Hacking on Rainbow Delimiters 2

Testing

A test setup must meet the following criteria:

• Test definitions must be run by with Neovim as the Lua interpreter to get access to all Neovim APIs
• Tests must not be affected by the user's own plugins and configuration
• Each test which mutates editor state must run in its own Neovim process

The first two points are achieved through a small command-line interface adapter script (a shim). The shim exposes the command-line interface of a Lua interpreter, and internally it sets up environment variable to point Neovim at a prepared blank directory structure. Neovim is then called with the -l flag.

We do have to use some plugins though:

• This plugin itself
• nvim-treesitter to install parsers for some languages

Both plugins are stored under the $XDG_DATA_HOME directory, the former as a symlink and the latter as a Git submodule.

As for process isolation, this is achieved inside the tests. We start a headless embedded Neovim instance which we control through MsgPack RPC from inside the test. We can control and probe this process only indirectly, which is awkward, but this is the best solution I could find.

Unit testing

We use busted for unit testing. A unit is a self-contained module which can be used on its own independent of the editor. Execute make unit-test to run unit tests. The busted binary must be available on the system $PATH.

End to end testing

End-to-end tests run in a separate Neovim instance which we control via RPC. These are tests which mutate the state of the editor, such as adding highlighting on changes. Execute make e2e-test to run all end to end tests.

Running tests with Neotest-busted

To run tests the g:bustedprg variable must be set to './test/busted', which is the path to the shim script. If the exrc option is set the variable will be set automatically.

Design decisions

Tables over strings for configuration

Strategies are given as a complex table, but a string identifier would have been much more pleasant on the eye. Which of these two is easier to read and write?

-- This?
settings = {
   strategy = {
      'global'
      html = 'local'
   }
}

-- Or this?
settings = {
   strategy = {
      require 'ts-rainbow.strategy.global'
      html = require 'ts-rainbow.strategy.local'
   }
}

Using strings might seem like the more elegant choice, but it it makes the code more complicated to maintain and less flexible for the user. With tables a user can create a new custom strategy and assign it directly without the need to "register" them first under some name.

More importantly though, we have unlimited freedom where that table is coming from. Suppose we wanted to add settings to a strategy. With string identifiers we now need much more machinery to connect a string identifier and its settings. On the other hand, we can just call a function with the settings are arguments which returns the strategy table.

settings = {
    strategy = {
        require 'ts-rainbow.strategy.global',
        -- Function call evaluates to a strategy table
        latext = my_custom_strategy {
            option_1 = true,
            option_2 = 'test'
        }
    }
}

Strategies

On container nodes

Every query has to define a container capture in addition to opening and closing captures. As humans we understand the code at an abstract level, but Tree-sitter works on a more concrete level. To a human the HTML tag <div> is one atomic object, but to Tree-sitter it is actually a container with further elements.

Consider the following HTML snippet:

<div>
  Hello
</div>

The tree looks like this (showing anonymous nodes):

element [0, 0] - [2, 6]
  start_tag [0, 0] - [0, 5]
    "<" [0, 0] - [0, 1]
    tag_name [0, 1] - [0, 4]
    ">" [0, 4] - [0, 5]
  text [1, 1] - [1, 6]
  end_tag [2, 0] - [2, 6]
    "</" [2, 0] - [2, 2]
    tag_name [2, 2] - [2, 5]
    ">" [2, 5] - [2, 6]

We want to highlight the lower-level nodes like tag_name or start_tag and end_tag, but we want to base our logic on the higher-level nodes like element. The @container node will not be highlighted, we use it to determine the nesting level or the relationship to other container nodes.

Determining the level of container node

In order to correctly highlight containers we need to know the nesting level of each container relative to the other containers in the document. We can use the order in which matches are returned by the iter_matches method of a query. The iterator traverses the document tree in a depth-first manner according to the visitor patter, but matches are created upon exiting a node.

Let us look at a practical example. Here is a hypothetical tree:

A
├─B
│ └─C
│   └─D
└─E
  ├─F
  └─G

The nodes are returned in the following order:

1. D
2. C
3. B
4. F
5. G
6. E
7. A

We can only know how deeply nodes are nested relative to one another. We need to build the entire tree structure to know the absolute nesting levels. Here is an algorithm which can build up the tree, it uses the fact that the order of nodes never skips over an ancestor.

Start with an empty stack s = []. For each match m do the following:

1. Keep popping matches off s up until we find a match m' whose @container node is not a descendant of the container node of m. Collect the popped matches (excluding m') onto a new stack s_m (order does not matter)
2. Set s_m as the child match stack of m
3. Add m to s

Eventually s will only contain root-level matches, i.e. matches of nesting level one. To apply the highlighting we can then traverse the match tree, incrementing the highlighting level by one each time we descend a level.

The order of matches among siblings in the tree does not matter. The above algorithm uses a stack when collecting children, but any unordered one-dimensional sequence will do. The stack s is important for determining the relationship between nodes: since we know that no ancestors will be skipped we can be certain that we can stop checking the stack for descendants of m once we encounter the first non-descendant match. Otherwise we would have to compare each match with each other match, which would tank the performance.

The local highlight strategy

Consider the following bit of contrived HTML code:

<div id="Alpha">
  <div id="Bravo">
     <div id="Charlie">
     </div>
  </div>
  <div id="Delta">
  </div>
</div>

Supposed the cursor was inside the angle brackets of Bravo, which tags should we highlight? From eyeballing the obvious answer is Alpha, Bravo and Charlie. Obviously Alpha and Bravo both contain the cursor within the range, but how do we know that we need to highlight Charlie? Charlie is contained inside Bravo, which contains the cursor, but on the other hand Delta is contained inside Alpha, which also contains the cursor. We cannot simply check whether the parent contains the cursor.

When working with the Tree-sitter API and iterating through matches and captures we have no way of knowing that any of the captures within Charlie are contained within Bravo. However, due to the order of traversal we do know that Bravo is the lowest node to still contain the cursor.

Therefore we that the first match which contains the cursor is the lowest one. If a match does not contain the cursor we can check whether it is a descendant of the cursor container match.

The problem with nested languages

The language tree of a buffer is a tree of parsers. Some languages like Markdown can contain other languages, which complicates things.

Foreign extmarks

Extmarks move along with the text they belong to. This is generally a good thing, but it can become a problem if we move text from one language to another. Consider the following Markdown code:

Hello world

```lua
print {{{{}}}}
print {{{{}}}}
```

We can move the cursor to line 4 and move that line out of the Lua block by executing :move 1 to move it to the second line. However, this will preserve the extmarks and we will end up with Lua delimiter highlighting inside Markdown.

My solution is on every change to delete all rainbow delimiter extmarks which do not belong to the current language.

Overwritten extmarks

Take the following Markdown code:

Hello world

```c
puts("This is an injected language")
{
    {
        {
            {
                {
                    return ((((((2)))))) + ((((3))))
                }
            }
        }
    }
}
```

If we put the cursor on the line with the puts statement and move it up one line (:move -2) we get the following changes:

• Markdown - { 2, 0, 3, 0 }

This means lines 3 and 4 of the Markdown tree have changed; we have changed the contents of the fifth line and added one more line. This is all as expected. However, let us now move the line back down by executing :move +1. We get the following changes:

• Markdown - { 3, 0, 15, 0 }
• C - { 3, 0, 4, 0 }

The changes to the C tree are what we expect. However, the changes to the Markdown tree span the code block as well. This is a problem when we start deleting foreign extmarks (see above). If we work from the outside we wipe out all non-Markdown extmarks in the range, which includes the C extmarks. Then we apply the C extmarks inside the C block, but the C change does not span the entire C tree. Thus we will only apply highlighting to the changed C line, but not the remainder of the C block.

The solution at the moment is to overwrite the changes of nested languages. If the changes belong to a language tree with parent language we replace all the changes with a range that spans the entire tree for that language.