list. The result is [“a””b”] which can’t be parsed.

The answer to both of these problems is to use CRDTs which understand the shape of your data structure. Eg, use a json OT/crdt system for json data (like sharedb or automerge). Likewise, if the user is editing rich text in prosemirror then you want a rich text crdt like peritext. Rich text CRDTs add the concept of annotations - so if two users bold overlapping regions of text, the crdt understands that the result should be that the entire region is bolded. And that can be translated back to markdown if you want.

The ink & switch people did a great write up of how this sort of crdt works here: https://www.inkandswitch.com/peritext/

> I'm looking out for practical CRDT ideas that works well with richtext.

Have you seen Peritext from Ink & Switch? https://www.inkandswitch.com/peritext/ It's relatively new, but is a CRDT aimed at rich text!

To put it in a different perspective, plain text editing has well-solved CRDT patterns. But, semantic data-structures like rich-text or syntax trees is what's tricky and has unsolved challenges.

Peritext[1] is the only one that came close to solving rich-text, but even that one left out important aspect of rich-text editing like handling list & table operations as "work to be done later".

For people interested on why it's difficult to build CRDTs for richtext, here's a piece I wrote a year back: https://writer.zohopublic.com/writer/published/grcwy5c699d67...

Related HN discussion: https://news.ycombinator.com/item?id=29433896

[1] https://github.com/inkandswitch/peritext

Anyone unsure of what a CRDT is, this is the perfect intro: https://www.inkandswitch.com/peritext/

The two most widely used CRDT implementations (combining JSON like general purpose types and rich text editing types) are:

- Automerge https://github.com/automerge/automerge

- Yjs https://github.com/yjs/yjs

Svelte is, but that is your smallest problem. You want to look into CRDTs (conflict-free replicated data types) to offer true (offline) collaboration. A popular JS library to solve this complex problem is called [automerge](Conflict-free replicated data type). A rather recent development in that area specifically for text-based content is Peritext. Also check out this interactive tutorial about CRDTs.

> The goog version seems to work well but I have had nothing but frustration with ms word. Bad merges and weird states are typical, particularly from the fat client.

Argh not getting this stuff right is really frustrating. I've been working on collaborative editing for over a decade now, and I still can't implement any of these algorithms correctly without the help of a fuzz testing. But fuzz testing done right finds all of these problems! There's no excuse!

Fuzzers work so well here because all of these algorithms have a clear correctness criteria: After syncing, state should always converge to the same result. So its pretty easy to write code which does this in a loop:

1. Generates some random changes on some fake "peers"

2. Picks 2 peers at random and sync their changes, using your new fancy synchronization algorithm

3. Assert that the state has converged between the peers

I've been working on this stuff for over a decade. I've implemented dozens of these algorithms. And every single time I write a fuzzy boi to check my work I find convergence bugs. Playing whack-a-mole with a fuzzer is a rite of passage for implementing systems like this.

When your fuzzer runs all night, you should never have lingering convergence bugs like you're describing with Word.

As an example, here's a simple fuzzer for a reference list CRDT implementation: https://github.com/josephg/reference-crdts/blob/9f4f9c3a97b4...

The code is so small it almost fits on my laptop screen.

> In this case, the bottleneck at 9 million LoC is not CPU cycles but memory usage. That's where I am considering pushing down into WebAssembly

How often does this come up in practice? I can't think of many files I've opened which were 9 million lines long. And you say "LoC" (lines of code). Are you doing syntax highlighting on 9 million lines of source code in javascript? Thats impressive!

> I guess my point is why do you need balanced trees? Is this a CRDT specific thing? Can you implement CRDT with just an array of lines / gap buffer?

Of course! Its just going to be slower. I made a simple reference implementation of Yjs, Automerge and Sync9's list types in javascript here[1]. This code is not optimized, and it takes 30 seconds to process an editing trace that diamond types (in native rust) takes 0.01 seconds to process. We could speed that up - yjs does the same thing in 1 second. But I don't think javascript will ever run as fast as optimized rust code.

The b-tree in diamond types is used for merging. If you're merging 2 branches, we need to map insert locations from the incoming branch into positions in the target (merged) branch. As items are inserted, the mapping changes dynamically. The benchmark I've been using for this is how long it takes to replay (and re-merge) all the changes in the most edited file in the nodejs git repository. That file has just shy of 1M single character insert / delete operations. If you're curious, the causal graph of changes looks like this[2].

Currently it takes 250ms to re-merge the entire causal graph. This is much slower than I'd like, but we can cache the merged positions in about 4kb on disk or something so we only need to do it once. I also want to replace the b-tree with a skip list. I think that'll make the code faster and smaller.

A gap buffer in javascript might work ok... if you're keen, I'd love to see that benchmark. The code to port is here: [3]

> Undo support -> In which case, you only have to stack / remember the set of commands and not have to store the state on every change. I'm not sure if this overlaps with the data structure choice, other than implementation details.

Yeah, I basically never store a snapshot of the state. Not on every change. Not really at all. Everything involves sending around patches. But you can't just roll back the changes when you undo.

Eg: I type "aaa" at position 0 (the start of the document). You type "bbb" at the start of the document. The document is now "bbbaaa". I hit undo. What should happen? Surely, we delete the "aaa" - now at position 3.

Translating from position 0 to position 3 is essentially the same algorithm we need to run in order to merge.

> I was just looking into TypedArrays.

I tried optimizing a physics library a few years ago by putting everything in typedarrays and it was weirdly slower than using raw javascript arrays. I have no idea why - but maybe thats fixed now.

TypedArrays are useful, but they're no panacea. You could probably write a custom b-tree on top of a typedarray in javascript if you really want to - assuming your data also fits into typedarrays. But at that point you may as well just use wasm. It'll be way faster and more ergonomic.

[1] https://github.com/josephg/reference-crdts

[2] https://home.seph.codes/public/node_graph.svg

[3] https://github.com/josephg/diamond-types/tree/master/src/lis...