From b87daa72aa4f833001cd60be0d1f5cfe4064f717 Mon Sep 17 00:00:00 2001
From: Philip Kaludercic <philipk@posteo.net>
Date: Sat, 19 Aug 2023 11:47:54 +0200
Subject: [PATCH] Add command to copy contents in a diff-mode buffer

* lisp/vc/diff-mode.el (diff-mode-shared-map): Bind 'diff-kill-ring-save'.
(diff-mode-map): Ensure the "w" binding does not get prefixed.
(diff-beginning-of-hunk-position, diff-end-of-hunk-position):
New utility functions.
(diff-kill-ring-save): Add the command.
* etc/NEWS: Mention 'diff-kill-ring-save'.  (Bug#65380)
---
 etc/NEWS             |  9 +++++++
 lisp/vc/diff-mode.el | 58 +++++++++++++++++++++++++++++++++++++++++++-
 2 files changed, 66 insertions(+), 1 deletion(-)

diff --git a/etc/NEWS b/etc/NEWS
index b89a80aa14d..e493c1bb035 100644
--- a/etc/NEWS
+++ b/etc/NEWS
@@ -210,6 +210,15 @@ The host name for Kubernetes connections can be of kind
 used.  This overrides the setiing in 'tramp-kubernetes-namespace', if
 any.
 
+** Diff
+
+---
+*** New command 'diff-kill-ring-save'
+This command copies out the modified contents out of a diff, without
+having to apply it first.  If the selected range extends a hunk, the
+commands attempts to look up and copy the text between from the
+referenced file.
+
 
 * New Modes and Packages in Emacs 31.1
 
diff --git a/lisp/vc/diff-mode.el b/lisp/vc/diff-mode.el
index 81e8b23ee33..0f3b4c6b4d1 100644
--- a/lisp/vc/diff-mode.el
+++ b/lisp/vc/diff-mode.el
@@ -196,6 +196,7 @@ diff-mode-shared-map
   "RET" #'diff-goto-source
   "<mouse-2>" #'diff-goto-source
   "W" #'widen
+  "w" #'diff-kill-ring-save
   "o" #'diff-goto-source                ; other-window
   "A" #'diff-ediff-patch
   "r" #'diff-restrict-view
@@ -208,7 +209,7 @@ diff-mode-map
           ;; We want to inherit most bindings from
           ;; `diff-mode-shared-map', but not all since they may hide
           ;; useful `M-<foo>' global bindings when editing.
-          (dolist (key '("A" "r" "R" "g" "q" "W" "z"))
+          (dolist (key '("A" "r" "R" "g" "q" "W" "w" "z"))
             (keymap-set map key nil))
           map)
   ;; From compilation-minor-mode.
@@ -630,6 +631,22 @@ diff-end-of-hunk
     ;; The return value is used by easy-mmode-define-navigation.
     (goto-char (or end (point-max)))))
 
+(defun diff-beginning-of-hunk-position (&optional try-harder)
+  "Call `diff-end-of-hunk' without moving.
+The optional argument TRY-HARDER is passed on to
+`diff-beginning-of-hunk'."
+  (save-excursion
+    (save-window-excursion
+      (diff-beginning-of-hunk try-harder))))
+
+(defun diff-end-of-hunk-position (&optional style donttrustheader)
+  "Call `diff-end-of-hunk' without moving.
+The optional arguments STYLE and DONTTRUSTHEADER are passed on to
+`diff-end-of-hunk'."
+  (save-excursion
+    (save-window-excursion
+      (diff-end-of-hunk style donttrustheader))))
+
 ;; "index ", "old mode", "new mode", "new file mode" and
 ;; "deleted file mode" are output by git-diff.
 (defconst diff-file-junk-re
@@ -2108,6 +2125,45 @@ diff-goto-source
       (goto-char (+ (car pos) (cdr src)))
       (when buffer (next-error-found buffer (current-buffer))))))
 
+(defun diff-kill-ring-save (beg end &optional reverse)
+  "Save contents of the region between BEG and END akin to `kill-ring-save'.
+By default the command will copy the text that applying the diff would
+produce, along with the text between hunks.  If REVERSE is non-nil, or
+the command was invoked with a prefix argument, copy the deleted text."
+  (interactive (list (region-beginning) (region-end) current-prefix-arg))
+  (unless (derived-mode-p 'diff-mode)
+    (user-error "Command can only be invoked in a diff-buffer"))
+  (let ((parts '()))
+    (save-excursion
+      (goto-char beg)
+      (catch 'break
+        (while t
+          (let ((hunk (diff-hunk-text
+                       (buffer-substring
+                        (diff-beginning-of-hunk-position)
+                        (min (diff-end-of-hunk-position) end))
+                       (not reverse)
+                       (- (point) (diff-beginning-of-hunk-position)))))
+            (push (substring (car hunk) (cdr hunk))
+                  parts))
+          ;; check if we have copied everything
+          (diff-end-of-hunk)
+          (when (<= end (point)) (throw 'break t))
+          ;; copy the text between hunks
+          (let ((inhibit-message t) start)
+            (save-window-excursion
+              (save-excursion
+                (forward-line -1)
+                (diff-goto-source t)
+                (forward-line +1)
+                (setq start (point))))
+            (save-window-excursion
+              (diff-goto-source t)
+              (push (buffer-substring start (point))
+                    parts))))))
+    (kill-new (string-join (nreverse parts)))
+    (setq deactivate-mark t)
+    (message (if reverse "Copied original text" "Copied modified text"))))
 
 (defun diff-current-defun ()
   "Find the name of function at point.
-- 
2.46.0