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.

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

1. quilljs/delta - original JS library for rich text OT. Entire public API of text_delta is based upod it.
2. jclem/ot_ex - implementation of this library is heavily influenced by ot_ex. Though this library pursues slightly different design goals than ot_ex, it wouldn't be possible without it.

If you are searching for a library matching Quill's Delta format 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 text_delta and ot_ex. Here are the key differences from ot_ex that might help you make the decision:

1. text_delta is heavily based on Quill Delta. That includes the public API and the delta format itself. This results in a more verbose format than ot_ex.
2. ot_ex uses fully reversible operations format, while text_delta is a one-way. If reversibility is a must, ot_ex is your only option.
3. text_delta allows arbitrary attributes to be attached to insert or retain operations. This allows 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.

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

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 QuickCheck installed. If so, simply 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