Is there an Emacs function or package to visually compare two text strings?

Question

Searching on Google, I found this website: https://www.dcode.fr/differences-2-texts. It allows "visually" comparing two text strings at the character level,

returning a result like this:

My question is: is there any function or package that allows doing the same thing directly in Emacs?

The function should take the two strings as arguments and return the "fontified" comparison string or show the result in a dedicated buffer.

I would need to use the function something like this:

(emacs-text-compare-bla-bla "String1" "String2" 'character-level)

without the need to manually select the strings to compare.

Note. I would really prefer to have a fontified string as output, like in the example, so that I can easily view/insert it without the need for additional buffers. If I were to have the output in a buffer, it should be a single window, not two as in the case of *diff* functions.

I thought that perhaps I could use a function used internally by some tool used for diffing... but since I don't use Emacs for this type of operation, I have no idea where to start.

Answer 1

Based on the answer above by @db48x, this seems to roughly work:

(defun malb/visual-diff-strings (old-string new-string)
  (let ((old-buffer (get-buffer-create (make-temp-name "old-buffer-")))
        (new-buffer (get-buffer-create (make-temp-name "new-buffer-")))
        (output-buffer (get-buffer-create (make-temp-name "*output-diff*"))))
    (with-current-buffer old-buffer
      (insert old-string))
    (with-current-buffer new-buffer
      (insert new-string))
    (diff-no-select old-buffer new-buffer "-u" t output-buffer)
    (kill-buffer old-buffer)
    (kill-buffer new-buffer)
    (display-buffer-pop-up-frame output-buffer '((pop-up-frame-parameters (width . 80))))))

Answer 2

Yes, just call the function diff.

It only takes buffers though, not strings. You’ll want to turn your strings into temporary buffers first by using with-temp-buffer. (Or, depending on how you are writing your code, you might already have buffers and can just skip the step of turning them into strings.)

Edit:

The difference between strings and buffers is pretty trivial. A buffer is mostly just a string with some ancillary data, such as the cursor location, stored alongside it. If you just want the text that would be displayed in a diff buffer, you can call the function diff-no-select instead. This will create the “Diff” buffer without making it visible, unless you pass in a buffer of your own. Thus, if you want a string, all you have to do is make a temporary buffer, render the diff into it, and then fetch the content of the buffer as a string. Like this:

(with-temp-buffer
  (diff-no-select old-buffer new-buffer nil t (current-buffer))
  (buffer-string))

Of course, that is such a common idiom that there is macro that does exactly that called with-output-to-string:

(with-output-to-string
  (diff-no-select old-buffer new-buffer nil t (current-buffer)))

Another edit:

I must have forgotten to mention this, but the resulting buffer will be in diff-mode, which can do refinement to a character–based diff. If you customize the variable diff-refine, you can turn this on or off.