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?

  • 1
    It's not very clear what you're trying to do.
    – Drew
    Commented Dec 12, 2019 at 5:28
  • AFAIU, the goal is to transform the contents of buffer A to match the contents of buffer B without changing the text which is identical in both buffers. Hence "Something like creating a diff and applying it, instead of replacing the entire buffer".
    – phils
    Commented Dec 12, 2019 at 5:58
  • I would expect a solution based on diff and diff-apply-hunk to be a sensible approach here. I don't know that it would be a lighter approach as such, but I think it would tend to achieve the aim of not using so much undo data.
    – phils
    Commented Dec 12, 2019 at 6:07
  • 1
    I just learned something I didn't know. When calling (diff OLD NEW &optional SWITCHES NO-ASYNC), OLD and NEW can each be buffers (despite the docstring indicating they must be files). I discovered this courtesy of the diff-buffer-with-file implementation, which passes a buffer object. The diff function uses diff-file-local-copy to create a local temp file as necessary.
    – phils
    Commented Dec 12, 2019 at 6:11
  • 2
    (Documentation bug report raised. Hopefully fixed for 27.1)
    – phils
    Commented Dec 12, 2019 at 6:48

1 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.

  • Works perfectly!
    – ideasman42
    Commented Dec 13, 2019 at 0:11

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