nuwiki/lua/nuwiki/folding.lua

-- lua/nuwiki/folding.lua — `foldexpr` fallback when the LSP isn't
-- attached. Same heading-block model as the server-side `foldingRange`
-- provider in the nuwiki-rs LSP server, but driven from a
-- pure regex over `getline()` so it works without a running server.
--
-- Activated by `setlocal foldmethod=expr foldexpr=v:lua.require'nuwiki.folding'.expr()`
-- in `ftplugin/nuwiki.vim` when `folding` is enabled in the user config.

local M = {}

--- Heading level from one line of source. Returns 0 for non-headings.
local function heading_level(line)
  local lead = line:match('^%s*(=+)%s')
  if not lead then return 0 end
  local lvl = #lead
  if lvl > 6 then return 0 end
  -- Require a matching trailing run of `=`s.
  local trail = line:match('%s(=+)%s*$')
  if not trail or #trail ~= lvl then return 0 end
  return lvl
end

--- `foldexpr` body. Returns a fold-marker string per `:help fold-expr`.
function M.expr()
  local lnum = vim.v.lnum
  local line = vim.fn.getline(lnum)
  local lvl = heading_level(line)
  if lvl > 0 then
    return '>' .. lvl
  end
  return '='
end

--- Build a heading-only `foldtext` showing the heading title trimmed.
function M.foldtext()
  local fs = vim.v.foldstart
  local line = vim.fn.getline(fs):gsub('=', ''):gsub('^%s+', ''):gsub('%s+$', '')
  local count = vim.v.foldend - vim.v.foldstart + 1
  return string.format('▸ %s … (%d lines)', line, count)
end

--- LSP-backed foldexpr with a safe fallback.
---
--- Wraps `vim.lsp.foldexpr` so a missing client / un-attached buffer /
--- transient evaluation error falls back to the regex implementation
--- instead of throwing E5108 for every line in the buffer.
function M.lsp_expr()
  if not (vim.lsp and vim.lsp.foldexpr) then
    return M.expr()
  end
  local ok, val = pcall(vim.lsp.foldexpr)
  if ok then
    return val
  end
  return M.expr()
end

return M