Published in Technology on A Place For My Head. Tagged , , & .

By default, Git diffs use the perfectly-serviceable, decades-old unified diff format. A drop-in replacement like delta offers various improvements: most relevantly to me, it adds syntax highlighting (not coming to Magit any time soon) and fine-grained differences (possible in Magit, but in a less useful form). When I tried magit-delta a year ago, I eventually had to disable it because of performance issues. Although those haven’t been resolved yet, I just took another stab at it, prompted by diff envy. I re-enabled magit-delta, then added a command to disable it when required: 

Emacs Lisp(defun aankh/toggle-magit-delta ()
    (interactive)
    (magit-delta-mode
     (if magit-delta-mode
         -1
       1))
    (magit-refresh))

  (transient-append-suffix 'magit-diff '(-1 -1 -1)
    '("l" "Toggle magit-delta" aankh/toggle-magit-delta))

Now I could always revert—pardon the pun—to quick, regular diffs by pressing d l in a Magit buffer.

Having come this far, I thought, why not try something even more cutting-edge? difftastic is a structural diff that understands syntax, which serves as a complement to ordinary diffs. I followed Tassilo Horn’s guide to using it with Magit, allowing # d in a Magit buffer to pop up an automatic difftastic diff, or # s to do the same for a specific range. Unfortunately, the builtin colours were hard to read:

The only control offered by the program is --light and --dark, which select dimmer and brighter shades, respectively. I tried giving just the difftastic buffer a dark background, which necessitated creating a new frame entirely:

It was awkward and brittle. b-fuze suggested rewriting the colours in the output, even simply via a tool like sed. Fortunately, I ultimately didn’t need to resort to modifying the ANSI colour codes. Instead, I examined the results of the ansi-color-apply-on-region function, which creates overlays with anonymous faces, and wrote some inelegant Emacs Lisp to alter just those colours (which I’ll need to update if difftastic starts using backgrounds): 

Emacs Lisp(defun aankh/recolor-difftastic ()
  (let ((ovs (overlays-in (point-min) (point-max))))
    (dolist (ov ovs)
      (let ((face (overlay-get ov 'face)))
        (when (and (not (null face)) (listp face))
          (when (plist-get face :foreground)
            (plist-put face :foreground (aankh/get-remapped-difftastic-colour (plist-get face :foreground))))
          (when-let ((existing (cl-find :foreground face :key (lambda (x) (if (consp x) (car x) nil)))))
            (setf face
                  (cl-subst `(:foreground ,(aankh/get-remapped-difftastic-colour (plist-get existing :foreground)))
                            :foreground
                            face
                            :key (lambda (x) (if (consp x) (car x) nil)))))
          (overlay-put ov 'face face))))))

(defun aankh/get-remapped-difftastic-colour (original)
  (alist-get original +aankh/difftastic-colour-remapping+ nil nil 'string=))

(defconst +aankh/difftastic-colour-remapping+
  `(("red2" . "#a8353e") ;; https://oklch.com/#50,0.15,20,100
    ("green2" . "#107823")
    ("yellow2" . "#2f3b97")))

I picked my colours using the OKLCH colour picker & converter to try to keep them in harmony with each other. When called at the end of th/magit--with-difftastic’s sentinel lambda, this function produces a more legible structural diff:

While I’d like to incorporate these into the regular Magit status view—probably requiring either unified diff–style output or JSON output—I’m satisfied for now. As a final step, I moved the three commands to a new subsection of the magit-diff transient: 

Emacs Lisp;; For some reason, this was being called twice without the guard.
(unless (boundp 'aankh/added-magit-diff-suffixes)
  (transient-append-suffix 'magit-diff '(-1 -1)
    [("l" "Toggle magit-delta" aankh/toggle-magit-delta)
     ("D" "Difftastic Diff (dwim)" th/magit-diff-with-difftastic)
     ("S" "Difftastic Show" th/magit-show-with-difftastic)]))
(setf aankh/added-magit-diff-suffixes t)

For reference, here’s a visual comparison between regular Magit diffs, Magit diffs with hunk refinement, and my new options:

Plus, how difftastic… compares to delta: