How to replace the buffer without slowing down the undo system?

Question

Say I have a command that operates on the entire buffer.

The simple solution is to:

• Run the command taking the entire buffer as input.
• Store the output.
• Clear the current buffer.
• Insert the new contents.

The problem with this is it's quite slow and it seems the undo system stores a lot of data for this operation.

In cases where only a few changes are made - is there a way to only apply changes - in a way doesn't require the heavy operation of replacing the entire contents?

Something like creating a diff and applying it, instead of replacing the entire buffer, however it need not use the diff format.

Or do I need to write my own code to detect differences and apply them as edits?

Answer 1

Emacs-26 introduced replace-buffer-contents specifically to do that job.

But use it with care: it works well when there are few differences, but if the new content happens to be completely different from the old it can be a lot more costly.