TextDelta

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Elixir counter-part for the Quill.js Delta library. It provides a baseline for Operational Transformation of rich text.

Here’s Delta pitch from the Delta.js repository:

Deltas are a simple, yet expressive format that can be used to describe contents and changes. The format is JSON based, and is human readable, yet easily parsible by machines. Deltas can describe any rich text document, includes all text and formatting information, without the ambiguity and complexity of HTML.

A Delta is made up of an Array of Operations, which describe changes to a document. They can be an insert, delete or retain. Note operations do not take an index. They always describe the change at the current index. Use retains to “keep” or “skip” certain parts of the document.

Don’t be confused by its name Delta—Deltas represents both documents and changes to documents. If you think of Deltas as the instructions from going from one document to another, the way Deltas represent a document is by expressing the instructions starting from an empty document.

More information on original Delta format can be found here. The best starting point for learning Operational Transformation is likely this article.

Acknowledments

This library is heavily influenced by two other libraries and wouldn’t be possible without them:

  1. quilljs/delta - original JS library entire public API of text_delta is based upod. text_delta was created to provide a server counter-part to frontend Delta’s.
  2. jclem/ot_ex - implementation of this library is heavily influenced by ot_ex and though this library pursues slightly different avenue of OT, it wouldn’t be possible without it.

Differences with ot_ex

If you are searching for a library matching Quill’s Delta format, but on the server side, this library is pretty much a direct match. If, however, you’re looking for a more general Operational Transformation library, you should consider both this library and its alternative - ot_ex. Here are key differences from ot_ex that might help you make the decision:

  1. text_delta is heavily based on Quill Delta, including the public API and the delta format itself. This results in a more verbose format than what ot_ex uses.
  2. ot_ex uses fully reversible operations format, while text_delta is a one-way. If reversibility is a must, ot_ex is the only option.
  3. text_delta allows arbitrary attributes to be attached to insert or retain operations. This would allow you to transform rich text alongside plain. With ot_ex you pretty much stuck with plain text format, which might not be a big deal if your format of choice is something like Markdown.

Installation

TextDelta can be installed by adding text_delta to your list of dependencies in mix.exs:

def deps do
  [{:text_delta, "~> 1.1.0"}]
end

Documentation

Documentation can be found at https://hexdocs.pm/text_delta.

Testing & Linting

This library is test-driven. In order to run tests, execute:

$> mix test

If this command fails, it is most likely due to that you don’t have EQC installed, try:

$> mix eqc.install --mini

TextDelta uses property tests to validate that composition, transformation and compaction work as expected.

The library also uses Credo and Dialyzer. To run both, execute:

$> mix lint