Bind SPC g d to difftastic

As I don't see a new binding in the Magit arena that makes sense.
2022-08-09 21:27:29 -07:00 · 2022-08-09 21:27:29 -07:00 · 3b8cfa1f73
commit 3b8cfa1f73
parent 89446f001b
1 changed files with 128 additions and 0 deletions
--- a/ha-config.org
+++ b/ha-config.org
@ -1464,6 +1464,134 @@ I also need to append the following to my [[file:~/.gitconfig][~/.gitconfig]] fi
        plus-emph-style               = syntax "#009000"
        plus-empty-line-marker-style  = normal "#006800"
 #+end_src
+*** Git with Difftastic
+I’m stealing the code for this section from [[https://tsdh.org/posts/2022-08-01-difftastic-diffing-with-magit.html][this essay]] by Tassilo Horn, and in fact, I’m going to lift a lot of his explanation too, as I may need to remind myself how this works. The idea is based on using Wilfred’s excellent [[https://github.com/Wilfred/difftastic][difftastic]] tool to do a structural/syntax comparison of code changes in git. To begin, install the binary:
+#+begin_src sh
+  brew install difftastic # and the equivalent on Linux
+#+end_src
+Next, we can do this, to use this as a diff tool for everything.
+#+begin_src emacs-lisp
+  (setenv "GIT_EXTERNAL_DIFF" "difft")
+#+end_src
+But perhaps integrating it into Magit and selectively calling it (as it is slow). Tassilo suggests making the call to =difft= optional by first creating a helper function to set the =GIT_EXTERNAL_DIFF= to =difft=:
+#+begin_src emacs-lisp :tangle no
+  (defun th/magit--with-difftastic (buffer command)
+    "Run COMMAND with GIT_EXTERNAL_DIFF=difft then show result in BUFFER."
+    (let ((process-environment
+           (cons (concat "GIT_EXTERNAL_DIFF=difft --width="
+                         (number-to-string (frame-width)))
+                 process-environment)))
+      ;; Clear the result buffer (we might regenerate a diff, e.g., for
+      ;; the current changes in our working directory).
+      (with-current-buffer buffer
+        (setq buffer-read-only nil)
+        (erase-buffer))
+      ;; Now spawn a process calling the git COMMAND.
+      (make-process
+       :name (buffer-name buffer)
+       :buffer buffer
+       :command command
+       ;; Don't query for running processes when emacs is quit.
+       :noquery t
+       ;; Show the result buffer once the process has finished.
+       :sentinel (lambda (proc event)
+                   (when (eq (process-status proc) 'exit)
+                     (with-current-buffer (process-buffer proc)
+                       (goto-char (point-min))
+                       (ansi-color-apply-on-region (point-min) (point-max))
+                       (setq buffer-read-only t)
+                       (view-mode)
+                       (end-of-line)
+                       ;; difftastic diffs are usually 2-column side-by-side,
+                       ;; so ensure our window is wide enough.
+                       (let ((width (current-column)))
+                         (while (zerop (forward-line 1))
+                           (end-of-line)
+                           (setq width (max (current-column) width)))
+                         ;; Add column size of fringes
+                         (setq width (+ width
+                                        (fringe-columns 'left)
+                                        (fringe-columns 'right)))
+                         (goto-char (point-min))
+                         (pop-to-buffer
+                          (current-buffer)
+                          `(;; If the buffer is that wide that splitting the frame in
+                            ;; two side-by-side windows would result in less than
+                            ;; 80 columns left, ensure it's shown at the bottom.
+                            ,(when (> 80 (- (frame-width) width))
+                               #'display-buffer-at-bottom)
+                            (window-width . ,(min width (frame-width))))))))))))
+#+end_src
+The crucial parts of this helper function are that we "wash" the result using =ansi-color-apply-on-region= so that the function can transform the difftastic highlighting using shell escape codes to Emacs faces. Also, note the need to possibly change the width, as difftastic makes a side-by-side comparison.
+
+The functions below depend on [[help:magit-thing-at-point][magit-thing-at-point]], and this depends on the [[https://sr.ht/~pkal/compat/][compat]] library, so let’s grab that stuff:
+#+begin_src emacs-lisp :tangle no
+  (use-package compat
+    :straight (:host github :repo "emacs-straight/compat"))
+
+  (use-package magit-section
+    :commands magit-thing-at-point)
+#+end_src
+Next, let's define our first command basically doing a =git show= for some revision which defaults to the commit or branch at point or queries the user if there's none.
+#+begin_src emacs-lisp :tangle no
+  (defun th/magit-show-with-difftastic (rev)
+    "Show the result of \"git show REV\" with GIT_EXTERNAL_DIFF=difft."
+    (interactive
+     (list (or
+            ;; Use if given the REV variable:
+            (when (boundp 'rev) rev)
+            ;; If not invoked with prefix arg, try to guess the REV from
+            ;; point's position.
+            (and (not current-prefix-arg)
+                 (or (magit-thing-at-point 'git-revision t)
+                     (magit-branch-or-commit-at-point)))
+            ;; Otherwise, query the user.
+            (magit-read-branch-or-commit "Revision"))))
+    (if (not rev)
+        (error "No revision specified")
+      (th/magit--with-difftastic
+       (get-buffer-create (concat "*git show difftastic " rev "*"))
+       (list "git" "--no-pager" "show" "--ext-diff" rev))))
+#+end_src
+And here the second command which basically does a =git diff=. It tries to guess what one wants to diff, e.g., when point is on the Staged changes section in a magit buffer, it will run =git diff --cached= to show a diff of all staged changes. If it can not guess the context, it'll query the user for a range or commit for diffing.
+#+begin_src emacs-lisp :tangle no
+  (defun th/magit-diff-with-difftastic (arg)
+    "Show the result of \"git diff ARG\" with GIT_EXTERNAL_DIFF=difft."
+    (interactive
+     (list (or
+            ;; Use If RANGE is given, just use it.
+            (when (boundp 'range) range)
+            ;; If prefix arg is given, query the user.
+            (and current-prefix-arg
+                 (magit-diff-read-range-or-commit "Range"))
+            ;; Otherwise, auto-guess based on position of point, e.g., based on
+            ;; if we are in the Staged or Unstaged section.
+            (pcase (magit-diff--dwim)
+              ('unmerged (error "unmerged is not yet implemented"))
+              ('unstaged nil)
+              ('staged "--cached")
+              (`(stash . ,value) (error "stash is not yet implemented"))
+              (`(commit . ,value) (format "%s^..%s" value value))
+              ((and range (pred stringp)) range)
+              (_ (magit-diff-read-range-or-commit "Range/Commit"))))))
+    (let ((name (concat "*git diff difftastic"
+                        (if arg (concat " " arg) "")
+                        "*")))
+      (th/magit--with-difftastic
+       (get-buffer-create name)
+       `("git" "--no-pager" "diff" "--ext-diff" ,@(when arg (list arg))))))
+#+end_src
+
+What's left is integrating the new show and diff commands in Magit. For that purpose, Tasillo created a new transient prefix for all personal commands. Intriguing, but I have a hack that I can use on a leader:
+#+begin_src emacs-lisp :tangle no
+  (defun ha-difftastic-here ()
+    (interactive)
+    (call-interactively
+     (if (eq major-mode 'magit-log-mode)
+         'th/magit-show-with-difftastic
+       'th/magit-diff-with-difftastic)))
+
+  (ha-leader "g d" '("difftastic" . ha-difftastic-here))
 #+end_src
 *** Time Machine
 The [[https://github.com/emacsmirror/git-timemachine][git-timemachine]] project visually shows how a code file changes with each iteration: