-- `map` is just my custom frontend with sane defaults for `vim.keymap.set`:
map({
  mode = "n",
  lhs = "<localleader>x",
  rhs = function()
    local bufnr = vim.api.nvim_get_current_buf()
    local row, _ = unpack(vim.api.nvim_win_get_cursor(0))
    local lines = vim.api.nvim_buf_get_lines(bufnr, 0, -1, false)

    -- 1. Start row is always the current line (checkbox line)
    local start_row = row

    -- 2. End row expands downward while lines are indented
    local end_row = row
    while end_row < #lines and lines[end_row + 1]:match("^%s") do
      end_row = end_row + 1
    end

    local first_line = lines[start_row]

    -- 3. Detect checkbox state on first line of task.
    local unchecked = first_line:match("^%s*%- %[ %]") ~= nil
    local checked = first_line:match("^%s*%- %[[xX]%]") ~= nil

    -- 4. Toggle the checkbox.
    local new_checked
    if unchecked then
      lines[start_row] = first_line:gsub("^(%s*%-+%s*)%[ %]", "%1[x]") -- check
      new_checked = true
    elseif checked then
      lines[start_row] = first_line:gsub("^(%s*%-+%s*)%[[xX]%]", "%1[ ]") -- uncheck
      new_checked = false
    else
      vim.notify("Not a valid Markdown task", vim.log.levels.WARN, { title = "Markview" })
      return
    end

    -- 5. Handle the completion comment on the last line.
    local last_line = lines[end_row]
    local comment_pattern = "%s*<!%-%- completed: %d%d%d%d%-%d%d%-%d%d %-%->"
    local comment = " <!-- completed: " .. os.date("%Y-%m-%d") .. " -->"

    if new_checked then
      if not last_line:match(comment_pattern) then
        lines[end_row] = last_line .. comment
      end
    else
      lines[end_row] = last_line:gsub(comment_pattern, "")
    end

    -- 6. Write back updated lines to buffer.
    -- stylua: ignore
    vim.api.nvim_buf_set_lines(
      bufnr,
      start_row - 1,
      end_row,
      false,
      vim.list_slice(lines, start_row, end_row)
    )
  end,
  desc = "Markdown: toggle checkbox (append completion date)",
})