Subject: Syntactic fontification of diff hunks
From: Juri Linkov <[email-masked]>
To: [email-masked]
Date: Sat, 01 Dec 2018 23:55:40 +0200
List-Id: <debbugs-submit.debbugs.gnu.org>

Tags: patch
Severity: wishlist

For a long time after announcing this feature in
https://lists.gnu.org/archive/html/emacs-devel/2018-08/msg00537.html
I received requests in private mails asking when I'll submit a complete patch.
I'm sorry, it took much time addressing all concerns raised in that thread,
and testing in many possible scenarios.  Based on the feedback, I rewrote it
several times, and now finally it's optimized to be fast and reliable.

diff --git a/lisp/vc/diff-mode.el b/lisp/vc/diff-mode.el
index 4adef02984..68b2f9e522 100644
--- a/lisp/vc/diff-mode.el
+++ b/lisp/vc/diff-mode.el
@@ -103,6 +103,21 @@ diff-font-lock-prettify
   :version "27.1"
   :type 'boolean)
 
+(defcustom diff-font-lock-syntax 'vc
+  "If non-nil, diff hunk font-lock includes syntax highlighting.
+If `vc', highlight syntax only in Diff buffers created by a version control
+system that provides all necessary context for reliable highlighting.
+If t, additionally try to get more context from existing files, or when
+source files are not found, still try to highlight hunks without enough
+context that sometimes might result in wrong fontification.
+If `hunk-only', fontification is based on hunk alone, without full source.
+This is the fastest, but less reliable."
+  :version "27.1"
+  :type '(choice (const :tag "Don't highlight syntax" nil)
+                 (const :tag "Only under version control" vc)
+                 (const :tag "Hunk-based only" hunk-only)
+                 (const :tag "Without full source or get it from files" t)))
+
 (defvar diff-vc-backend nil
   "The VC backend that created the current Diff buffer, if any.")
 
@@ -406,6 +421,7 @@ diff-font-lock-keywords
      (1 font-lock-comment-delimiter-face)
      (2 font-lock-comment-face))
     ("^[^-=+*!<>#].*\n" (0 'diff-context))
+    (,#'diff--font-lock-syntax)
     (,#'diff--font-lock-prettify)
     (,#'diff--font-lock-refined)))
 
@@ -2316,6 +2333,195 @@ diff--font-lock-prettify
                              'display "")))))
   nil)
 
+;;; Syntax highlighting from font-lock
+
+(defun diff--font-lock-syntax (max)
+  "Syntax highlighting from font-lock."
+  (when diff-font-lock-syntax
+    (when (get-char-property (point) 'diff--font-lock-syntax)
+      (goto-char (next-single-char-property-change
+                  (point) 'diff--font-lock-syntax nil max)))
+    (let* ((min (point))
+           (beg (or (ignore-errors (diff-beginning-of-hunk))
+                    (ignore-errors (diff-hunk-next) (point))
+                    max)))
+      (while (< beg max)
+        (let ((end
+               (save-excursion (goto-char beg) (diff-end-of-hunk) (point))))
+          (if (< end min) (setq beg min))
+          (unless (or (< end beg)
+                      (get-char-property beg 'diff--font-lock-syntax))
+            (diff-syntax-fontify beg end)
+            (let ((ol (make-overlay beg end)))
+              (overlay-put ol 'diff--font-lock-syntax t)
+              (overlay-put ol 'diff-mode 'syntax)
+              (overlay-put ol 'evaporate t)
+              (overlay-put ol 'modification-hooks
+                           '(diff--font-lock-syntax--refresh))))
+          (goto-char (max beg end))
+          (setq beg (or (ignore-errors (diff-hunk-next) (point)) max))))))
+  nil)
+
+(defun diff--font-lock-syntax--refresh (ol _after _beg _end &optional _len)
+  (delete-overlay ol))
+
+(defun diff-syntax-fontify (start end)
+  (save-excursion
+    (diff-syntax-fontify-hunk start end t)
+    (diff-syntax-fontify-hunk start end nil)))
+
+(defvar diff-syntax-fontify-revisions (make-hash-table :test 'equal))
+
+(defun diff-syntax-fontify-hunk (beg end old)
+  "Highlight language syntax in diff hunks."
+  (remove-overlays beg end 'diff-mode 'syntax)
+  (goto-char beg)
+  (let* ((hunk (buffer-substring-no-properties beg end))
+         (text (or (ignore-errors (diff-hunk-text hunk (not old) nil)) ""))
+	 (line (if (looking-at "\\(?:\\*\\{15\\}.*\n\\)?[-@* ]*\\([0-9,]+\\)\\([ acd+]+\\([0-9,]+\\)\\)?")
+		   (if old (match-string 1)
+		     (if (match-end 3) (match-string 3) (match-string 1)))))
+         (line-nb (and line (string-match "\\([0-9]+\\),\\([0-9]+\\)" line)
+                       (list (string-to-number (match-string 1 line))
+                             (string-to-number (match-string 2 line)))))
+         props)
+    (cond
+     ((and diff-vc-backend (not (eq diff-font-lock-syntax 'hunk-only)))
+      (let* ((file (diff-find-file-name old t))
+             (revision (and file (if (not old) (nth 1 diff-vc-revisions)
+                                   (or (nth 0 diff-vc-revisions)
+                                       (vc-working-revision file))))))
+        (if file
+            (if (not revision)
+                ;; Get properties from the current working file
+                (when (and (not old) (file-exists-p file))
+                  ;; Try to reuse an existing buffer
+                  (if (get-file-buffer (expand-file-name file))
+                      (with-current-buffer (get-file-buffer (expand-file-name file))
+                        (setq props (diff-syntax-fontify-props nil text line-nb t)))
+                    ;; Get properties from the file
+                    (with-temp-buffer
+                      (insert-file-contents file t)
+                      (setq props (diff-syntax-fontify-props file text line-nb)))))
+              ;; Get properties from a cached revision
+              (let* ((buffer-name (format " diff-syntax:%s.~%s~"
+                                          (expand-file-name file) revision))
+                     (buffer (gethash buffer-name diff-syntax-fontify-revisions))
+                     (no-init t))
+                (unless (and buffer (buffer-live-p buffer))
+                  (let* ((vc-find-revision-no-save t)
+                         (vc-buffer (save-window-excursion
+                                      ;; Restore restore previous window configuration
+                                      ;; because when vc-find-revision can't find a revision
+                                      ;; (e.g. for /dev/null), it jumps to another window
+                                      ;; using pop-to-buffer in vc-do-command when
+                                      ;; the buffer name doesn't begin with a space char.
+                                      (ignore-errors
+                                        (vc-find-revision (expand-file-name file)
+                                                          revision diff-vc-backend)))))
+                    (when vc-buffer
+                      (with-current-buffer (get-buffer-create buffer-name)
+                        (insert-buffer-substring-no-properties vc-buffer)
+                        (setq buffer (current-buffer) no-init nil))
+                      (puthash buffer-name buffer diff-syntax-fontify-revisions)
+                      (kill-buffer vc-buffer))))
+                (when buffer
+                  (with-current-buffer buffer
+                    (setq props (diff-syntax-fontify-props file text line-nb no-init))))))
+          ;; If file is unavailable, get properties from the hunk alone
+          (setq file (car (diff-hunk-file-names old)))
+          (with-temp-buffer
+            (insert text)
+            (setq props (diff-syntax-fontify-props file text line-nb nil t))))))
+     ((eq diff-font-lock-syntax 'hunk-only)
+      (setq file (car (diff-hunk-file-names old)))
+      (with-temp-buffer
+        (insert text)
+        (setq props (diff-syntax-fontify-props file text line-nb nil t))))
+     ((not (eq diff-font-lock-syntax 'vc))
+      (let ((file (car (diff-hunk-file-names old))))
+        (if (and file (file-exists-p file))
+            ;; Try to get full text from the file
+            (with-temp-buffer
+              (insert-file-contents file t)
+              (setq props (diff-syntax-fontify-props file text line-nb)))
+          ;; Otherwise, get properties from the hunk alone
+          (with-temp-buffer
+            (insert text)
+            (setq props (diff-syntax-fontify-props file text line-nb nil t)))))))
+
+    ;; Put properties over the hunk text
+    (when props
+      (goto-char beg)
+      (while (< (progn (forward-line 1) (point)) end)
+        (when (or (and (not old) (not (looking-at-p "[-<]")))
+                  (and      old  (not (looking-at-p "[+>]"))))
+          (if (and old (not (looking-at-p "[-<]")))
+              ;; Fontify context lines only from new source,
+              ;; don't refontify context lines from old source.
+              (pop props)
+            (let ((line-props (pop props))
+                  (bol (1+ (point))))
+              (dolist (prop line-props)
+                (let ((ol (make-overlay (+ bol (nth 0 prop))
+                                        (+ bol (nth 1 prop))
+                                        nil 'front-advance nil)))
+                  (overlay-put ol 'evaporate t)
+                  (overlay-put ol 'face (nth 2 prop)))))))))))
+
+(defun diff-syntax-fontify-props (file text line-nb &optional no-init hunk-only)
+  "Get font-lock properties from the source code."
+  (unless no-init
+    (buffer-disable-undo)
+    (font-lock-mode -1)
+    (let ((enable-local-variables :safe) ;; to find `mode:'
+          (buffer-file-name file))
+      (set-auto-mode)
+      (generic-mode-find-file-hook)))
+
+  (let ((font-lock-defaults (or font-lock-defaults '(nil t)))
+        props beg end)
+    (goto-char (point-min))
+    (if hunk-only
+        (setq beg (point-min) end (point-max))
+      (forward-line (1- (nth 0 line-nb)))
+      ;; non-regexp looking-at to compare hunk text for verification
+      (if (search-forward text (+ (point) (length text)) t)
+          (setq beg (- (point) (length text)) end (point))
+        (goto-char (point-min))
+        (if (search-forward text nil t)
+            (setq beg (- (point) (length text)) end (point)))))
+
+    (when (and beg end)
+      (goto-char beg)
+      (when (text-property-not-all beg end 'fontified t)
+        (if file
+            ;; In a temporary or cached buffer
+            (save-excursion
+              (font-lock-fontify-region beg end)
+              (put-text-property beg end 'fontified t))
+          ;; In an existing buffer
+          (font-lock-ensure beg end)))
+
+      (while (< (point) end)
+        (let* ((bol (point))
+               (eol (line-end-position))
+               line-props
+               (searching t)
+               (from (point)) to
+               (val (get-text-property from 'face)))
+          (while searching
+            (setq to (next-single-property-change from 'face nil eol))
+            (when val (push (list (- from bol) (- to bol) val) line-props))
+            (setq val (get-text-property to 'face) from to)
+            (unless (< to eol) (setq searching nil)))
+          (when val (push (list from eol val) line-props))
+          (push (nreverse line-props) props))
+        (forward-line 1)))
+    (set-buffer-modified-p nil)
+    (nreverse props)))
+
+
 (defun diff--filter-substring (str)
   (when diff-font-lock-prettify
     ;; Strip the `display' properties added by diff-font-lock-prettify,