Fold API

[lewis6991]

In order to render a scrollbar, we need to know information about the visible (aka virtual) lines.

Currently Neovim only provides one function we can use to get this information: vim.fn.foldclosedend(). From this we can create a lazy virtual line index map so we can translate from buffer lnum to virtual lnum.

This is very inefficient as it has to be updated every time these is a change to folds. This lookup function can be made much more efficient if either:

1. a dedicated function is added to core to translate to virtual line numbers.
2. An API is provided to get information on the current buffer folds.

Finally we will also need information on virtual lines provided by earmarks as these should influence the rendering of a scrollbar.

[clason]

Is this related or can be repurposed: https://github.com/neovim/neovim/pull/15248?

[lewis6991]

Kind of, but not really. That PR adds nvim_win_get_viewport() which is used to get information about the visible bounds of the buffer. For the size of the scrollbar and decorations we need to be able to translate a buffer line number to a virtual line number.

For reference this plugin currently uses these functions that an API in core would need to supplant:

-- Returns the count of virtual lines between the specified start and end lines
-- (both inclusive), in the specified window. A closed fold counts as one
-- virtual line.
function M.virtual_line_count(winid, start, vend)
  ... -- Very inefficient code
end

function M.height_to_barheight(winid, row, row2)
  local vlinecount0 = M.virtual_line_count(winid, 1) - 1
  local vheight = M.virtual_line_count(winid, row, row2)
  local winheight0 = api.nvim_win_get_height(winid) - 1
  return round(winheight0 * vheight / vlinecount0)
end

-- Calculate virtual line number from a real line number
function M.row_to_barpos(winid, row)
  return M.height_to_barheight(winid, 1, row)
end

[clason]

Couldn't this be an option to get_viewport, controlling how to count virtual lines?

[justinmk]

Related?:

• https://github.com/neovim/neovim/issues/16166
• https://github.com/neovim/neovim/issues/15247
• https://github.com/neovim/neovim/issues/8538

[lewis6991]

Couldn't this be an option to get_viewport, controlling how to count virtual lines?

To me, that doesn't seem like the best approach. get_viewport is more related to the bounds of the window.

For example say you have:

• 1 window with 100 lines in height
• a buffer in that window with a line count of 1000
• a fold from line 201-999
• a diagnostic on lines 200 and 1000

When the fold is:

• open:
  • the virtual line count is 1000
  • the virtual line number of the diagnostics is 200 and 1000
  • in a scrollbar the diagnostics would appear on roughly lines 20 and 100
• closed:
  • the virtual line count is 202
  • the virtual line number of the diagnostics is 200 and 202
  • in a scrollbar the diagnostics would both appear on line 100

This information isn't possible to calculate with only knowing the viewport bounds.

[clason]

OK, that does indeed sound like an orthogonal problem. Thanks for the explanation!

[lewis6991]

Potential alternative solution: https://github.com/neovim/neovim/pull/24236