bug#25410: 26.0.50; Refine an unified diff hunk only if adds lines

[Tino Calancha <tino.calancha@gmail.com>]

npostavs@users.sourceforge.net writes:

> Tino Calancha <tino.calancha@gmail.com> writes:
>
>> After deletion of a large file from CVS, a diff shows
>> a very large hunk with just deleted lines.  Then, for unified diffs, a call
>> to `diff-refine-hunk' on that hunk takes a huge time.
>> Instead, it's better to first check if the hunk adds new lines: only when
>> this is true, then proceed with the hunk refinement.
>
> What about a diff that adds a very large file?  Perhaps we should only
> refine if there added lines *and* deleted lines?
I have updated the patch.  Now it checks before the `pcase' that
the hunk adds and removes lines.  Only when this is true, we enter
in the `pcase'.

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From 0ff79ca6f106f121a05b8c5de55990b88fecb4d2 Mon Sep 17 00:00:00 2001
From: Tino Calancha <tino.calancha@gmail.com>
Date: Wed, 11 Jan 2017 11:42:56 +0900
Subject: [PATCH] Only refine diff hunks that both remove and add lines

* lisp/vc/diff-mode.el (diff-refine-hunk): Refine the hunk
only if it adds and removes some lines (Bug#25410).
---
 lisp/vc/diff-mode.el | 77 ++++++++++++++++++++++++++--------------------------
 1 file changed, 39 insertions(+), 38 deletions(-)

diff --git a/lisp/vc/diff-mode.el b/lisp/vc/diff-mode.el
index 9dfcd944bb..f0c53e6f8a 100644
--- a/lisp/vc/diff-mode.el
+++ b/lisp/vc/diff-mode.el
@@ -2075,44 +2075,45 @@ diff-refine-hunk
            (props-c '((diff-mode . fine) (face diff-refine-changed)))
            (props-r '((diff-mode . fine) (face diff-refine-removed)))
            (props-a '((diff-mode . fine) (face diff-refine-added))))
-
-      (remove-overlays beg end 'diff-mode 'fine)
-
-      (goto-char beg)
-      (pcase style
-        (`unified
-         (while (re-search-forward
-                 (eval-when-compile
-                   (let ((no-LF-at-eol-re "\\(?:\\\\.*\n\\)?"))
-                     (concat "^\\(?:-.*\n\\)+" no-LF-at-eol-re
-                             "\\(\\)"
-                             "\\(?:\\+.*\n\\)+" no-LF-at-eol-re)))
-                 end t)
-           (smerge-refine-subst (match-beginning 0) (match-end 1)
-                                (match-end 1) (match-end 0)
-                                nil 'diff-refine-preproc props-r props-a)))
-        (`context
-         (let* ((middle (save-excursion (re-search-forward "^---")))
-                (other middle))
-           (while (re-search-forward "^\\(?:!.*\n\\)+" middle t)
-             (smerge-refine-subst (match-beginning 0) (match-end 0)
-                                  (save-excursion
-                                    (goto-char other)
-                                    (re-search-forward "^\\(?:!.*\n\\)+" end)
-                                    (setq other (match-end 0))
-                                    (match-beginning 0))
-                                  other
-                                  (if diff-use-changed-face props-c)
-                                  'diff-refine-preproc
-                                  (unless diff-use-changed-face props-r)
-                                  (unless diff-use-changed-face props-a)))))
-        (_ ;; Normal diffs.
-         (let ((beg1 (1+ (point))))
-           (when (re-search-forward "^---.*\n" end t)
-             ;; It's a combined add&remove, so there's something to do.
-             (smerge-refine-subst beg1 (match-beginning 0)
-                                  (match-end 0) end
-                                  nil 'diff-refine-preproc props-r props-a))))))))
+      ;; Only refine the hunk if both adds and removes lines (Bug#25410).
+      (when (and (save-excursion (re-search-forward "^-.*\n" end t))
+                 (re-search-forward "^\\+.*\n" end t))
+        (remove-overlays beg end 'diff-mode 'fine)
+        (goto-char beg)
+        (pcase style
+          (`unified
+           (while (re-search-forward
+                   (eval-when-compile
+                     (let ((no-LF-at-eol-re "\\(?:\\\\.*\n\\)?"))
+                       (concat "^\\(?:-.*\n\\)+" no-LF-at-eol-re
+                               "\\(\\)"
+                               "\\(?:\\+.*\n\\)+" no-LF-at-eol-re)))
+                   end t)
+             (smerge-refine-subst (match-beginning 0) (match-end 1)
+                                  (match-end 1) (match-end 0)
+                                  nil 'diff-refine-preproc props-r props-a)))
+          (`context
+           (let* ((middle (save-excursion (re-search-forward "^---")))
+                  (other middle))
+             (while (re-search-forward "^\\(?:!.*\n\\)+" middle t)
+               (smerge-refine-subst (match-beginning 0) (match-end 0)
+                                    (save-excursion
+                                      (goto-char other)
+                                      (re-search-forward "^\\(?:!.*\n\\)+" end)
+                                      (setq other (match-end 0))
+                                      (match-beginning 0))
+                                    other
+                                    (if diff-use-changed-face props-c)
+                                    'diff-refine-preproc
+                                    (unless diff-use-changed-face props-r)
+                                    (unless diff-use-changed-face props-a)))))
+          (_ ;; Normal diffs.
+           (let ((beg1 (1+ (point))))
+             (when (re-search-forward "^---.*\n" end t)
+               ;; It's a combined add&remove, so there's something to do.
+               (smerge-refine-subst beg1 (match-beginning 0)
+                                    (match-end 0) end
+                                    nil 'diff-refine-preproc props-r props-a)))))))))
 
 (defun diff-undo (&optional arg)
   "Perform `undo', ignoring the buffer's read-only status."
-- 
2.11.0

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In GNU Emacs 26.0.50.1 (x86_64-pc-linux-gnu, GTK+ Version 3.22.5)
 of 2017-01-10
Repository revision: fa0a2b4e7c81f57aecc1d94df00588a4dd5c281d