How to design a comment system like in Google Docs?

Question

Software design

Good day!
There is such a problem. An internal system for working with documents is being developed, similar to Google Docs. In the backlog there is a task to create a system of comments to certain parts of the document. And nothing good comes to my mind, except for two options: adding comments to a line in the document and recalculating this line during the change, or to certain characters and, again, recalculating the location of the characters in the text.
However, I am not sure about the correctness of this option.
In our case, the document is a separate entry in the database, however, at the current moment, it is possible to change the structure of the database, up to a complete rewrite.