Give us a way to efficiently work with cursor character positions

[mcclure]

Half a feature request, half a support request.

I am improving the vim compatibility of this library's "vim.rs" example.. So far I've added arrow keys, J, S, r, and R, and I've improved the end-of-line behavior of a and x. I will send a PR soon¹, but I'm hitting a problem:

For a ,x, and R to work properly, they must have different behavior when the cursor is on the final character of the line. I implement this with:

    // True if the textarea cursor is at the end of its given line
    fn is_before_line_end(textarea: &TextArea<'_>) -> bool {
        let (cursor_line, cursor_char) = textarea.cursor();
        let lines = textarea.lines();
        let line = &lines[cursor_line];
        let line_length = line.chars().count(); // FIXME: Not acceptable-- O(N)

        cursor_char < line_length
    }

The problem is that when retrieving the cursor from the library, the offset is in characters², but when accessing the strings in the buffer, you cannot random-access a character index. Your only option is to use the chars() iterator, which will become slow for very long lines.

My proposal is: In addition to .cursor(), offer a .cursor_bytes(), which gives a (line_idx, byte_idx) position instead of (line_idx, char_idx). This would be practical for two reasons:

1. It would easily, efficiently, and safely solve my problem, because I could compare the cursor_bytes to line_string.len().
2. If someone was willing to use unsafe {}, I think they could do some string operations more efficiently even when the cursor is not at the end of the line— and despite the use of unsafe {} it would be safe, because we can trust that tui-textarea will only allow the cursor to stop on a unicode character boundary.

However, what is important to me is solving the problem, there may be other proposals that solve it in a different or better way. What do you think?

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¹ The added characters in my patch work correctly in normal mode, but still have some FIXMEs when pressed in visual mode; they do not do the same thing in that case as standard vim.

² This is what the documentation says, and what my testing with multibyte characters confirms 👍, but by the way, can you confirm that the cursor() character index really is chars, not graphemes?