helix/docs/architecture.md

| Crate        | Description                                            |
| -----------  | -----------                                            |
| helix-core   | Core editing primitives, functional.                   |
| helix-syntax | Tree-sitter grammars                                   |
| helix-lsp    | Language server client                                 |
| helix-view   | UI abstractions for use in backends, imperative shell. |
| helix-term   | Terminal UI                                            |
| helix-tui    | TUI primitives, forked from tui-rs, inspired by Cursive |


This document contains a high-level overview of Helix internals.

> NOTE: Use `cargo doc --open` for API documentation as well as dependency
> documentation.

## Core

The core contains basic building blocks used to construct the editor. It is
heavily based on [CodeMirror 6](https://codemirror.net/6/docs/). The primitives
are functional: most operations won't modify data in place but instead return
a new copy.

The main data structure used for representing buffers is a `Rope`. We re-export
the excellent [ropey](https://github.com/cessen/ropey) library. Ropes are cheap
to clone, and allow us to easily make snapshots of a text state.

Multiple selections are a core editing primitive. Document selections are
represented by a `Selection`. Each `Range` in the selection consists of a moving
`head` and an immovable `anchor`. A single cursor in the editor is simply
a selection with a single range, with the head and the anchor in the same
position.

Ropes are modified by constructing an OT-like `Transaction`. It's represents
a single coherent change to the document and can be applied to the rope.
A transaction can be inverted to produce an undo. Selections and marks can be
mapped over a transaction to translate to a position in the new text state after
applying the transaction.

> NOTE: `Transaction::change`/`Transaction::change_by_selection` is the main
> interface used to generate text edits.

`Syntax` is the interface used to interact with tree-sitter ASTs for syntax
highling and other features.

## View

The `view` layer was supposed to be a frontend-agnostic imperative library that
would build on top of `core` to provide the common editor logic. Currently it's
tied to the terminal UI.

A `Document` ties together the `Rope`, `Selection`(s), `Syntax`, document
`History`, language server (etc.) into a comprehensive representation of an open
file.

A `View` represents an open split in the UI. It holds the currently open
document ID and other related state.

> NOTE: Multiple views are able to display the same document, so the document
> contains selections for each view. To retrieve, `document.selection()` takes
> a `ViewId`.

The `Editor` holds the global state: all the open documents, a tree
representation of all the view splits, and a registry of language servers. To
open or close files, interact with the editor.

## LSP

A language server protocol client.

## Term

The terminal frontend.

The `main` function sets up a new `Application` that runs the event loop.

`commands.rs` is probably the most interesting file. It contains all commands
(actions tied to keybindings). 

`keymap.rs` links commands to key combinations.


## TUI / Term

TODO: document Component and rendering related stuff