Peritext

This is a prototype implementation of Peritext, a CRDT for rich text with inline formatting. The algorithm is described in the following publications:

• Online essay, Peritext: A CRDT for Rich Text Collaboration
• Geoffrey Litt, Sarah Lim, Martin Kleppmann, and Peter van Hardenberg. Peritext: A CRDT for Collaborative Rich Text Editing. Proceedings of the ACM on Human-Computer Interaction (PACMHCI), Volume 6, Issue CSCW2, Article 531, November 2022. doi:10.1145/3555644

This repo includes:

• A Typescript implementation of the core Peritext CRDT algorithm
• A prototype integration with the Prosemirror editor library
• An interactive demo UI where you can try out the editor
• A test suite

Try the editor demo

To see a basic interactive demo where you can type rich text into two editors and periodically sync them:

npm install

npm run start

Code tour

Algorithm code: The main algorithm implementation is in src/peritext.ts. Because the goal of this work is to eventually implement a rich text type in Automerge, we implemented Peritext as an extension to a codebase called Micromerge, which is a simplified implementation of Automerge that has mostly the same behavior but is less performance-optimized.

The essay describes the algorithm in three main parts:

• Generating operations: happens in changeMark
• Applying operations: happens in applyAddRemoveMark
• Producing a document: there are two places this logic is defined. getTextWithFormatting is a "batch" approach which iterates over the internal document metadata and produces a Prosemirror document. There is also a codepath that produces incremental patches representing changes (which is actually what powers the editor demo); these patches get emitted directly while applying the op, within applyAddRemoveMark.

Prosemirror integration: src/bridge.ts contains the code for the integration between the CRDT and the Prosemirror library. There are two main pieces to the integration:

• Prosemirror to CRDT: when a change happens in the editor, Prosemirror emits a Transaction. We turn that transaction into a list of InputOperation commands for the CRDT, inside the applyProsemirrorTransactionToMicromergeDoc function.
• CRDT to Prosemirror: when a change happens in the Micromerge CRDT, the CRDT emits a Patch object representing what changed. We turn this into a Prosemirror transaction with the extendProsemirrorTransactionWithMicromergePatch function.

Each direction of this transformation is straightforward, because the external interface of InputOperations and Patches provided by the CRDT closesly matches the Prosemirror Transaction format.

Tests

npm run test will run the manual tests defined in test/micromerge.ts. These tests correspond to many of the specific examples explained in the essay.

You can also run a generative fuzz tester using npm run fuzz. This will randomly generate edit traces and check for convergence.

Build demo artifact for essay

This repo also contains a UI that plays back a preset trace of edit actions, which is included in the Ink & Switch essay about Peritext.

To see that UI, you can run npm run start-essay-demo.

To build an artifact for including in the essay, run npx parcel build src/essay-demo.ts, and then copy the resulting ./dist/essay-demo.js file to content/peritext/static/peritext-demo.js in the essays repo. Also copy over any CSS changes from static/essay-demo.css to content/peritext/static/peritext-styles.css if needed.