# TextDelta
[](https://travis-ci.org/everzet/text_delta)
Elixir counter-part for the Quill.js [Delta](https://quilljs.com/docs/delta/)
library. It provides a baseline for [Operational
Transformation](https://en.wikipedia.org/wiki/Operational_transformation) of
rich text.
Here's Delta pitch from the [Delta.js repository](https://github.com/quilljs/delta):
> 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](https://quilljs.com/docs/delta/). The best starting point for learning
Operational Transformation is likely [this
article](http://www.codecommit.com/blog/java/understanding-and-applying-operational-transformation).
## Acknowledments
This library is heavily influenced by two other libraries and wouldn't be
possible without them:
1. [`quilljs/delta`](https://github.com/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`](https://github.com/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`:
```elixir
def deps do
[{:text_delta, "~> 1.1.0"}]
end
```
## Documentation
Documentation can be found at [https://hexdocs.pm/text_delta](https://hexdocs.pm/text_delta).
## Testing & Linting
This library is test-driven. In order to run tests, execute:
```bash
$> mix test
```
If this command fails, it is most likely due to that you don't have EQC
installed, try:
```bash
$> mix eqc.install --mini
```
TextDelta uses property tests to validate that composition, transformation and
compaction work as expected.
The library also uses [Credo](http://credo-ci.org) and
[Dialyzer](http://erlang.org/doc/man/dialyzer.html). To run both, execute:
```bash
$> mix lint
```
