How to handle no content for a custom text object?

Question

I am building custom text object in lua. With the following code, I am able to change content within certain symbols. This is my code:

local in_symbol = function(start_symbol, end_symbol, mode)
  local v = vim.api
  local currentLineNum = v.nvim_win_get_cursor(0)[1]
  local end_match = vim.api.nvim_call_function("searchpos", {start_symbol, "", currentLineNum})
  local start_match = vim.api.nvim_call_function("searchpos", {start_symbol, "b", currentLineNum})
  if start_match[1] == 0 then
    return
  end
  if end_match[1] == 0 then
    end_match = vim.api.nvim_call_function("searchpos", {start_symbol, "n", currentLineNum})
  end
  if end_match[1] == 0 then
    return
  end
  local end_symbol_length = string.len(end_symbol)
  local end_offset = mode == 'i' and end_match[2] - end_symbol_length - 1 or end_match[2] - 1

  local start_symbol_length = string.len(start_symbol)
  local start_offset = mode == 'i' and start_match[2] or start_match[2]
  if end_match[2] - start_match[2] == 1 then
    v.nvim_win_set_cursor(0, { currentLineNum, start_offset - 1})
    print('no content')
    return
  end
  v.nvim_win_set_cursor(0, { currentLineNum, start_offset})
  vim.cmd('normal! v')
  v.nvim_win_set_cursor(0, { currentLineNum, end_offset})
end

return {
  in_symbol = in_symbol,
}

For a custom text object for *(i.e. ci*), I have the following mapping

vim.cmd [[xnoremap <silent> i* :lua require('plugins.custom_text_objects').in_symbol('*' ,'*', 'i')<cr>]] 
vim.cmd [[onoremap <silent> i* :lua require('plugins.custom_text_objects').in_symbol('*' ,'*', 'i')<cr>]] 

This works well with a sentence like this:

*hello*

But when there is no content (i.e. **), it will just eat up the first or the second * on ci*. How can I fix this behavior? I cannot think of a good solution right now.

Answer 1

An onoremap will not act on anything if you exit the function without moving the cursor, so you should try to do that.

Your initial calls to searchpos() don't include the n flag, so they're doing that.

Here's a very simple and naive proof of concept (in pure Vimscript) to show how this works:

function! FindStar()
  let [_, s] = searchpos('*', 'bcn', line('.'))
  if s <= 0
    return
  endif
  let [_, e] = searchpos('*', 'n', line('.'))
  if e <= 0 || e <= s+1
    return
  endif
  call cursor('.', s+1)
  execute 'normal! v'
  call cursor('.', e-1)
endfunction

onoremap <silent> i* :<C-u>call FindStar()<CR>

Note how both calls to searchpos() are passing the n flag, so they're not moving the cursor. The initial one also passes c to possibly match at the current position.

They both check for the result being zero (or negative, which shouldn't happen) to cover for the case of no match being found in the current line.

But then the second case also checks for e <= s+1, which is exactly the case where the stars are next to each other and the content is empty in that case. Simply going ahead and returning in that case, without having moved the cursor, will ensure nothing will have been selected.

The code then goes ahead to move the cursor and prepare a visual selection.

Take that for a spin (or perhaps convert this to Lua if you like), you'll see it covers that corner case nicely.