[Qustion]: How to get the col number of byte index about ropey

[pascalkuthe]

Depends on how you define the column number. If you define it as the utf-8 offset from the start of the line:

let col = byte - rope.line_to_byte(rope.byte_to_line(byte))

If you define it as the utf-16 offset from the start of the line:

let line = doc.byte_to_line(byte);
let line_start = doc.char_to_utf16_cu(doc.line_to_char(line));
let col = doc.char_to_utf16_cu(doc.byte_to_char(byte)) - line_start;

If you define it as the utf-32 offset from the start of the line:

let line = doc.byte_to_line(byte);
let line_start = doc.line_to_char(line);
let col = doc.byte_to_char(byte) - line_start;

If you define it as the actual visual column (for example for vertical movement in the editor) that will depend on how you render the text and will likely require some form of grapheme segmentation and using something like the unicode-width crate. The first three options are already possible with the current API. The last option is not uniformly defined and out of scope (but also possible with the current API by using the chunks iterator, its just more work)